eutrophication in the baltic sea - s...

Eutrophication in the Baltic Sea

from area-specific biological effects to interdisciplinary consequences

Cecilia Lundberg

Environmental and Marine Biology, Department of Biology Åbo Akademi University

Åbo, Finland

2005

Supervised by Prof Erik Bonsdorff Environmental and Marine Biology, Department of Biology Åbo Akademi University Akademigatan 1 FI-20500 Åbo Finland Reviewed by PhD Sif Johansson Swedish Environment Protection Agency SE-106 48 Stockholm Sweden Prof Daniel Conley Danish National Environmetal Research Institute (NERI) Box 358 DK- 4000 Roskilde Denmark Faculty opponent Prof Michael Elliott Department of Biological Sciences University of Hull Hull, HU6 7RX United Kingdom

© Cecilia Lundberg ISBN: 952-12-1537-2

Åbo Akademis tryckeri

Åbo 2005

Eutrophication in the Baltic Sea - from area-specific biological effects to interdisciplinary consequences Cecilia Lundberg Environmental and Marine Biology, Department of Biology, Åbo Akademi University, FI-20500 Åbo, Finland Abstract The semi-enclosed topography and the strong anthropogenic pressures make the Baltic Sea vulnerable to environmentally induced ecological changes. The large-scale nutrient over-enrichment is one of the most serious threats against the health and future perspectives of the Baltic Sea ecosystem. To combat the eutrophication in a complex marine ecosystem, an interdisciplinary agenda is needed. Co-operation must function between the fields of natural science and socio-economy, and between science and management. For a complete understanding of the eutrophication process and possibilities for remedies, the linkages between ecology and economy is essential. Gaps in communication between science and decision-makers are other obstacles to overcome. The DPSIR-approach (drivers, pressure, state, impact, response) is one way to conceptualize environmental changes and present information in a comprehensive way for society. For a comprehensive analysis of the eutrophication-related processes in the Baltic Sea, the problem is here confronted both in general terms on a basinwide scale and in detail on regional scales. The complexness of eutrophication is illustrated in a general conceptual model, where the pathways from natural science to human society and management options are presented. The model is intended as an interdisciplinary communication base. To pin-point regional differences, the Baltic Sea is divided into nine sub-regions for area-specific analyses. Based on publicly available papers and reports, the effects and duration of the nutrient over-enrichment are described for each sub-region. The present state of any particular area is summarised in regional conceptual models, which are updated to cover the present situation (2004). For further detection of eutrophication-related trends on a site-specific level, monitoring programmes are important sources of information. Eutrophication is a problem in the entire Baltic Sea area, but the effects and consequences varies between sub-basins. Positive measurements and potential recovery require action both on basinwide and regional scales. To widen the awareness between scientific disciplines and to present the problem to society, principal and site-specific conceptual models are important starting points. Keywords: eutrophication, area-specific changes, conceptual models, interdisciplinary, DPSIR, Baltic Sea.

Övergödningen i Östersjön - småskaliga biologiska effekter ger tvärvetenskapliga följder Svensk sammanfattning – Summary in Swedish Östersjön är ett litet, grunt och instängt havsområde med ett begränsat vattenutbyte till Nordsjön. Skarpa skillnader i salthalt och klimat skiljer de södra och norra delarna från varandra. Ett invånarantal på 85 miljoner i hela avrinningsområdet gör belastningstrycket betydande. Dessa faktorer inverkar väsentligt på Östersjöns känslighet för miljöförändringar, speciellt sådana orsakade av människan. Eutrofiering innebär en ökad tillgänglighet av fosfor och kväve i vattnet. Näringsämnena härstammar från luftburet kväve, avrinning från jord- och skogsbruksmark samt orenat eller otillräckligt renat avloppsvatten från hushåll och industrier. Överskottsnäringen gynnar snabb-växande vegetation, som på sikt konkurrerar ut långsamt växande fleråriga arter med nyckel-funktioner för hela ekosystemet. Ute på öppet vatten leder övergödningen till massföre-komster av cyanobakterier (blågröna alger) under varma och vindstilla perioder av sommaren. Nedbrytningen av alla eutrofieringsgynnade arter är syrekrävande.Vid syrebrist produceras svavelväte, och stora bottenarealer förvandlas till undervattensöknar för kortare eller längre perioder då allt djurliv slås ut. I det sammansatta marina ekosystemet påverkas samtliga arter, biodiversiteten minskar då känsliga växter och djur minskar i antal eller försvinner helt. Eutrofiering är därtill inte Östersjöns enda problem, utan måste ställas i relation till andra gissel såsom miljögifter, överfiskning och etableringen av främmande arter. Åtgärder för en förbättring av Östersjöns tillstånd kräver en mångfacetterad syn och hand-lingskraft genom ett tvärvetenskapligt engagemang. Ett fungerande samarbete mellan natur-vetenskap och socio-ekonomi, liksom mellan vetenskap och beslutsfattande är nödvändigt. Övergödning har inte verkningar enbart på en biologisk och fysikalisk-kemisk nivå. Problemet kräver också ett ekonomiskt, tekniskt och sociologiskt nytänkande. Vilken teknik är mest hållbar ur miljöns synpunkt utan att hindra människan från att utnyttja naturens resurser? Vad kostar det att rena Östersjön och vilken strategi är mest prisvärd? Kom-munikationsförbistringar kan överkommas med hjälp av begreppsmässiga, s.k. konceptuella, modeller där förändringar i miljön kartläggs på ett lättförståeligt sätt, samtidigt som möjligast många aspekter av problemet framkommer. I min avhandling strävar jag till att framställa Östersjöns eutrofiering ur flera olika synvinklar. Problemet presenteras dels på ett allmänt plan, dels regionalt mer i detalj. Jag har gjort ett försök att kartlägga övergödningsproblemet i Östersjön i sin helhet i en konceptuell modell. Modellen visar hur både naturliga och antropogena drivmedel åstadkommer fysikalisk-kemiska och biologiska verkningar i ekosystemet, samt hur dessa kopplas till ekonomiska, hälsomässiga och estetiska konsekvenser på ett samhälleligt plan. För vidtagande av positiva åtgärder krävs restriktioner och bestämmelser från beslutsfattare. Min modell kan jämföras med det s.k. DPSIR-tankesättet, som visar hur drivkrafter (Drivers), påtryckning (Pressures), tillstånd (State), effekter (Impact) och reaktioner (Response) kopplas till varandra för att för-klara orsakerna till miljöförändringar. Övergödningen är ett hot mot hela Östersjöns framtid, men olika kusttyper och skillnaden mellan kustområde och öppet hav påverkar händelseförloppet. Skärgårdar fungerar som filter mellan kustområden och öppna Östersjön. Näringsämnen bibehålls, omformas och lagras på plats istället för att snabbt föras ut till havs. Längs en öppen kustlinje är förhållandet det motsatta. För att peka på regionala skillnader har jag delat in Östersjön i nio delområden. På

basen av tillgänglig litteratur, från vetenskapligt granskade publikationer till s.k. grå rapporter och webbinformation, har jag sammanställt eutrofieringsgraden och när problemen uppstod skilt för varje delområde. Det rådande tillståndet för varje region är sammanfattad i en konceptuell modell. För noggrannare analyser av utveckling i näringstillstånd, syresituation etc. på regional nivå har långtidsdata från nationella och internationella övervakningsprogram värdefullt informationsvärde. I det här arbetet har mönster i fysikaliska och biologiska para-metrar uppmätta längs den finska kustzonen av Finska viken studerats närmare. Resultaten av min avhandling visar att Östersjöns tillstånd och återhämtningsmöjligheter ifrågan om övergödning kräver handlingsprogram både på en storskalig nivå som täcker hela Östersjön och på regionala plan, där specifika problem för specifika områden beaktas. För positiva resultat krävs en medvetenhet och vilja till förbättringar som sträcker sig över vetenskapliga ämnesdomäner och ut till beslutsfattare och allmänheten. Användarvänliga modeller som beskriver problemet är en viktig grund för samarbetet. Nyckelord: eutrofiering, områdesspecifika förändringar, konceptuella modeller, tvär-vetenskap, DPSIR-tankesätt, Östersjön

Table of contents List of orginal publications.......................................................................................................... 11 Contributions to the publications, list of abbreviations ............................................................... 12 1. INTRODUCTION................................................................................................................ 13 1.1 The Baltic Sea .......................................................................................................... 14 1.2 Eutrophication – a global problem ........................................................................... 16 1.3 Ecology meets society .............................................................................................. 17 1.3.1 The DPSIR-approach ................................................................................... 18 1.4 Eutrophication – measurements in the Baltic Sea region ......................................... 18 1.5 Conceptualizing the eutrophication .......................................................................... 19 1.5.1 The EU Water Framework Directive............................................................ 20 1.6 Area-specific eutrophication .................................................................................... 20 1.7 Aims of the work ...................................................................................................... 22 2. ASSESSING EUTROPHICATION..................................................................................... 23 2.1 Eutrophication parameters ........................................................................................ 26 2.1.1 Secchi depth.................................................................................................. 26 2.1.2 Salinity.......................................................................................................... 26 2.1.3 Oxygen.......................................................................................................... 27 2.1.4 Nutrients ....................................................................................................... 27 2.1.5 Chlorophyll a and primary production......................................................... 28 2.1.6 Phytoplankton and harmful algal blooms..................................................... 29 2.1.7 Macrovegetation........................................................................................... 30 2.1.8 Drifting algal mats ....................................................................................... 30 2.1.9 Zoobenthos ................................................................................................... 31 2.1.10 Ichthyofauna ................................................................................................ 31 3. THE GULF OF BOTHNIA.................................................................................................. 33 3.1 Secchi depth ............................................................................................................. 34 3.2 Salinity ..................................................................................................................... 34 3.3 Oxygen ..................................................................................................................... 35 3.4 Nutrients ................................................................................................................... 36 3.5 Primary production and chlorophyll a ..................................................................... 37 3.6 Phytoplankton and algal blooms .............................................................................. 37 3.7 Macrovegetation ...................................................................................................... 38 3.8 Zoobenthos .............................................................................................................. 39 3.9 Ichthyofauna ............................................................................................................ 39 3.10 The present situation and suggestions for measurements ........................................ 40 4. THE ARCHIPELAGO REGION; Archipelago Sea, Åland Islands and Stockholm archipelago ........................................................................................................ 42 4.1 Secchi depth ............................................................................................................ 43 4.2 Salinity .................................................................................................................... 44 4.3 Oxygen .................................................................................................................... 44 4.4 Nutrients .................................................................................................................. 45 4.5 Primary production and chlorophyll a ..................................................................... 47 4.6 Phytoplankton and algal blooms............................................................................... 48 4.7 Macrovegetation ...................................................................................................... 49 4.8 Ephemeral algae and drifting algal mats .................................................................. 50 4.9 Zoobenthos .............................................................................................................. 51 4.10 Ichthyofauna ............................................................................................................ 52 4.11 The present situation and suggestions for measurements ........................................ 53 5. THE GULF OF FINLAND .................................................................................................. 56 5.1 Secchi depth ............................................................................................................ 57 5.2 Salinity ..................................................................................................................... 58 5.3 Oxygen .................................................................................................................... 58 5.4 Nutrients .................................................................................................................. 59

5.5 Primary production and chlorophyll a ..................................................................... 61 5.6 Phytoplankton and algal blooms .............................................................................. 61 5.7 Macrovegetation ...................................................................................................... 62 5.8 Ephemeral algae and algal mats ............................................................................... 63 5.9 Zoobenthos .............................................................................................................. 63 5.10 Ichthyofauna ............................................................................................................ 64 5.11 The present situation and suggestions for measurements ........................................ 65 5.12 Multiple use of monitoring data; case studies from the Gulf of Finland ................. 67 6. THE GULF OF RIGA ............................................................................................................ 71 6.1 Secchi depth ............................................................................................................ 72 6.2 Salinity ..................................................................................................................... 72 6.3 Oxygen .................................................................................................................... 73 6.4 Nutrients .................................................................................................................. 73 6.5 Chlorophyll a and primary production .................................................................... 74 6.6 Phytoplankton and algal blooms .............................................................................. 75 6.7 Macrovegetation ...................................................................................................... 75 6.8 Ephemeral algae and drifting algal mats................................................................... 76 6.9 Zoobenthos .............................................................................................................. 76 6.10 Ichthyofauna ............................................................................................................ 77 6.11 The present situation and suggestions for measurements ........................................ 78 7. THE GULF OF GDANSK, including the Curonian and Vistulan Lagoons ........................ 81 7.1 Secchi depth ............................................................................................................ 82 7.2 Salinity ..................................................................................................................... 83 7.3 Oxygen .................................................................................................................... 83 7.4 Nutrients .................................................................................................................. 84 7.5 Chlorophyll a and primary production .................................................................... 84 7.6 Phytoplankton and algal blooms .............................................................................. 85 7.7 Macrovegetation ...................................................................................................... 86 7.8 Zoobenthos .............................................................................................................. 87 7.9 Ichthyofauna ............................................................................................................ 87 7.10 The present situation and suggestions for measurements ........................................ 88 8. THE SWEDISH EAST-COAST; from the Himmerfjärden Bay to the Hanö Bay .............. 91 8.1 Secchi depth ............................................................................................................ 92 8.2 Salinity ..................................................................................................................... 92 8.3 Oxygen .................................................................................................................... 92 8.4 Nutrients .................................................................................................................. 93 8.5 Chlorophyll a and primary production .................................................................... 94 8.6 Phytoplankton and algal blooms .............................................................................. 94 8.7 Macrovegetation ...................................................................................................... 94 8.8 Ephemeral algae and drifting algal mats................................................................... 95 8.9 Zoobenthos .............................................................................................................. 95 8.10 Ichthyofauna ............................................................................................................ 96 8.11 The present situation and suggestions for measurements ........................................ 97 9. THE CENTRAL BALTIC SEA .......................................................................................... 99 9.1 Secchi depth ............................................................................................................ 100 9.2 Salinity ..................................................................................................................... 101 9.3 Oxygen .................................................................................................................... 101 9.4 Nutrients .................................................................................................................. 102 9.5 Chlorophyll a and primary production .................................................................... 103 9.6 Phytoplankton and algal blooms .............................................................................. 104 9.7 Macrovegetation ...................................................................................................... 104 9.8 Zoobenthos ............................................................................................................... 105 9.9 Ichthyofauna ............................................................................................................ 106 9.10 The present situation and suggestions for measurements ........................................ 107

10. THE BELT SEA REGION; including Great Belt, Little Belt and the Sound, Fehmarn Belt, and the Bights of Kiel, Mecklenburg and Pomerania ............................... 109 10.1 Secchi depth ............................................................................................................ 111 10.2 Salinity ..................................................................................................................... 111 10.3 Oxygen .................................................................................................................... 111 10.4 Nutrients .................................................................................................................. 113 10.5 Chlorophyll a and primary production .................................................................... 114 10.6 Phytoplankton and algal blooms .............................................................................. 115 10.7 Macrovegetation ...................................................................................................... 115 10.8 Ephemeral algae and drifting algal mats................................................................... 117 10.9 Zoobenthos .............................................................................................................. 117 10.10 Ichthyofauna ............................................................................................................ 118 10.11 The present situation and suggestions for measurements ........................................ 119

11. KATTEGAT ...................................................................................................................... 122 11.1 Secchi depth ............................................................................................................ 123 11.2 Salinity ..................................................................................................................... 123 11.3 Oxygen .................................................................................................................... 123 11.4 Nutrients .................................................................................................................. 124 11.5 Chlorophyll a and primary production .................................................................... 125 11.6 Phytoplankton and algal blooms .............................................................................. 126 11.7 Macrovegetation ...................................................................................................... 126 11.8 Ephemeral algae and drifting algal mats................................................................... 127 11.9 Zoobenthos .............................................................................................................. 127 11.10 Ichthyofauna ............................................................................................................ 128 11.11 The present situation and suggestions for measurements ........................................ 129 12. CONCLUSIONS ................................................................................................................. 131 13. ACKNOWLEDGEMENTS ................................................................................................ 135 14. REFERENCES .................................................................................................................... 136

List of original publications This thesis is a monograph, but it is also based on four original publications. N.B. Lundberg (né Rönnberg). The publications are referred to by their Roman numerals in the text: I. BONSDORFF, E., C. RÖNNBERG & K. AARNIO, 2002. Some ecological properties

in relation to eutrophication in the Baltic Sea. Hydrobiol. 475/476: 371-377. II. RÖNNBERG, C. & E. BONSDORFF, 2004. Baltic Sea eutrophication: area-specific

ecological consequences. Hydrobiol. 514: 227-241. III. RÖNNBERG, C. 2005. Conceptualizing the Baltic Sea ecosystem – an inter-

disciplinary tool for environmental decision making. Ambio, 35 (6), in print. IV. LUNDBERG, C., M. LÖNNROTH, M. VON NUMERS & E. BONSDORFF. A multi-

variate assessment of coastal eutrophication. Examples from the Gulf of Finland, northern Baltic Sea. Mar. Poll. Bull, accepted.

The papers are reprinted with permission from the respective publishers; I, II: Kluwer Academic Publisher, III: Royal Swedish Academy of Sciences and IV: Elsevier Ltd.

11

Contributions to the publications: I II III IV Concept EB, CL, KA CL, EB CL CL Data collection CL CL CL ML Data analysis KA CL CL ML, CL, MvN Manuscript preparation CL, EB CL, EB CL CL, MvN, EB CL = Cecilia Lundberg (Rönnberg), EB = Erik Bonsdorff, KA = Katri Aarnio, ML = Malin Lönnroth, MvN = Mikael von Numers List of abbrevations used in the text: BOD Biological oxygen demand DIN Dissolved inorganic nitrogen DIP Dissolved inorganic phosphorus POM Particulate organic material Tot-N Total nitrogen Tot-P Total phosphorus

12

Introduction

1. INTRODUCTION

This thesis is an attempt to summarise, analyse and interpret ecological information

on the state of the Baltic Sea in relation to the escalating process of eutrophication.

The state of knowledge is presented in a format aimed at facilitating environmental

decision-making by society. Hence, this study is based on the existing information

from numerous sources, and gives a comprehensive overview of the current situation

in the Baltic Sea, as well as suggestions for remedies for continued sustainable use of

the sea ecosystem.

A healthy ecosystem is stable and sustainable, maintaining organisation and

productivity over time and resilience to stress (RAPPORT et al. 1998, BOESCH 2000).

Signs of large-scale eutrophication of aquatic ecosystems were detected by biologists

in the 1960s, but the problem reached the minds of managers and the general public

only after several decades. As DUNLAP (1993) states, natural scientists debated the

causes of ecological problems in the early 1970s, but the social scientists were slow to

recognise the societal significance of pollution and related problems. Solutions for

environmental problems, such as eutrophication, need measures on an

interdisciplinary level. Marine ecologists contribute with monitoring and

interpretation of long-term data, but the final measures and financial considerations

will be the task for politicians (BONSDORFF et al. 1997a). For a more complete

understanding and sustainable decision-making, interlinkage between natural and

social sciences is necessary.

To be able to learn and understand new information, we must understand the

past and the processes that regulate the systems – in this case the Baltic Sea. The

eutrophication-related changes are described both in general (paper I) and in more

detail (Chapters 3-11, paper II). Based on these results, parameters and species of

special indicator-value can be identified and used for further investigation and

analysis where the actual links between nutrient loads and ecological effects are

studied, towards an interdisciplinary approach (paper III), and more site-specific

(paper IV). This thesis aims primarily at identifying eutrophication-related problems

by constructing conceptual models of the process at different environmental scales.

This enables us to further analyse the quantitative dose-response effects when nutrient

loadings and concentrations are coupled to ecological effects.

13

Introduction

This thesis is a part of the research programme MARE (Marine Research on

Eutrophication – a scientific base for cost-effective measures for the Baltic Sea;

WULFF et al. 2001a, MARE 2005). MARE has developed a decision-support system,

NEST, for testing cost-effective strategies to reduce eutrophication and its negative

effects in the Baltic Sea. MARE involves ecology, oceanography, economy and

modelling, and aims at narrowing the gap between scientists and decision-makers.

1.1 The Baltic Sea The Baltic Sea is one of the largest brackish water bodies on earth, situated between

54°-66°N and 10°30´-31°E (Fig. 1.1). It has steep gradients in topography,

hydrography and climate, as well as a permanent stratification of the water mass

(LEPPÄKOSKI & BONSDORFF 1989, paper I). The salinity is decreasing from 15-25 psu

in the surface water in the Kattegat, to 0-2 psu in the northern Bothnian Bay

(ELMGREN & LARSSON 2001). These characteristics make the system very species

poor with a limited number of ecological functional groups (BONSDORFF & PEARSON

1999). Tides are virtually non-existent and during winter the sea is partially covered

by ice (HELCOM 1996). Basic environmental data on the Baltic Sea are presented in

Table 1.1.

Table 1.1. Some basic data for the Baltic Sea (after JANSSON & VELNER 1995).

Statistics Measure Area 415.000 km²

Length (N-S) 1300 km Width (W-E) 1200 km Average depth 60 m Maximum depth 459 m Sill depth 17 m Volume 21.700 km³ Residence time of water 25 yrs Total catchment area 1.641.650 km²

The anthropogenic stress to the Baltic is severe, as 16 million people live on or

in immediate victincy of the coast and a total of 85 million within the catchment area.

The large riverine inflow and the shallow and narrow entrance to the North Sea

restrict the water exchange and give the Baltic Sea its brackish character. The largest

rivers; Neva (flow: 77.6 km3 yr-1), Vistula (33.6 km3 yr-1), Daugava (20.8 km3 yr-1),

14

Introduction

Njemen/Nemunas (19.9 km3 yr-1) and Odra/Oder (18.1 km3 yr-1), supply most of the

freshwater, and thereby also most of the nutrients to the system (STÅLNACKE 1996,

SCHERNEWSKI & NEUMANN 2005). Also, the five largest European lakes; Ladoga and

Fig. 1.1. The Baltic Sea and its catchment area. The surrounding nations (BEL=Belarus, CZE REP=Czech republic, EST=Estonia, DEN=Denmark, FIN=Finland, GER=Germany, LAT=Latvia, LIT=Lithuania, NOR=Norway, POL=Poland, RUS=Russia, SWE=Sweden, SLO REP=Slovak republic UKR=Ukraine) and the five biggest rivers and lakes are included, as well as the 9 sub-regions used in Chapters 3-11. Modified from GRIDA (2001).

15

Introduction

Onega in Russia, Vänern and Vättern in Sweden and Peipsi on the Estonian and

Russian boarder, are all situated within the Baltic drainage area (STÅLNACKE 1996;

Fig. 1.1).

More comprehensive information on the Baltic Sea is found in e.g.

FALANDYSZ et al. (2000) and KAUTSKY & KAUTSKY (2000). The special issue of

Ambio 30 (no. 4-5, 2001), with the title “Man and the Baltic Sea” is also

recommended for further reading. The Baltic system in relation to integrated coastal

zone management is found in SCHERNEWSKI & SCHIEWER (2002), and a system-

oriented analysis is presented in WULFF et al. (2001b).

1.2 Eutrophication – a global problem Eutrophication is defined as an increased input of nutrients or organic matter into an

aquatic ecosystem, resulting in an increase in primary production (e.g. NIXON 1995).

Marine eutrophication is well described, both globally and regionally. The

anthropogenic, or cultural, nutrient over-enrichment in marine systems are discussed

in e.g. JØRGENSEN & RICHARDSON (1996), CLOERN (2001), BOESCH (2002), DE JONGE

et al. (2002), and WASSMANN & OLLI (2004). BRICKER et al. (1999), give valuable

information on assessment of coastal eutrophication in the U.S.

For the Baltic Sea, LARSSON et al. (1985) have estimated a four-fold increase

in nitrogen (N) and eight-fold in phosphorus (P) during the 20th century. According to

ELMGREN & LARSSON (2001), the total annual nutrient load to the Baltic Sea is

1.249.000 tons nitrogen and 56.000 tons phosphorus.

The main sources of eutrophication are agriculture and husbandry, industries,

aquaculture, municipal sewage water, river run-off and erosion, atmospheric

deposition and nitrogen fixation (ELMGREN & LARSSON 2001, WASSMANN & OLLI

2004). Besides the Baltic Sea, large-scale nutrient over-enrichment is occurring in e.g.

the Black Sea area (e.g. MEE 1992), the North Sea and Wadden Sea (e.g. VAN

BEUSEKOM & DE JONGE 2002, DE GALAN et al. 2004), Chesapeake Bay (e.g. HERBST

2002) and the northern Gulf of Mexico (e.g. RABALAIS et al. 2002). In most cases

eutrophication seems to be more serious for coastal ecosystems than the open sea

(DIAZ & ROSENBERG 1995).

16

Introduction

1.3 Ecology meets society The marine ecosystem is considered to be the most complex (ELLIOTT 2002a).

Besides the linkages between physical, chemical and biological processes,

interrelations to sociology, economy and technology are evident. For an ecosystem to

be sustainable, it is dependent on how management is run and how political decisions

are made. To achieve a general understanding of a complex issue, the scientific

community working in detail must meet the holistic approach of the managers

(ELLIOTT 2002a). The aim of science is to serve society. Communication problems

form the largest obstacles in the co-operation between scientists and decision-makers.

Common failures are the use of different standards, lack of available information and

difficulties in quantifying uncertain data (KINZIG & STARRETT et al. 2003). Therefore,

the demand for an active interdisciplinary communication between different

disciplines of science and between science and management is important (WALTERS

1997). Then, transdisciplinary new theories, tools and techniques can be developed

(COSTANZA 1996).

The interdisciplinary linkages between ecology and economy is an important

factor in the eutrophication process. The socio-economic system with human

demographic, social, cultural and economic trends is a part of the ecosphere. The

carrying capacity of the whole system also involves the growth of the human

population and its activities (COSTANZA 1996, FOLKE et al. 1996). Even from a

sociological perspective, humans have exceeded the limit for the carrying capacity

during the 20th century (DUNLAP 1993, DUNLAP & CATTON 2002).

Fertilizer use and fossil-fuel consumption are examples of economic factors

driving the human decisions that further impact the nutrient over-enrichment.

Therefore, knowledge of economic forces provides identification and predictions of

trends in the nutrient cycles, both on national and global scales (SEGERSON &

WALKER 2002). The search for cost-effectiveness and choose of management

programmes play a role both nationally and internationally. ELOFSSON (2002) shows

that cost-effective reductions, where they are most needed, are more effective than

uniform reduction rates for all countries around the Baltic Sea. Theory and

methodology of eutrophication-management in the Baltic Sea area is found in the

special issue “Management of eutrophied coastal ecosystems” of Ecological

Economics (volume 47, 2003).

17

Introduction

1.3.1 The DPSIR-approach

The DPSIR-approach is a tool used to explain the causes of environmental change for

interdisciplinary problems. DPSIR stands for the Drivers of change causing Pressure

on the environment. The Status has to be assessed, giving the definition of Impacts.

All these indicate the human Response (TURNER et al. 1999, ELLIOTT 2002a, BOING

2005). The framework is developed from a conceptual identification of environmental

indicators by OECD, with the purpose to link human activities and environmental

degradation (BLANC et al. 2004). According to ELLIOTT (2002a), the human response

has to meet six tenets for environmental management. In other words, the response

has to be environmentally sustainable, including technology and economy, as well as

sociology, legislation and administration support an environmentally friendly

management (ELLIOTT 2002a).

1.4 Eutrophication – measurements in the Baltic Sea region Besides the coastal zone, also rivers and the atmosphere transport nutrients into the

Baltic Sea. In the year 2000, about 28.000 tons of phosphorus and 660.000 tons of

nitrogen were brought to the Baltic by rivers (HELCOM 2003a). The atmospheric

deposition is the major nitrogen source, 35 % of the total load. In the 1980s, 324.000

t y-1 entered the Baltic Sea through atmospheric deposition, which was reduced to

210.000 t in 1997 (HELCOM 2002).

Clear reductions in diffuse loads from agriculture and scattered dwellings are

harder to obtain compared to point-source reductions from industries and

municipalities. Policy governing preventive measures in agriculture has to be

prioritised for a reduction of diffuse loading. Efficiency in the recycling of nutrients,

with a decreased use of mineral fertilizers and an increase in the residence time of

water in river catchments are some examples (JANSSON 1997). Wetlands have a

capacity to reduce about 20 % of the nitrogen loading to the sea, while ecological

farming should potentially recycle most of the phosphorus (ŁYSIAK-PASTUSZAK et al.

2004).

The development of networks and organizations between the Baltic countries

are, according to KERN & LÖFFELSEND (2004), more dynamic compared to the rest of

Europe. The Helsinki Convention, HELCOM, was signed in 1974 of the nine riparian

states to the Baltic Sea with the intention to undertake measures to minimize the land-

18

Introduction

based pollution to the Baltic. By the Ministerial Declaration in 1988, the countries set

a reduction goal of 50 % of the nutrient loads until the year 1997. After only seven

years, in 1995, the total nitrogen and phosphorus load was reduced by 35 %

(SCHERNEWSKI & NEUMANN 2002). The fast drop was mostly reached in the post-

socialistic republics as the reductions co-occurred with the collapse of their political

structure. The results of the load reduction by the Ministerial Declaration are

evaluated in LÄÄNE et al. (2002).

Even if all Baltic countries, except Russia, are members of the EU since May

2004, the Baltic Sea region can still be divided into two disparate worlds due to

economic possibilities. JAHN & KUITTO (2005) found strong correlations between the

development of Gross Domestic Product (GDP) and CO2 emissions and fertilizer

consumption in the countries in the Baltic Sea region. In KERN & LÖFFELSEND (2004)

Germany and the Nordic countries are classified as environmental pioneers, whereas

Poland and the Baltic republics still lag behind European standards in environmental

issues. However, the fertilizer consumption is much lower in the eastern Baltic Sea

region than in the western part (JAHN & KUITTO 2005).

Mathematical models are useful and fast methods for predicting the

environmental status, both from long-term and short-term perspectives. For the Baltic

Sea, quantitative models for calculations of biogeochemical nutrient cycles (SAVCHUK

& WULFF 2001, SAVCHUK 2002), phytoplankton biomass (KIIRIKKI et al. 2001,

KORPINEN et al. 2004), and nutrient budgets (WULFF et al. 2001c) are used. On a

global scale, fuzzy logic modelling is used for predictions of eutrophication (e.g.

CHEN & MYNETT 2003). Fuzzy logic can be a useful tool, as environmental data

consists of heterogeneous and large data sets with a high range of uncertainty

(ADRIAENSSENS et al. 2004).

1.5 Conceptualizing the eutrophication Eutrophication is an interdisciplinary problem. The excessive input of nutrients,

mainly phosphorus and nitrogen, will lead to a chain of consequences and effects, not

only in the aquatic environment, but also on a socio-economic level and concretely

affect the well-being of humans. CLOERN (2001) has conceptualized the

eutrophication problem on a general level. A somewhat more detailed attempt to show

the complexness of eutrophication is presented in a conceptual model (so called

19

Introduction

“horrendogram” in ELLIOTT (2002a)) in paper III, where the pathways from natural

science to human society and management options are presented. Various kinds of

end-users can use the model for their mutual communication. A conceptual model

helps to formulate questions, identify gaps in knowledge, and act as a platform for

ideas from various scientific perspectives (READ et al. 2001, HEEMSKERK et al. 2003).

However, a complex model can never fully reflect reality. By increasing the

resolution and complexity in a model, the accuracy and uncertainty set limitations

(COSTANZA 1996). 1.5.1 The EU Water Framework Directive

The ongoing implementation of the European Union Water Framework Directive

(WFD) has contributed to raise the awareness for environmental issues in Europe. The

WFD shows the need for clear and precise information on an interdisciplinary level.

By promoting a sustainable use of water, the objective of the WFD legislation

is to achieve good ecological status of surface water by the year 2015 (ANON. 2000).

The EU member states have to develop classification systems for describing the

ecological status of water bodies and establish reference conditions (ANON. 2000,

ANDERSEN et al. 2004a). Methods from paleoecology (ANDERSEN et al. 2004a,

WECKSTRÖM et al. 2004) to modelling and expert judgement are under development

(ANON. 2000). The WFD raises the need for joint programmes, both for national and

international monitoring. In BÄCK et al. (2002) the phytobenthos programme for the

Finnish coast is assessed, and in PERUS et al. (2004) the corresponding for

macrozoobenthos.

In order to assess habitats at risk, the definition of Ecological Quality (EcoQ)

and Ecological Quality Objectives (EcoQO) is a useful tool as a basis for monitoring

(ELLIOTT & DE JONGE 2002). Recommendations for eutrophication-related risk

assessment by different kinds of models, monitoring and management are found in

READ et al. (2001).

1.6 Area-specific eutrophication When discussing the state of the environment, the Baltic Sea is often regarded as one,

uniform water body (DE JONGE et al. 1994, CLOERN 2001, paper I). In the case of

eutrophication, topography, exposure, upwelling and other physical factors influence

the process, and make it function differently in the various parts of the Baltic Sea.

20

Introduction

Open coasts with good water circulation behave differently from shallow

archipelago areas, which act as buffers between the coast and the open sea. Also the

climate and salinity status differ in the Baltic Sea system. The almost limnic and

species-poor Bothnian Bay can be classified as middle-boreal, whereas the

Archipelago Sea, Gulf of Finland, Baltic proper and Gulf of Riga are hemi-boreal.

The more marine and biologically diverse southern Baltic belongs to the temperate

vegetation zone (RYDIN et al. 1999, HUMBORG et al. 2003). The overall information

on transformation and retention of nutrients in estuaries is often based on eutrophied

north temperate estuaries, whereas the nutrient dynamics in boreal and arctic waters

are scarce (HUMBORG et al. 2003).

Hence, eutrophication is an area-specific problem, and the choice of

appropriate management strategies or policy varies among estuaries (SEGERSON &

WALKER 2002). In the Baltic Sea, regional differences occur especially in the coastal

waters (NEUMANN & SCHERNEWSKI 2005).

Based on this reasoning, the Baltic Sea is divided into nine sub-regions in

Chapters 3-11, where the state and changes due to eutrophication is presented and

evaluated. Chapter 2 presents the parameters studied, and Chapters 3-11 describe the

sub-regions: 1) the Gulf of Bothnia, 2) the Archipelago region, 3) the Gulf of Finland,

4) the Gulf of Riga, 5) the Gulf of Gdansk, 6) the East coast of Sweden, 7) the Central

Baltic, 8) the Belt Sea region, and 9) the Kattegat (Fig. 1.1).

The presentations of the eutrophication situation and related changes in the

sub-regions of the Baltic Sea are presented with data on both primary parameters and

secondary effects. The analysis is based on public and available information only, and

no attempt has been made to re-analyse reported numerical information. In that

respect, the present thesis is not to be confused with the large and valuable assessment

reports compiled regularly by HELCOM (HELCOM 1990, 1996, 2002), which describe

trends in concentrations and biotic conditions. The presented analysis is an updated

version of RÖNNBERG (2001).

21

Introduction

1.7 Aims of the work

The specific aims of this thesis are:

1. To present a comprehensive review of the knowledge of changes in and

responses to eutrophication in the Baltic Sea, when and where the first obvious

signs were detected and how severe the problems are at present (Chapters 3-

11, papers I, II).

2. To detect and describe differences in the effects and consequences of

eutrophication in the Baltic Sea, at the level of nine sub-regions (Fig. 1.1;

Chapters 3-11, papers I, II).

3. To view the Baltic eutrophication from an interdisciplinary perspective and

show the relations and interlinkages between ecological and social processes

(paper III).

4. To use long-term monitoring data for detection of spatial and temporal trends,

and for short-term predictions of the state of the environment (paper IV).

22

Assessing eutrophication

2. ASSESSING EUTROPHICATION Threats to the environment are described by a variety of parameters. Every parameter

contributes to the assessment of the state of the environment. Regarding the negative

effects of eutrophication, total phosphorus and nitrogen, chlorophyll a and

transparency (Secchi depth) are used as the key-parameters. Other parameters, such as

oxygen concentration, zoobenthos and ichthyofauna, also provide information for the

estimation of the degree of nutrient over-enrichment (ANON. 1999a). The assessment

of the state of the environment, in this case to eutrophication, needs a standardised

and uniform classification system. Despite the ongoing implementation of the EU

Water Framework Directive, such a system is still non-existing. In this thesis I have

used a three level classification scheme (Table 2.1, paper II). Note that it is not based

on reference conditions as required by the WFD (ANDERSEN et al. 2004a).

In Chapters 3-11, the eutrophication is illustrated for the nine sub-areas of the

Baltic Sea using a uniform conceptual model (Fig. 2.1). This model only describes the

environmental processes affected by eutrophication. The model is based on the model

in paper III. The estimates for the assessment and classification in the conceptualised

eutrophication models are found in Table 2.1 and in paper II. Fig. 2.2 shows the

legend for the colouring of the models in Chapters 3-11.

In the following, the parameters used for the assessment of the area-specific

descriptions and couplings to eutrophication in Chapters 3-11 are presented.

23


Table 2.1. Guidelines for the degree of changes in the parameters considered in Chapters 3-11. The degree of changes corresponds to the scale presented in Fig. 2.2. The classification is based on JUMPPANEN & MATTILA (1994), RUMOHR et al. (1996), ÆRTEBJERG et al. (1998), ANON. (1999a), and DAHLGREN & KAUTSKY (2002). The values for the first six parameters represent summer values (August value for chlorophyll a). The table is modified from paper II. Effects: Parameter: Small-moderate Serious Very serious Transparency 3-5 m 2-3 m < 2 m Oxygen 4-6 ml l-1 2-4 ml l-1 < 2 ml l-1 Tot-P 15-19 µg l-1 19-24 µg l-1 > 24 µg l-1 Tot-N 250-310 µg l-1 310-360 µg l-1 > 360 µg l-1 Chlorophyll a 1.5-2.2 µg l-1 2.2-3.2 µg l-1 > 3.2 µg l-1 Harmful Algal Blooms Few colonies Formation of Bloom-areas and (HAB) floating algae layer of cyano-

bacteria 1-5 g m-3 5-11 g m-3 > 11 g m-3

Zooplankton Diverse group, not used as an indicator for eutrophication. The

assessments in Fig. 3-11.2 are based on rough estimates. For mesozooplankton a biomass of 2-5 mg C m-3 = low, a biomass

> 30 mg C m-3 = high. Macrovegetation Fucus species, Filamentous algae No Fucus present. meadows of Zostera as epiphytes on Filamentous algae with associated Fucus, sporadic dominate. charophytes, relatively occurrences of Drifting algal mats sparse abundances Zostera, no charo- and sulphur- of filamentous algae. phytes. Filamentous bacteria. algae dominate. Zoobenthos Dominated by Animals live close to No macrofauna. molluscs and long- or at the sediment. Lack of bioturbation lived polychaetes. Small worms (e.g. → lamination of the Increased total Capitella capitata and sediment. biomass production. chironomids) dominate.

Low species richness, high adundance, low biomass. Fish Decrease in Decrease in cod, increase in herring, sprat,

flatfishes. pikeperch and cyprinides.

24


Fig. 2.1. A conceptual model of the ecological pathways of eutrophication (modified from BERNES 1988).

Fig. 2.2. Scheme that explains the colouring in the conceptual models, Figs. 3-11.2, in Chapters 3-11. The classification is based on the classification in Table 2.1. If an upper left corner (Fig. 4.2, 5.2, 6.2, 7.2, 10.2, 11.2) is coloured differently, it indicates changes to the assessment in RÖNNBERG (2001). The corner has the former colour of the state of the parameter.

25


2.1 Eutrophication parameters 2.1.1 Secchi depth

Secchi depth, i.e. transparency, is dependent on the amount of planktonic algae and

other particles in the water (JUMPPANEN & MATTILA 1994, BONSDORFF et al. 1997a).

The Secchi depth-method has advantages, as it is easy to conduct, and has been used

for about a century, allowing analysis of long-term trends (SANDÉN & HÅKANSSON

1996). However, the readings are semi-quantitative, and in the Bothnian Bay, the

Secchi depth is not reliable due to the humus and resuspended material in the water

(JOHAN WIKNER, pers. com.).

2.1.2 Salinity

Salinity variations in the history of the Baltic Sea have mainly been caused by

changes in the size of the sounds and in the overall freshwater supply (GUSTAFSSON &

WESTMAN 2002). The Baltic is dependent on inflow of marine water from the North

Sea, which occurs in wind- and current-driven irregular fluxes. The duration and

mean salinity of the inflowing water regulate the state in the Baltic Sea. Small

quantities of inflowing saline water and long intervals between the inflows cause

stagnation of the deep sub-halocline water. The inflows in 1913, 1920, 1921, 1951

and 1975-1976 are classified as the strongest recorded during the 20th century

(MATTHÄUS & FRANCK 1992).

In January 2003, 200 km3 of saline water entered through the Danish Straits,

which was the first larger inflow since 1993 (NAUSCH et al. 2003, PIECHURA &

BESZCZYŃSKA-MÖLLER 2003). Both inflows and runoff are also regulated by climatic

factors in the Atlantic, such as the NAO (Northern Atlantic Oscillation; HÄNNINEN et

al. 2000). Much research is today focused on climate change (see for example Ambio

33 (4-5), 2004; special issue on the Swedish regional climate modelling programme).

Salinity plays an important role for the general species distribution in the

Baltic Sea. The proportion of larger zooplankton, cod (Gadus morhua) abundance and

biomass, herring (Clupea harengus membras) biomass and weight at ages all correlate

positively with salinity (FLINKMAN et al. 1998). Many marine organisms, such as the

brown algae Fucus vesiculosus and the blue mussel Mytilus edulis reach their critical

salinity limit at about 5-6.5 psu (KUKK et al. 1997). Therefore, fluctuations in salinity

have consequences for the whole ecosystem of the Baltic Sea.

26


2.1.3 Oxygen

The concentration of oxygen in the deep water is partly influenced by the saltwater

inflows and partly by eutrophication. The saline inflows push nutrient-rich bottom

waters from the deep basins in the Central Baltic further north. The effects are

dependent on the size of the inflows (ANDERSIN & SANDLER 1991, CEDERWALL &

SJÖBERG 1995). Old, anoxic water is generally not pushed north of the Fårö Island,

because of the large volume of the Eastern Gotland Basin (CEDERWALL & SJÖBERG

1995). The sedimentation increases due to an increase in the algal production and

subsequent transport of organic material to the bottom. The decay of the organic

material consumes oxygen (RICHARSON & JØRGENSEN 1996, GRAY et al. 2002),

causing large-scale severe hypoxia and anoxia (KARLSON et al. 2002).

Benthic organisms are negatively affected by oxygen concentrations below 2

ml l -1, a value which is often used as the threshold for hypoxia (DIAZ & ROSENBERG

1995, CONLEY et al. 2002). In case of anoxia, hydrogen sulphide, H2S, is formed by

bacterial reduction of sulphate ions in the sediment-water interface. The hydrogen

sulphide is chemically dissolved in the anoxic water layers, mainly as HS- ions, and

when the water is re-oxygenated, sulphide is again formed (HELCOM 1990).

In a period of 30 years, from 1970 to 2000, the total area with oxygen

concentrations < 2 ml l-1 varied from < 12.000 km2 to 70.000 km2, which corresponds

to 5-27 % of the bottom area of the Baltic (CONLEY et al. 2002).

A comprehensive numerical modelling of long-term trends in oxygen and

hydrogen sulphide in the central Baltic Sea is presented in UNVERZAGT (2001). In

KARLSON et al. (2002) the couplings between oxygen deficiency and zoobenthos in

the Baltic Sea is reviewed, and estimates of potentially missing benthic biomass

presented.

2.1.4 Nutrients

The coupling of benthic and pelagic processes are complicated in the Baltic, as the

buffering capacities for different nutrients vary, the salt water inflows are irregular

and the length of the growth season differs between the sub-basins (KUPARINEN &

TUOMINEN 2001). A considerable part of the nutrients in the Baltic Sea is originating

from external, anthropogenic sources (HEISKANEN 1998). Nitrate and ammonia are the

dominant inorganic nitrogen compounds, and phosphate the dominating form of

27


phosphorus. In the transition between oxic and anoxic conditions, nitrate and

ammonia are denitrified into free N2. Phosphorus can bind to the sediment under oxic

conditions, whereas it is released back to the water column under anoxia

(LEHTORANTA 2003). Phosphate is more bioavailable in coastal marine water

compared to freshwater. This is a matter of salinity, which promotes nitrogen

limitation in marine coastal waters in contrast to dilute estuaries, where phosphorus is

often limiting (BLOMQVIST et al. 2004). For more detailed discussions of nutrient

cycling and sedimentation in the Baltic, see e.g. HEISKANEN (1998) and LEHTORANTA

(2003).

In this compilation I have, where possible, concentrated on summer values of

the total amounts of nitrogen and phosphorus in the different sub-regions. Tot-N and

Tot-P include all forms of the particular nutrient in the water, both as dissolved and

bound to particles and biomass (ANON. 1999a).

Contrary to the increasing amounts of nitrogen and phosphorus, the pool of

silica (Si) is decreasing, especially in the southern Baltic. This is partly due to

eutrophication, but most likely to the damming of rivers. Dams convert rivers to lakes,

and the sedimentation of Si occurs already in the lake-phase. The consequence is a

reduction in diatoms in the sea, in favour for e.g. toxic dinoflagellates (HUMBORG et

al. 2000). Silica is not included as a parameter in this text, as sufficient information

for all sub-regions is lacking.

2.1.5 Chlorophyll a and primary production

The chlorophyll a content in the productive layer gives an estimate of the amount of

phytoplankton in the water (JUMPPANEN & MATTILA 1994, KONONEN et al. 1998).

The concentration of chlorophyll a generally increases from the open sea towards the

coast, depending on the input from land and degree of mixing (PITKÄNEN & KANGAS

1987).

The primary production is the rate at which carbon is fixed by photosynthesis.

The amount varies with light and temperature. WASMUND et al. (2001) have estimated

the annual phytoplankton primary production in the entire Baltic Sea to 62 x 106 t C.

N.B. The concentrations of primary production are given both as g C m-2 and g C m-3 in Chapters 3-11,

depending on different methods used in different investigations and areas.

28


2.1.6 Phytoplankton and harmful algal blooms

The number of phytoplankton species identified in the Baltic Sea is about 2000

(GUNDERSEN 2002). Generally, diatoms have larger cells than dinoflagellates, which

are able to grow also under low nutrient concentrations. Cyanobacteria are only

limited by phosphate, as they can fix atmospheric nitrogen (SCHERNEWSKI &

NEUMANN 2005). The diatom:flagellate ratio is a potential indicator for

eutrophication, since an increase in flagellates cause harmful microalgal blooms

(HAB; SMAYDA 2004). However, the N:P ratio has raised more interest, because

nitrogen and phosphorus are the primary nutrients controlling cyanobacterial blooms

of e.g. Nodularia spp and Aphanizomenon spp (NIEMI 1979).

Cyanobacteria are able to fix dissolved molecular N2, and are therefore able to

grow in the summer period when other phytoplankton species are limited by nitrogen

(KAHRU et al. 1994). These species also have the ability to store phosphorus and

avoid grazing by forming filamentous cells (FEUERPFEIL et al. 2004). The filaments of

cyanobacteria are positively buoyant and, as an inverted sedimentation, aggregate in

the surface layer (Horstmann et al. 1986 in KAHRU et al. 1994). For a cyanobacterium

bloom to develop, the requirements are a water temperature > 16° C, a daily radiation

over 120 W m-2 and wind < 6 m s-1 (LARSSON et al. 1998). There are different scales

for estimation of the duration and quantity of blooms, for example in KANOSHINA et

al. (2003) and Reimers (1990) in OLENINA & OLENIN (2002).

The increase in frequency and magnitude of the cyanobacterial biomass during

the latest decades is related to the man-induced eutrophication, but the nitrogen

limitation of phytoplankton in the Baltic proper is not anthropogenic, rather a natural

phenomenon, and has endured for about 7000 years (BIANCHI et al. 2000). In FINNI et

al. (2001) the history of the cyanobaterial blooms in the Baltic is described.

ANON. (2001a) gives a brief guide to the nitrogen fixation in the Baltic Sea.

Mechanisms regulating the planktonic ecosystems are described in KONONEN (2001).

Continuously updated information of the algal situation in different parts of the Baltic

Sea during the summer season is found at the BALTIC SEA PORTAL (2004).

The diversity of zooplankton with respect to size, life cycles and behaviour,

make them difficult to use as indicators for eutrophication (ÆRTEBJERG et al. 2003).

Therefore, they are not included as a parameter in this thesis. However, as

zooplankton is an important component in the flow-chart of eutrophication-related

effects, they are included in the conceptual model in Figs. 2.1, 3-11.2. The estimation

29


of their changes due to eutrophication is rough, but some general traits are found in

Table 2.1.

2.1.7 Macrovegetation

The benthic macrovegetation plays an important regulating role in shallow areas. The

single perennial brown algae in the northern Baltic Sea, Fucus vesiculosus, and the

aquatic vascular plant, Zostera marina, can be regarded as key-species in the Baltic as

they host and protect various amounts of other species. Both species are adapted to

oligotrophic ecosystems (ANON. 1991), and nutrient over-enrichment affects them

negatively through decreased light penetration and stronger competition with

filamentous fast-growing algae (WALLENTINUS 1981). An extreme growth of

epiphytes reduces the structural complexity of Z. marina and the plant is pressed

down onto the sediment by the weight of the filamentous algae. This may further lead

to reduced water circulation, which may cause local anoxia (PIHL et al. 1995, BADEN

& BOSTRÖM 2001). According to BERGSTRÖM (2005), the adaptation of some

macroalgae to low salinity may infer with the capacity to utilize nutrients, which

indicates that effects of eutrophication cannot be assessed uniformly for the whole

Baltic Sea. Negative effects of eutrophication on different life stages of F. vesiculosus

are presented in BERGER et al. (2004).

A literature review of changes in the benthic macrovegetation in various parts

of the Baltic Sea is presented in DAHLGREN & KAUTSKY (2002).

2.1.8 Drifting algal mats Eutrophication favours the growth of filamentous algae. Mechanical loss of algal

material is transported to benthic systems where they aggregate and form loose-lying

mats (ÓLAFSSON 1988, BONSDORFF 1992). The mats may be over a hectare in size and

consist of several different algal species (BONSDORFF 1992, BONSDORFF et al. 1995,

NORKKO & BONSDORFF 1996a, b, c). When light conditions are sufficient, the algal

mats are productive and may provide habitat and food for benthic organisms

(NORKKO et al. 2000). The decomposition of the algae is rapid and may induce

hypoxia, anoxia and even hydrogen sulphide in the sediment below the algal mats

(NORKKO & BONSDORFF 1996a, b, c, VAHTERI et al. 2000).

The drifting algae mats in the northern Baltic Sea are dominated by the brown

algae Pilayella littoralis and Ectocarpus siliculosus (NORKKO & BONSDORFF 1996a,

30


MARTIN et al. 2003). In the Kattegat green algae, such as Cladophora glomerata, are

common also (HÅKANSSON 2003). P. littoralis has an active growth period from

February to December in areas without sea ice, resulting in drifting algae also during

winter. In cold, offshore water the detached algae may remain intact for long periods,

when the microbial activity is low and the colonisation of invertebrates slow. Thus,

the over-wintering capacity is good in favourable conditions. Algal degradation by

mesograzers is negligible in the northern Baltic Sea (SALOVIUS & BONSDORFF 2004).

2.1.9 Zoobenthos Knowledge of species composition and biomass of the zoobenthic community gives

valuable information of the state of eutrophication. This ecological parameter is

central as the species differ in sensitivity and act differently to the supply of organic

matter, oxygen conditions and presence of hydrogen sulphide in the bottom water and

the sediments (CEDERWALL & ELMGREN 1980, 1990).

The temporal and spatial changes of the benthic fauna in the Baltic Sea due to

eutrophication and oxygen deficiency are comprehensively described in KARLSON et

al. (2002). A functional analysis of zoobenthos throughout the Baltic Sea gradient is

presented in BONSDORFF & PEARSON (1999), and some specific species distribution

patterns are shown in LAINE (2003).

2.1.10 Ichthyofauna

The fish-catches in the Baltic Sea started to increase in the early 1950s, partly due to

an intensification of the fishery and the decreased predation by seals, and partly due to

eutrophication (HANSSON & RUDSTAM 1990, ROSENBERG et al. 1990). Today,

overfishing is a global problem (ELLIOTT 2002b, HJERNE 2003), and the

eutrophication has changed the fish stock abundances and community compositions

(OJAVEER, H. 2002). Of the economically most important species in the Baltic Sea –

cod (Gadus morhua), herring (Clupea harengus membras) and sprat (Clupea sprattus)

– cod is the most vulnerable to eutrophication. The cod eggs need a salinity of 11 psu

and an oxygen concentration > 2 ml l-1 for successful hatching (HANSSON & RUDSTAM

1990, NISSLING & WESTIN 1991).

According to HJERNE & HANSSON (2002), the fishery in the Baltic counteracts

eutrophication by removing approximately 15.000 t nitrogen and 3000 t phosphorus

31


every year. On the other hand, the fishes sequester high quantities of phosphorus

(HJERNE & HANSSON 2002). The environmental impact on the Baltic fish and fishery

is reviewed in OJAVEER, H. (2002). Reproductive disturbances in Baltic Sea fish is the

theme for the special issue of Ambio 23 (no. 1), 1999.

---

Various publications about the Baltic Sea and eutrophication, from refereed scientific

papers to regional environmental reports, are never-ending sources of information.

The papers analysed in Chapters 3-11 are therefore subjective in the sense that they

are the results of my requests to scientists and authorities in the Baltic region, and

otherwise accessible in scientific journals and on the Internet (all the web-sites quoted

were, with a few exceptions, available in December 2004). The language is another

aspect of priority, and this work is based on literature in English, Danish, Finnish,

German, and Swedish. Hence, any limitations to this work are set by the sources of

information used.

32

Gulf of Bothnia

3. THE GULF OF BOTHNIA The Gulf of Bothnia has two major basins, the Bothnian Sea (60.5°N-63.5°N), and the

Bothnian Bay (63.5°N-66°N). The Bothnian Sea has an average depth of 68 m, and

the average depth of the Bothnian Bay is 43 m. These two basins are separated by the

Northern Quark, a shallow sill of only 20 m (Fig. 3.1). The maximum depth in the

Bay and in the Sea is 147 and 230 m, respectively. The Finnish coast to the Gulf of

Bothnia slopes gently towards the central, deeper section, whereas the Swedish coast

is more steep. The land elevation in the Bothnian Bay is 8-9 mm per year and 5-8 mm

in the Bothnian Sea (HELCOM 1996, HÅKANSSON et al. 1996).

The inflow of fresh water from the rivers, together with the precipitation, gives

the Bothnian Bay its estuarine character (HELCOM 1996). The rivers discharging into

the Bothnian Bay are still classified as pristine (SCHERNEWSKI & NEUMANN 2005).

The Bothnian Bay has a three times higher freshwater discharge per volume compared

to the Bothnian Sea (SANDBERG et al. 2004). The brackish water from the surface of

the Baltic proper promotes more “marine” conditions in the Bothnian Sea (HELCOM

1996). Fig. 3.1. The Gulf of Bothnia sub-region.

33

Gulf of Bothnia

The annual ice-sheet is a specific character for the Gulf of Bothnia. The ice

thickness is approximately 0.8-1.0 m in the northern Bothnian Bay. The general

freezing period lasts for about 120 days, from January to April/May. In the Bothnian

Sea an ice cover of ~ 0.4 m is normally formed a month later and lasts for

approximately 60 days (HÅKANSSON et al. 1996). The upper 60 m water layer in the

Gulf of Bothnia turns over every spring and autumn, which promotes good oxygen

concentrations in the entire water column throughout the year (HELCOM 1996,

HUMBORG et al. 2003). The natural year-to-year variation make trends for the Gulf of

Bothnia hard to assess (KAJRUP 1999).

Many industries are situated along the coast and shipping is intense all year

round, despite the ice-cover during the winter period (HÅKANSSON et al. 1996). In

February 2004, only the mining waste from the Swedish River Dalälven is classified

as a HELCOM hot spot in the Gulf of Bothnia (HELCOM-HOTSPOTS 2004).

3.1 Secchi depth The Bothnian Bay is the only sub-region in the Baltic Sea with a stable trend in water

transparency. The high N:P ratio does not induce cyanobacterial blooms and the high

amount of humic substances causes a natural turbidity (ANDERSSON & WIKNER 2004,

LAAMANEN et al. 2004). Resuspended material and humus affect the Secchi

recordings negatively in the Bothnian Bay, therefore Secchi depth is not a reliable

indicator for eutrophication in this area (JOHAN WIKNER pers. com.). However, the

mean recordings for August in 1986-2000 were 5.7 m (KVARKEN COUNCIL 2003).

In the Bothnian Sea the transparency has decreased by 35 % during the last

one-hundred years (LAAMANEN et al. 2004). In the offshore areas and in the outer

archipelago, the visibility in the water lays in the range of 3-7 m in August. In coastal

areas, the Secchi depth in late summer varies from < 1 to 2.5 m, according to the

degree of eutrophication (BERGSTRÖM & BERGSTRÖM 1999, ÅDJERS & SANDSTRÖM.

1999, DANILOV & EKELUND 2001, SANDSTRÖM & KARÅS 2002, KVARKEN COUNCIL

2003).

3.2 Salinity

The salinity in the Gulf of Bothnia is relatively stable. The salinity varies from 5-7

psu in the Bothnian Sea to 3-4 psu in the Bothnian Bay (HÅKANSSON et al. 1996,

34

Gulf of Bothnia

WULFF et al. 1996, HELCOM 2002, FONSELIUS & VALDERAMA 2003). The proportion

of freshwater outside the river mouths is high (HÅKANSSON et al. 1996).

In the Bothnian Bay, the salinity trend is slightly decreasing since the mid

1980s. The surface salinities declined from 3.5 psu in 1985 to 2.8 psu in 2002, and the

bottom salinities from 4.5 to 3.5 psu (STOCKHOLM UNIVERISTY 2004a, AXE &

BJERKEBÆK LINDBERG 2004).

In the well-mixed, upper 50 m of the Bothnian Sea, the salinity variations are

small (HELCOM 1996, HÅKANSSON et al. 1996). A slight decrease was seen in the

period 1980 to 1995, to about 6.2 psu in the bottom water and 5.5 psu in the surface

layer (STOCKHOLM UNIVERSITY 2004a, AXE & BJERKEBÆK LINDBERG 2004). In 1999

the salinity in the southern Bothnian Sea was 0.5 psu lower than the long-term mean

(KAJRUP 2000). The salinity rose somewhat in 2001 in both basins of the Gulf of

Bothnia (AXE & BJERKEBÆK LINDBERG 2004).

3.3 Oxygen The thermohaline convection in the Gulf of Bothnia gives a good oxygen situation

(ANDERSSON & WIKNER 2000, FONSELIUS & VALDERAMA 2003). This together with

the low BOD activity and the fact that cold and low saline water solve oxygen more

easily, makes the oxygen situation good in the Bothnian Bay (AXE & BJERKEBÆK

LINDBERG 2004). The oxygen has been stable between 7.5 and 9.0 ml l-1 during the

last century (FONSELIUS & VALDERAMA 2003). The oxygen saturation in the bottom

water in the Bothnian Bay varies between 80 and 95 % (ANDERSSON & WIKNER

2000).

The oxygen conditions in the deep waters of the Bothnian Sea have decreased

with about 0.13 ml l-1 per year since the 1960s (WIKNER 1994, HELCOM 1996), even

if the situation is not alarming. The long-term mean oxygen concentration in the

Bothnian Sea is 6.2 ml l-1 (KAJRUP 2000). The oxygen values in 2003 ranged between

5.4 and 6.8 ml l-1 (AXE & BJERKEBÆK LINDBERG 2004). The lowest oxygen value in

the northern Bothnian Sea is recorded in late summer 1999; 4.6 ml l-1 (KAJRUP 2000).

The oxygen saturation is 70-95 % in the bottom water (ANDERSSON & WIKNER 2000).

Laminated sediments are found between 15 and 25 m in the Bothnian Sea. The

lamination is due to natural conditions in the ecosystem; e.g. the land elevation and

35

Gulf of Bothnia

the amount of material transported via the river, and is not eutrophication-induced

(JONSSON 2003).

3.4 Nutrients The concentrations of phosphorus and nitrogen (especially DIN) in the Gulf of

Bothnia are twice as high as in the early 1970s. During the 1980s, the values stabilised

at a relatively high level (HELCOM 1996, WULFF et al. 1996). There is no direct sign

of eutrophication in the open sea areas of the Gulf of Bothnia. In the coastal zones

long-term measurements are lacking, but areas with a high nutrient load in

combination with low water exchange show clear symptoms of eutrophication

(HELCOM 1996, WIKNER 1999).

In the Bothnian Sea, the N:P ratio varies seasonally from 10:1 to 40:1. The

ratio also shows variation between areas. Generally, the Bothnian Sea is N-limited,

but some coastal areas at the Swedish side of the Northern Quark are P-limited

(ANDERSSON et al. 1996, WIKNER 1999). The trends for both inorganic phosphorus

and nitrogen in the deep water in the Bothnian Sea have increased three times since

the 1930s (FONSELIUS & VALDERAMA 2003). The tot-N and tot-P in late summer were

measured to 240 µg l-1 and 12 µg l-1 respectively in the offshore area in the beginning

of the 2000s (HUMBORG et al. 2003, KVARKEN COUNCIL 2003).

A weaker vertical stability may influence the increasing phosphorus

concentration (HELCOM 1996). The high nitrogen concentration may be due to a

higher freshwater supply from rivers and an inflow of P-rich water from the Baltic

proper along the Finnish coast (HELCOM 1996, WULFF et al. 1996, WIKNER 1999).

The total amount of nutrients in the Bothnian Sea in autumn 2002 was ~ 60.000 t

phosphorus and 1.000.000 t nitrogen (SMHI 2004).

The Bothnian Bay is during the whole growth season clearly phosphorus-

limited (ANDERSSON et al. 1996, HELCOM 1996, WULFF et al. 1996, KIRKKALA

1998). The concentration of phosphate is lower and that of nitrate is higher in the

Bothnian Bay than in the Bothnian Sea (SANDBERG et al. 2004). In the deep water of

the Bothnian Bay, the trend for phosphate is relatively stable, while the inorganic

nitrogen is increasing since the 1960s onwards (FONSELIUS & VALDERAMA 2003).

The late summer concentrations for tot-N is 260-270 µg l-1 and for tot-P 6.0-6.5 µg l-1

in the Bothnian Bay (HUMBORG et al. 2003, KVARKEN COUNCIL 2003), which is

classified as very low to low (Table 2.1). Lower salinities in the surface water may

36

Gulf of Bothnia

slightly decrease the tot-P, which is seen between 1985 and 2000 in the Bothnian Bay

(AXE & BJERKEBÆK LINDBERG 2004). The total amount of phosphorus in the

Bothnian Bay in autumn 2002 was 8000 t (SMHI 2004).

Almost all rivers discharging into the Bothnian Bay have somewhat lower

concentrations of DIN than the open sea area outside the river mouth during the

growth period. In the Bothnian Sea, the discharging rivers have slightly lower DIP

concentrations compared to the nearby open sea area during winter. Thus, the

estuaries in the Gulf of Bothnia are ineffective as sinks for riverborne nutrients and

organic material. In the southern Baltic the estuarine sediments are richer in nutrients

compared to offshore areas (HUMBORG et al. 2003).

3.5 Primary production and chlorophyll a Naturally, the concentrations of chlorophyll a are highest during the spring bloom,

around 8 µg l-1 in the Bothnian Sea, and 3-4 µg l-1 in the Bothnian Bay. The summer

values in the Bothnian Sea have doubled since the 1990s (HELCOM 1996). The

chlorophyll a content in the open sea areas of the Gulf is still low, not reaching a

mean value over 1.5 µg l-1 in August (DANILOV & EKELUND 2001, KVARKEN COUNCIL

2003).

The primary production has decreased in the entire Gulf of Bothnia in the

period 1991-2001. Recently, an increase in the Bothnian Sea and the Northern Quark

is seen, to the same levels as in 1998 (ANDERSSON & WIKNER 2004). In the northern

Bothnian Bay, the average value for the primary production is 48 g C m-2 yr –1

(WIKNER 1994), which is four times lower than that in the Bothnian Sea (ANDERSSON

et al. 1996).

3.6 Phytoplankton and algal blooms The dominating phytoplankton groups in the Gulf of Bothnia are generally diatoms

and dinoflagellates. Cyanobacteria account for 25 % of the phytoplankton biomass. In

late summer, unicellular cyanobacteria in open sea areas in the Bothnian Sea may

reach mass occurrences (SANDBERG et al. 2004).

The phytoplankton biomass in the Bothnian Bay is only one third compared to

the amounts in the Bothnian Sea. The limitation by phosphorus reduces the

phytoplankton production in the Bothnian Bay (ANDERSSON & WIKNER 2004). The

37

Gulf of Bothnia

spring bloom is also a month delayed compared to the southern Gulf of Bothnia.

Cyanobacteria are almost totally lacking in the Bothnian Bay (WIKNER 1994, 1995).

In the southern Bothnian Sea, the concentration of especially Aphanizomenon

flos-aquae has increased, probably due to increased concentrations of phosphorus in

the deep water (WIKNER 1994, HELCOM 1996, HAJDU et al. 2003). The nitrogen load

in the Bothnian Sea has increased due to the cyanobacteria (HAJDU et al. 2003). A.

flos-aquae has advantages to Nodularia spp in the Bothnian Sea. A. flos-aquae has

ability to store phosphorus and grow in cold temperature, low salinity and bad light

condition. The weak stratification takes the cells deeper, which is a disadvantage for

Nodularia spp. (HAJDU et al. 2003).

The potentially toxic dinoflagellate Dinophysis acuminata may cause

extensive algal blooms in the Bothnian Sea, as in the summer 1993 along the Finnish

coast (WIKNER 1994).

3.7 Macrovegetation So far, no drastic changes in macrovegetation in the Gulf of Bothnia seem to be

eutrophication-related. In the northern Bothnian Bay, both vegetation and fauna

communities are poor with low abundances and biomasses (KAUTSKY & FOBERG

2001). Although, according to KAUTSKY & FOBERG (2001), the biomass has increased

in the Råneå archipelago compared to earlier investigations. The perennial vegetation

in the Bothnian Bay consists of the green algae Cladophora aegagrophila and the

water moss Fontinalis spp (DAHLGREN & KAUTSKY 2002).

At the Finnish side of the Northern Quark, 40 different algal taxa are recorded.

The dominating species are the brown algae Fucus vesiculosus, Pilayella littoralis and

Sphacelaria arctica, and the green algae Cladophora glomerata (BERGSTRÖM &

BERGSTRÖM 1999). At the Swedish coast in the northern Bothnian Sea, F. vesiculosus

had a depth distribution to 10.5 m in 1998, which indicates a good water quality

(KAUTSKY & FOBERG 1999).

Drifting algal mats have occurred in coastal areas both on the Finnish and

Swedish side of the Bothnian Sea. At the Finnish coast, south of the river mouth of

Eurajoki, mats of Enteromorpha intestinalis were found on shallow bottoms in 1993-

1997. The thickness of the mats varied between 5 and 20 cm, and H2S was

occasionally present beneath the mats (BÄCK et al. 2000). In the northern Bothnian

38

Gulf of Bothnia

Sea at the Swedish coast, smaller occurrences of drifting algae were observed in

August 2000 (Minnhagen 2001 in DAHLGREN & KAUTSKY 2002).

3.8 Zoobenthos Generally, the diversity of macrofauna decreases from south to north in the Gulf of

Bothnia. This is due to the shorter productive season and decreasing salinity in the

Bothnian Bay. Freshwater fauna gives a higher species diversity in the northernmost

parts of the Gulf. Therefore, the zoobenthos in the Bothnian Sea are more similar to

the fauna in the northern Baltic proper (ELMGREN et al. 1984).

The deep bottoms in the Gulf of Bothnia are dominated by the crustaceans

Monoporeia affinis and Saduria entomon (LAINE 2003). About five species colonise

the deep areas in the Bothnian Sea and only two in the Bothnian Bay (ELMGREN et al.

1984). The crustaceans are the most important group in the Bothnian Bay, in lack of

bivalves (LAINE 2003). The macrobenthic biomass has increased significantly, 4-5

times, in the Bothnian Sea since the 1920s (ELMGREN et al. 1984, CEDERWALL &

BLOMQVIST 1991). Significant changes in the zoobenthic communities are observed in

the entire Gulf of Bothnia during the latest years. The invasive polychaete

Marenzelleria viridis has continued to increase since the mid 1990s, the populations

of M. affinis have decreased and molluscs have shell damages (KARLSSON &

LEONARDSSON 2004). M. affinis increased in the 1990s, probably due to an increase in

nutrients (LAINE & KANGAS 2004), but the reason for the decline is not clear

(KARLSSON & LEONARDSSON 2004).

In coastal areas the benthic communities consist of few marine species, and

the majority are of freshwater origin. In 2000, three archipelago areas at the Finnish

side of the Gulf of Bothnia were investigated. In the areas in the Bothnian Sea and

Northern Quark, 28 and 31 species were found, respectively. Macoma balthica, S.

entomon and M. viridis were most abundant. The archipelago area in the Bothnian

Bay was poorer with 18 species. M. affinis, M. viridis and oligochaets were most

common (WESTBERG & LAX 2003).

3.9 Ichthyofauna The number of fish species decreases to the north in the Gulf of Bothnia. Cold water

species as herring (Clupea harengus membras), whitefish (Coregonus spp) and

39

Gulf of Bothnia

vendace (Coregonus albula) dominate in the Bothnian Bay (BOTHNIAN BAY LIFE

2004). In the Bothnian Sea, perch (Perca fluviatilis), roach (Rutilus rutilus) and ruffe

(Acerina cernua) are the dominating species (WIKNER 1995, ÅDJERS et al. 2001,

HOLMQVIST et al. 2003). Ruffe is eutrophication-favoured, but there is no clear

explanation for the increasing ruffe stock in some areas during the 1990s (WIKNER

1995). Other eutrophication-favoured species are lacking or occur sparsely (SVEDÄNG

et al. 1997). The catches of perch, pike (Esox lucius), pikeperch (Sander lucioperca)

and whitefish have decreased, but an increase in vendace is seen during the latest

years (ÅDJERS et al. 2001, SÖDERBERG & GÅRDMARK 2004).

The fisheries for salmon (Salmo salar), herring, sprat (Clupea sprattus) and cod

(Gadus morhua) are limited in the Gulf of Bothnia (SÖDERBERG & GÅRDMARK 2004).

3.10 The present situation and suggestions for measurements The Gulf of Bothnia is the only sub-region in the Baltic Sea with no clear sign of

eutrophication. The concentrations of nutrients have increased to some extent in the

open sea areas during the latest decades. Point sources have caused eutrophication on

a local level in coastal areas. Agricultural productivity in combination with many

small rivers, a flack landscape and enclosed embayments favour local eutrophication

along the Finnish coast. The Swedish coast to the Gulf of Bothnia is more open and

steep and the large rivers are mainly oligotrophic, which makes local eutrophication

more rare (BOTHNIAN BAY LIFE 2004). The eutrophication model for the Gulf of

Bothnia (Fig. 3.2) only indicates some increasing tendencies, which hopefully can act

as warning signals for prevention of more serious changes in the ecosystem.

According to HELCOM (2003a), the average riverine load of nitrogen and

phosphorus to the Gulf of Bothnia and the Archipelago region in the year 2000 was

140.000 t y-1 and 6500 t y-1, respectively. The background load stands for 55 % of the

N and 62 % of the P. The diffuse load origins to 37 % from N and 35 % from P, the

rest (8 % N and 3 % P) are point source load (HELCOM 2003a). The majority of the

phosphorus sources to the Bothnian Bay from Finland come from natural leaching,

forestry and agriculture. In the Bothnian Sea, agriculture, natural leaching and

scattered dwellings are the main P-sources. For nitrogen, natural leaching, agriculture

and atmospheric deposition are of most importance in both areas (KAUPPILA & BÄCK

2001).

40

Gulf of Bothnia

BERNET (2000a) recommends measures in nitrogen reduction in sewage plants

and in diffuse loads from the agriculture and scattered dwellings for the Finnish side

of the Gulf. The risks for flooding should also be taken into consideration and

prevented. The river systems in the Gulf of Bothnia have a human impact in

hydrological alterations, or river dams (HUMBORG et al. 2000). Only the three

northernmost rivers in Sweden are unregulated (HUMBORG et al. 2003). Thus,

according to HUMBORG et al. (2003), most estuaries draining into the Gulf of Bothnia

may behave differently from well-studied nutrient enriched estuaries in the southern

Baltic Sea. Information on flooding in Swedish rivers and probable risk areas are

found at SMHI (2004).

The database of the Finnish and Swedish co-operation project, BOTHNIAN BAY

LIFE (2004), gives accurate information on monitoring data from several sampling

stations in the Gulf of Bothnia.

Fig. 3.2. The present eutrophication situation in the Gulf of Bothnia (see Table 2.1 and Fig. 2.2 for explainations).

41

Archipelago region

4. THE ARCHIPELAGO REGION; Archipelago Sea, Åland

Islands and Stockholm archipelago The Archipelago Sea (SW Finland), the archipelago of the Åland Islands (Finland),

the Åland Sea and the Stockholm archipelago (Sweden) are here included in the

Archipelago region (Fig. 4.1). In e.g. HELCOM reports, this area is a part of the Gulf of

Bothnia, but I have separated these two areas, as the characteristics and state of

eutrophication clearly differ in the Bothnian Sea and Bothnian Bay from the

archipelago areas south of the Southern Quark.

The Finnish side of the Åland Sea (59°45´-60°30Ń, 19°30´-23°00É) is

characterised by some 30.500 islands (total area ~ 2500 km2, the mainland of Åland

not included), forming a mosaic of more or less distinct zonation ranging from the

innermost sheltered coastal zones to the open sea areas in the middle of the

Archipelago Sea (JUMPPANEN & MATTILA 1994, BONSDORFF et al. 1997b, KIRKKALA

1998). Average water depth is 23 m (the deepest trench: 146 m) with a shoreline of

over 20.000 km (JUMPPANEN & MATTILA 1994, BONSDORFF et al. 1997b).

Fig. 4.1. The Archipelago re-gion consists of the Archipelago Sea, the Åland Islands, Åland Sea and the Stockholm ar-chipelago.

The Stockholm archipelago (58°50´-60°30Ń, 18°00´-19°30É) is the

collective term for the 100 km wide and 200 km long area from the inner city of

Stockholm to the islet Svenska Björn in the Åland Sea, and from the town of

Öregrund (1500 inh.) to the town of Nynäshamn (13.000 inh.) in the north-south

direction. The archipelago consists of more than 10.000 islands (total area ~ 500 km2)

with a total coastline of 10.000 km. The southern part of the Stockholm archipelago is

flat with long and narrow bays. The northern part is characterised by a few, deep

channels, sandy bottoms and moraine islands. The coastline to the Åland Sea is

42

Archipelago region

straight and open (KAUTSKY et al. 2000) steeping down to the depths of the Åland Sea

(max. depth: 294 m).

The water has an eastward net transport from the Baltic proper and the Gulf of

Finland to the Gulf of Bothnia through the Archipelago Sea. The Skiftet Strait with an

average mean depth of 40 m forms a corridor in a north-south direction (ERKKILÄ &

KALLIOLA 2004). From the Gulf of Bothnia, the water has a southward transport

through the deep Åland Sea.

Eutrophication is the main environmental problem in the Archipelago region

(BONSDORFF et al. 1997a, KIRKKALA 1998). A high background load in combination

with the increased nutrient concentrations accumulates in shallow archipelagos and

coastal waters. Archipelagos act as filters for nutrients, which incorporate in

organisms and sedimentate, therefore the eutrophication is accelerating much faster in

enclosed areas than in the open sea (MATTILA 2000). For example, the outflow from

Lake Mälaren is important for the state of the environment in the inner Stockholm

archipelago, whereas the outer archipelago is under the influence of the Baltic proper

(LÄNNERGREN 2001).

According to latest list of HELCOM-HOT SPOTS (2004), the agricultural runoff

from SW Finland is the only environmental hot spot in the Archipelago region.

4.1 Secchi depth In the Archipelago Sea, the transparency in the middle and outer areas has changed

from > 10 m in the 1950s (JUMPPANEN & MATTILA 1994) to 2-5 m at the end of the

1990s (ERKKILÄ & KALLIOLA 2004). The Secchi depth is gradually increasing from

the inner coastal bays towards the offshore areas (KIRKKALA 1998, ERKKILÄ &

KALLIOLA 2004). During the period 1999-2003, the innermost areas in the northern

Archipelago Sea had the lowest transparency, 0.9-2 m, while the rest of the inner

archipelago had values of 2-3 m. The best transparency values were found in the

southern archipelago and offshore, 4-4.5 m (SW FINLAND REGIONAL ENVIRONMENT

CENTRE 2004). The Secchi depth has decreased with 0.3-1.4 m in the outer and

middle archipelago zones since the early 1990s, but in the inner areas the situation is

stable (SUOMELA 2001).

The Secchi depth has a significantly decreasing trend in the Åland

archipelago, from 8 m in 1984 to 4.3 m in 1994 (BONSDORFF et al. 1997b, ÖSTMAN &

43

Archipelago region

BLOMQVIST 1997). The highest values are found offshore in the Åland Sea or south of

the mainland; 7 m in summer 2003. In the NW Åland, the outer archipelago had a

transparency of 4 m and the middle area 1.5-2 m in the summer 2003 (HUSÖ

BIOLOGICAL STATION 2004).

In the outer areas of the Stockholm archipelago, the water transparency has

decreased from 8 m in the 1940s (WÆRN 1952) to 7 m in the beginning of the 1980s

(KAUTSKY et al. 1986). In the inner archipelago zone, the Secchi depth has increased

since the 1960s, after the introduction of biological treatment and phosphorus

reduction in the sewage treatment plants in the Stockholm area (BRATTBERG 1986,

LÄNNERGREN 1995, LÄNNERGREN et al. 2003). The Secchi depths in the Stockholm

archipelago were good in the summer of 2003; ~ 4 m in the inner, 5 m in the middle

and 6-8 m in the outer areas (LÄNNERGREN et al. 2003). According to ANON. (1999a)

and LÄNNERGREN et al. (2003), the situation is classified as good to very good in the

whole archipelago zone.

4.2 Salinity The deep layer of the Archipelago region and the Gulf of Bothnia receives saline

water primarily from the Baltic proper surface layer. The deep water salinity trend has

been decreasing, from 7.2 psu in 1980 to 5.8 psu in 1994 (HELCOM 1996, VUORINEN

et al. 1998). Since then, a slight increase has taken place. In 2000, the salinity in the

Åland Sea was 6.5 psu in the bottom water (FONSELIUS & VALDERAMA 2003).

In the Stockholm archipelago, the changes in salinity are most obvious in the

innermost part. The salinity decreased in a step-like manner, from nearly 6 psu in the

beginning of the 1980s to about 5 psu at the end of the 1990s (LÄNNERGREN &

ERIKSSON 2000), due to fluctuations in received freshwater (KAUTSKY et al. 2000). In

the outer archipelago, the salinity is rather stable, varying from 5.2-5.8 in the northern

parts and 6-7 in the southern area (KAUTSKY et al. 2000, LÄNNERGREN et al. 2003).

4.3 Oxygen The oxygen content in the bottom water in the inlet to the town of Turku/Åbo has

decreased dramatically since the 1970s, even though the improvement in sewage

treatment has had a positive effect on the oxygen situation outside the urban areas

(JUMPPANEN & MATTILA 1994). The inner areas, except for the inlet to Åbo, also

44

Archipelago region

show a clear decrease in oxygen concentrations from the late 1990s to 2002. In the

rest of the Archipelago Sea, no changes in oxygen have occurred since the late 1990s

(LAIHONEN et al. 2003). The middle archipelago area had oxygen concentrations in

the ranged 3-6 ml l-1 in the summer 2000. In the outer archipelago and offshore, the

oxygen situation was good (SUOMELA 2001). One exception is the south-eastern part

of the Archipelago Sea, which is influenced by nutrient-rich water from the Gulf of

Finland together with loads from the local fish farms (JUMPPANEN & MATTILA 1994).

In 2000, oxygen concentrations of 1.5-3 ml l-1 were recorded (SUOMELA 2001).

Around the Åland Islands a non-significant decreasing trend is seen in the

minimum oxygen saturation at deep bottoms from 1904 to 2000 (FONSELIUS &

VALDERAMA 2003). In shallow bays and in deeper areas, where the water exchange is

poor, hypoxia and anoxia are common features. The situation is worst in late summer

due to stagnant water (ÖSTMAN & BLOMQVIST 1997).

The oxygen situation was good in the inner Stockholm archipelago the whole

productive season of 2003, with values over 4 ml l-1 (LÄNNERGREN et al. 2003). The

concentration is comparable with 0.2-0.8 ml l-1 in 1994-1998, and 2 ml l-1 in 1999

(LÄNNERGREN & ERIKSSON 2000). The oxygen concentration in summer is highly

related to the outflow from Lake Mälaren in spring (LÄNNERGREN 2001). In the

southern part of the Stockholm archipelago, the condition in the bottom water has

been poor and the concentrations of H2S have periodically been high. Since 1994, the

situation has improved and H2S seldom exceeds 20 µg l-1 (LÄNNERGREN & ERIKSSON

2000). In 2003, the oxygen situation in the middle and outer areas were good,

generally, but H2S was present from August to November at some localities

(LÄNNERGREN et al. 2003).

The amount of laminated bottoms in the Stockholm archipelago has steadily

increased during the 20th century until 1990. In the inner and middle archipelago, no

improvements are seen, even if there have been clear reductions in the sewage

treatment. In the outer archipelago, the laminated sediments were first observed in the

1950s-80s, depending on area (JONSSON 2003).

4.4 Nutrients The nutrient conditions in the Archipelago Sea and the Gulf of Bothnia clearly differ.

The general level and the annual variation of nitrogen and phosphorus concentrations,

45

Archipelago region

as well as the primary production, are much higher in the archipelago area. The

Archipelago Sea receives nutrient-rich surface water from the northern Baltic proper

and the Gulf of Finland. Besides, the complex geomorphology restricts the vertical

mixing in the water mass (HELCOM 1996).

In the 1980s, signs of eutrophication were detected in the outer archipelago

and the Åland area, while the inner Archipelago Sea already suffered from local

eutrophication in the 1960s (JUMPPANEN & MATTILA 1994). The nutrient

concentrations have increased significantly in most parts of the Archipelago Sea since

the 1970, even if better sewage treatment has lead to decreased loads of both nitrogen

and phosphorus (JUMPPANEN & MATTILA 1994, KIRKKALA 1998, KIRKKALA et al.

1998, HÄNNINEN et al. 2000).

The total nitrogen was higher in 2001 in the northern part of the Archipelago

Sea compared to the first half of the 1990s, while the situation was more or less the

same in the rest of the area. In the innermost areas even a decrease has taken place

(SUOMELA 2001). In the summers of 2003-2004, tot-N offshore gradually increased

from 275-300 µg l-1 (low value; Table 2.1) to 350-400 µg l-1 (high value; Table 2.1)

near the coast (SW FINLAND REGIONAL ENVIRONMENT CENTRE 2004). The northern

and south-western parts of the Archipelago Sea have the lowest phosphorus

concentrations. However, phosphorus has increased especially in the northern areas.

Also the internal phosphorus loading has increased during the latest years (SUOMELA

2001, SW FINLAND REGIONAL ENVIRONMENT CENTRE 2004). In the summer of 2004,

the outer and middle archipelago areas in the northern part had tot-P values of 20-24

µg l-1 (medium high; Table 2.1), while the values in the same type of area in the

southern Archipelago Sea was 28-32 µg l-1 (high-very high; Table 2.1, SW FINLAND

REGIONAL ENVIRONMENT CENTRE 2004).

Outside the town of Åbo, the total concentrations of nutrients were 850 µg l-1

for tot-N and 26 µg l-1 for tot-P (SUOMELA 2001, SW FINLAND REGIONAL

ENVIRONMENT CENTRE 2004). The concentrations can be comparable with the tot-N

values of 20 µg l-1 in late 1960s and 25-45 µg l-1 in early 1990s, and tot-P

concentrations of 300 µg l-1 in late 1960s and 400-900 µg l-1 in early 1990s

(JUMPPANEN & MATTILA 1994, KIRKKALA 1998, HÄNNINEN et al. 2000).

Since the late 1980s, the concentrations of nutrients have increased in the

Åland archipelago. The increase in the outer archipelago is higher for nitrogen than

for phosphorus (NUMMELIN 2000). Phosphorus has a decreasing trend with distance

46

Archipelago region

from land (ÖSTMAN & BLOMQVIST 1997). The most eutrophied areas in the

archipelago of Åland are outside the town of Mariehamn (11.000 inh.) and the

archipelago area of Föglö, south-east of the mainland. The westernmost areas next to

the Åland Sea have the best water conditions (APPELGREN & MATTILA 2002). In the

summer of 2003, tot-N and tot-P were ~ 400 µg l-1 and ~35 µg l-1 respectively in an

inner archipelago station in the NW. The concentrations in the outer archipelago were

250-300 for tot-N and 16-20 µg l-1 for tot-P (HUSÖ BIOLOGICAL STATION 2004).

The nutrient concentrations in the Stockholm archipelago had a peak in the

beginning of the 1970s (BRATTBERG 1986). Since then the concentrations of both

phosphorus and nitrogen have decreased in the drainage area of Lake Mälaren. The

amount of phosphorus was about 85-90 µg l-1 in 1970 and 25 µg l-1 in 1994. For

nitrogen the values were about 1220 µg l-1 in 1976 and 500 µg l-1 in 1994

(LÄNNERGREN 1995). In the inner archipelago, near the Stockholm city, tot-P and tot-

N have decreased with 20 % and 50 % respectively since 1990 (LÄNNERGREN et al.

2003). The nutrient concentrations in the summer of 2003 in the inner archipelago

were classified as very high-high (tot-P: 35 µg l-1, tot-N: 800 µg l-1), declining

successively to low-very low (tot-P: 15 µg l-1, tot-N: 280 µg l-1) in the outermost

areas. The annual internal phosphorus load in inner archipelago is estimated to 30-60

tons (LÄNNERGREN et al. 2003).

The discharges of waste water in the Stockholm inner archipelago cause a

nutrient-rich out-flowing current in the deep water. This results in the total-P

concentrations in the surface water being highest 7-12 km from Stockholm, in the

outer archipelago. The gradient for nitrogen is sharper. The concentration of tot-N has

slightly risen, when the surplus of inorganic nitrogen is unutilised in the phosphorus

limited inner archipelago (BRATTBERG 1986, LÄNNERGREN 1995).

4.5 Primary production and chlorophyll a The open Archipelago Sea has a spatial distribution of chlorophyll a in the range 2-4

µg l-1 (ERKKILÄ & KALLIOLA 2004). In the early 1990s, the limit for 3 µg l-1 moved 5-

10 km further out from the coastline (KIRKKALA 1998). During the last decade, the

concentrations of chlorophyll have increased in the innermost and northern parts of

the Archipelago Sea, whereas no changes or improvements have been detected in the

middle archipelago (SUOMELA 2001). The highest, 3-5 µg l-1 (high; Table 2.1),

47

Archipelago region

chlorophyll values are found in the south eastern part of the Archipelago Sea, in

comparison with 2-3 µg l-1, or medium high (Table 2.1), in the inner and middle areas

(KIRKKALA 1998, ERKKILÄ & KALLIOLA 2004, SW FINLAND REGIONAL

ENVIRONMENT CENTRE 2004). In the coastal areas, the range is 5-10 µg l-1, which is

very high (SW FINLAND REGIONAL ENVIRONMENT CENTRE 2004).

In the deep Åland Sea the values of both chlorophyll a and primary production

capacity show a slightly increasing tendency during the 1980s (HELCOM 1990).

During the 1990s, chlorophyll a has slightly decreased in the Åland archipelago, e.g.

in the NW part from about 2.3 µg l-1 to 1.8 µg l-1 (NUMMELIN 2000). In the summer

of 2003, the chlorophyll values in NW Åland were 2-6 µg l-1 in the inner archipelago

and 1-3 µg l-1 in the outer (HUSÖ BIOLOGICAL STATION 2004). The trend for the

primary production is similar, the concentration was 4.5 mg C m–3 h-1 in 1979

and increased to 8.5 mg C m–3 h-1 in 1987 (HELCOM 1990, SCHULZ et al. 1992).

In the 1970s, the chlorophyll a concentration in the Stockholm archipelago

indicated a eutrophic system with > 30 µg l-1 15 km out from the city of Stockholm,

which slightly decreased to < 10 µg l-1 offshore. Until 1985 the situation had

improved remarkably, when the concentrations were halved (BRATTBERG 1986). The

mean chlorophyll content in the period 1999-2003 was 5.7 µg l-1, and the inner,

middle and outer archipelago zones are classified to very high, high-very high and

low-medium high, respectively (LÄNNERGREN et al. 2003).

4.6 Phytoplankton and algal blooms According to JUMPPANEN & MATTILA (1994), the amount of phytoplankton in the

inlet to the town of Åbo was low in the 1970s and 1980s. During the 1990s the

amount of phytoplankton started to increase. The most common species in the

Archipelago Sea and Åland archipelago are Aphanizomenon spp and Anabaena spp.

In late summer Nodularia spp is abundant (JUMPPANEN & MATTILA 1994, ÖHMAN

1995). In the warm summers of 1997 and 1999, the inner and middle regions of the

whole Archipelago Sea suffered from dense algal blooms (ref. in RÖNNBERG 2001). In

late summer 2004, visible plankton accumulations were observed in the north-west

and south Åland as well as in the deep areas of the Archipelago Sea (BALTIC SEA

PORTAL 2004, SW FINLAND REGIONAL ENVIRONMENT CENTRE 2004).

In the inner parts of the Stockholm archipelago, Planktothrix agardhii has

been a frequently occurring cyanobacteria since the 1970s onwards (KAUTSKY et al.

48

Archipelago region

2000). In the summer of 2003, Aphanizomenon spp was the dominating cyanobacteria

(LÄNNERGREN et al. 2003).

4.7 Macrovegetation The Fucus vesiculosus belt in the Archipelago Sea declined drastically during the

1970s. Instead the amount of annual filamentous algae, e.g. Pilayella littoralis,

Cladophora glomerata and Ceramium tenuicorne, increased (RÖNNBERG 1981). In the

early 1980s the areas with apparently weakened Fucus-vegetation was estimated to be

3500 km². The decline occurred mainly in the middle and outer archipelago zones,

which are directly influenced by water from the open Baltic Sea (RÖNNBERG et al.

1985). An increased turbidity and sedimentation, together with the increased

production of filamentous algae are considered as the direct reasons for the

impoverished Fucus-vegetation (RÖNNBERG 1981). An increase in the occurrence of

F. vesiculosus was seen in the early 1990s (BÄCK et al. 1993), but aerial photography

from the summer 2000 shows a decreasing trend at the same places (HELMINEN et al.

2000). According to KOHONEN et al. (2004), the Fucus-belts further decreased in

width in the outer areas of the Archipelago Sea in the 1990s.

In the Åland waters, long-term changes are observed both in the eastern

archipelago areas and offshore. The macrovegetation in the 1970s and the turn of the

millennium were compared in ROOS et al. (2004). In the 1970s, 26 species of

macroalgae were recorded (RÖNNBERG 1981); three decades later, six species had

disappeared and two were new, which means a total turn-over of 31 %. The increasing

amount of filamentous algae seems to have out-competed more long-lived species

(ROOS et al. 2004).

Around the Åland mainland, 37 species of macrovegetation were detected in

the end of the 1990s. The filamentous algae Cladophora glomerata and Pilayella

littoralis dominated the biomass and abundance together with Chara aspera and

Potamogeton pectinatus (BERGLUND et al. 2003). South of the Åland islands, the

macrophytes at an offshore locality were studied in 1956 and 1993. In almost 40

years, the changes were of a minor extent compared with the studies from the

Archipelago Sea. However, fewer species were recorded in the 1990s. A decline in

the number of perennial deep-water algae and the depth distribution of the most

diverse vegetation has declined with about 8 m (RÖNNBERG & MATHIESEN 1998).

49

Archipelago region

At the Swedish side of the Åland Sea, the distribution of F. vesiculosus has

changed significantly the last century. The maximum depth for attached Fucus- plants

decreased with 3 m from the 1940s to 1984 (KAUTSKY et al. 1986). In the 1940s, the

maximum depth for F. vesiculosus varied between 8.8 and 11.5 m (WÆRN 1952), in

1984 and 1996 between 6.5 and 9 m (KAUTSKY et al. 1986, ERIKSSON et al. 1998).

Plausible reasons to the decline are all related to eutrophication: increased

sedimentation, overgrowth by epiphytic annual algae and increased shading by

plankton (ERIKSSON et al. 1998). According to ERIKSSON et al. (1998), light stress for

both young and mature plants and/or establishment problems for zygotes may be the

reason. The reduced Fucus-belt also corresponds to the declining Secchi depth during

the same period. After 1984 the situation has slightly improved (WULFF & HALLIN

1994). In ERIKSSON et al. (2004), the negative effects of recreational boating and

small ferryboats on the macrovegetation in the Stockholm archipelago are illustrated.

However, F. vesiculosus is positively favoured in that the wave actions keep the

bottoms free from sedimented material (ERIKSSON et al. 2004).

The state of shallow bays in the Stockholm archipelago, Åland Island and

western Gulf of Finland is presented in WALLSTRÖM et al. (2000). That report gives

recommendations for measurements of underwater vegetation and zoobenthos in

shallow areas.

4.8 Ephemeral algae and drifting algal mats Increased phytobenthic production in the archipelago has lead to an increase in

benthic drifting algal mats, presently covering large areas of intermediate depths.

These mats, with an average biomass of 300 g dry weight m-2, or 2000 g wet weight

m-2 (in 1995), are highly oxygen-demanding, and have serious effects on both

zoobenthos and fish (BONSDORFF 1992, NORKKO 1997).

In the Archipelago Sea and in the archipelago of Åland, drifting algal mats

have frequently occurred during the 1980s and 1990s (NORKKO 1997). The drifting

algal masses in the Archipelago Sea range from 1.5 m down to 35 m, with the

thickest, totally anaerobic mats deepest. The drifting algal masses consist mainly of

Cladophora glomerata, Pilayella littoralis and Ceramium tenuicorne. The occurrence

of drifting algal masses was most abundant in the SE corner of the Archipelago Sea.

This is due of the high nutrient load from the Gulf of Finland, good water

transparency and substrates suitable for algal growth (VAHTERI et al. 2000).

50

Archipelago region

The soft-bottoms in the archipelago of NW Åland were periodically, totally or

partly, covered with drifting algal mats at about 70 % of the bottoms down to 10 m

(HOLMSTRÖM 1998). In 1997-2000, mats of drifting algae occurred frequently in the

Åland archipelago (BERGLUND et al. 2003, ROOS et al. 2004). However, the regular

and strong water movements near ferry routes keep the bottoms free from sediments

and drift algae (ROOS et al. 2004).

4.9 Zoobenthos Better waste water treatment improved the situation for the zoobenthos in inner parts

of the Archipelago Sea from the 1950s to the 1980s. Since then the state of the bottom

fauna has reversed. The amphipod Monoporeia affinis has decreased remarkably and

the Macoma-Nereis communities have replaced the former Macoma-Monoporeia

communities. Temperature, dissolved oxygen and sediment organic carbon controls

the zoobenthos. High oxygen concentrations and a good food supply favour

communities dominating by M. affinis. High organic content in combination with

shallow and warmer waters are more suitable for M. balthica (MATTILA 1993,

JUMPPANEN & MATTILA 1994, HÄNNINEN & VUORINEN 2001, BONSDORFF et al. 2003,

O´BRIEN et al. 2003).

In the middle and outer parts studies are scarce, but in some areas both

abundance and biomass have decreased since the 1960s, mostly near fish farming

localities. In the same areas, an increase of the polychaete Marenzelleria viridis has

taken place (HÄKKILÄ et al. 1993, HÄNNINEN & VUORINEN 2001, BONSDORFF et al.

2003, O’BRIEN et al. 2003).

Between the early 1970s and the early 1990s the number of species has

decreased in the Åland archipelago, while faunal abundance and biomass have

increased above 30 m. Between 20 and 40 m depths, the trend has been similar in

inner areas, while the outer, more exposed areas have been in better condition. About

40 % of the species composition has changed since the 1970s and a shift from

suspension feeders to deposit feeders has taken place. M. balthica has increased while

M. affinis has decreased. The distribution of M. affinis has moved further towards the

open sea. (BONSDORFF et al. 1990, 1991, 1997b, NORKKO & BONSDORFF 1994). In the

1990s, the species abundances have been stable, except for the middle archipelago,

where the number of M. viridis and gastropods has increased (PERUS & BONSDORFF

51

Archipelago region

2004). On shallow bottoms drifting algae has caused problems for the fauna, by

inducing hypoxia (NORKKO et al. 2000).

Dead bottoms were found in the Stockholm archipelago since the 1970s

(LINDAHL et al. 1993). The extent of dead bottoms in the end of the 1990s was around

20 % in the inner archipelago (LÄNNERGREN & ERIKSSON 2000). In 1991, Beggiatoa-

mats were reported in inner areas of the Stockholm archipelago between 9 and 50 m

and the evidence for surface or subsurface faunal activity was sparse (ROSENBERG &

DIAZ 1993). The total biomass in the inner archipelago increased in the end of the

20th century (LÄNNERGREN & ERIKSSON 2000). Benthic fauna has been found down

to 20-30 m depths in the inner parts and to 40-60 m depths in the outer parts. The

fauna was dominated of small sized M. balthica, Saduria entomon, Potamopyrgus

antipodarum, M. affinis, oligochaetes and chironomides. M. viridis has increased up

to 10 times since 1996 (STEHN & DROMBERG 1999). According to LÄNNERGREN &

ERIKSSON (2000), the majority of the Stockholm inner archipelago was in good

condition down to 10 m.

4.10 Ichthyofauna The production of fish has increased in the Archipelago region as a consequence of

eutrophication. The catches of perch (Perca fluviatilis), pikeperch (Sander lucioperca)

and cyprinids have increased since the 1980s, also in outer areas both in the

Archipelago Sea and around Åland (JUMPPANEN & MATTILA 1994, ÅDJERS et al.

1997, 2001, HUSÖ BIOLOGICAL STATION 2004). During the same time, the catches of

flounder (Platichthys flesus) have increased compared with the situation in the early

1990s. However, the variations in catches are large, and the results are not significant

(ÅDJERS et al. 2001).

Eutrophication may influence spawning ground for fish negatively, which

seems to be the case for the three spined stickleback (Gasterosteus aculeatus) in the

Archipelago Sea (KOHONEN et al. 2004). Also the economically important Baltic

herring (Clupea harengus membras) today spawns in the outer archipelago instead of

the more eutrophied inner areas (KÄÄRIÄ 1999). In the 1980s, herring spawned down

to 6 m, and in 2000 the spawning depths had decreased to 0.5-2.5 m (VAHTERI &

VUORINEN 2001).

52

Archipelago region

Increases in turbidity and phytoplankton biomass together with a reduced

coverage of submerged vegetation have influenced the fish communities. Ruffe

(Gymnocephalus cernuus) and cyprinids are able to forage in turbid and dim

environment, e.g. in a eutrophied inlet on the Swedish coast of the Archipelago region

(SANDSTRÖM & KARÅS 2002). The recruitment of pike (Esox lucius) has decreased in

the outer archipelago of Åland Island, probably due to the diminishing Fucus

vesiculosus communities (LEHTONEN et al. 2000).

Fish farms (mainly Oncorhynchus mykiss) affect the aquatic ecosystem by a

constant, year-round input of nutrients, which prolongs the season for primary

production and eliminates the natural nutrient limitation (BONSDORFF et al. 1997a).

The distribution of fish farms in the archipelago of SW Finland and Åland cover the

entire area concentrated to the middle and outer regions (HAMMER 2000). According

to the SW FINLAND REGIONAL ENVIRONMENT CENTRE (2004), the annual loads of

nutrients from the ~100 aquacultures in SW Finland and Åland are approximately 274

t nitrogen and 35 t phosphorus. A reduction in production and number of farms, more

environmental friendly policy and development of the feeding system are factors

contributing to a 50 % nominal reduction in loads compared with the beginning of the

1990s (SW FINLAND REGIONAL ENVIRONMENT CENTRE 2004). In the Stockholm

archipelago the fish farms are only about ten and are on average small-scaled

(HAMMER 2000).

4.11 The present situation and suggestions for measurements

The Archipelago region is well studied, which makes the detection of eutrophication-

related changes relatively easy. Based on the available literature, the assessment of

eutrophication in the Archipelago region is presented in Fig. 4.2. Different changes in

macrovegetation can be considered as the most severe problem in the region. In

comparison with the state of chlorophyll a and the phytoplankton production in the

end of the 1990s and today, the situation has improved. The transparency indicates

that the area is in poor condition, which is primary based on the data from the

Archipelago Sea. The situation for the Stockholm archipelago alone would indicate a

“green box” (Fig. 4.2), or moderate changes.

The total load of phosphorus to the Archipelago Sea and the Åland

archipelago in the 1990s was 550 t yr-1 and derivered mainly from the agriculture (50

53

Archipelago region

%), aquaculture (20 %), and scattered dwellings (10 %). The corresponding value for

nitrogen is 7030 t yr-1 and the major sources area agriculture (45 %), municipalities

(25 %), natural leaching (25 %), and aquaculture (10 %; KAUPPILA & BÄCK 2001). In

the Archipelago Sea, the areas near the coast and in closeness to larger islands are

mostly influenced by agriculture. In the central archipelago, the fish farms are the

major source to the nutrient loads (HÄNNINEN et al. 2000). The annual production of

aquaculture in the Åland archipelago stands for 31 % of the local nitrogen load and 65

% of that from phosphorus (ANON. 2003). The environmental conflict and debate of

eutrophication and fish farming industry in the Archipelago Sea, SW Finland are

analysed from an sociological perspective in SALMI & NORDQUIST (2003), PEUHKURI

(2004) and SALMI et al. (2004). The results show a need for a balanced integration of

environmental, cultural and socio-economic factors to benefit water protection

(PEUHKURI 2004).

Fig. 4.2. The conceptual model of eutrophication in the Archipelago region. An improvement in the amount of phytoplankton is detected since 2001 (see Table 2.1 and Fig. 2.2 for explainations).

54

Archipelago region

Dumping and dredging in the Åbo harbour have had serious consequences for

the zoobenthos in the inlet to the town. A shift from suspension to deposit feeders has

taken place (HÄNNINEN & VUORINEN 2001). The intense ferry traffic in the region

causes extra mixing of water, especially in shallow areas. The consequence is an

occasional sedimentation of nutrients, and the material is more available for primary

production (SUOMELA et al. 2000). According to HÄNNINEN et al. (2000), the situation

in the outer Archipelago Sea could, on a long term basis, be improved by international

agreements for the Baltic proper. In the middle archipelago, restrictions for

aquaculture is the simplest measure to decrease local or regional nutrient input. Non-

point source loading from rivers is the main eutrophication inducing factor in the

coastal zones (HÄNNINEN et al. 2000).

The city of Stockholm is the main contributor of wastewater on the Swedish

side. In 2003, the discharges from waste water treatment plants to the Stockholm

archipelago were 1560 t of nitrogen and 23 t of phosphorus, which are less than in

previous years (LÄNNERGREN et al. 2003). Both the methods of cleaning and sewage

treatment need an increase in capacity. KAUTSKY et al. (2000) suggest surveys of the

local loads and long-range circulation planning in every municipality in the

Stockholm archipelago. This is a reasonable goal also for the Archipelago Sea and the

Åland Islands. The water circulation pattern in the Stockholm archipelago is found in

ENGQVIST (2002), and for parts of the Åland archipelago in EILOLA (2002).

55

Gulf of Finland

5. THE GULF OF FINLAND The Gulf of Finland (59°11Ń, 22°50É - 60°46Ń, 30°20É) is a direct extension of

the Baltic proper with no threshold at the mouth (Fig. 5.1). The Gulf is shallow (mean

depth: 38 m, max. depth: 123 m) and the depth decreases towards the east, from 60-80

m in the middle part to 20-40 m at the eastern end of the Gulf, before the Neva Bight.

The largest river in the Baltic Sea drainage area, River Neva, discharges into the

eastern end of the Gulf of Finland. The River Neva has an annual inflow of 2500

m3s-1 (PITKÄNEN et al. 2003).

The combination of the large freshwater inflow and the water exchange with

the Baltic proper gives the Gulf of Finland a strong salinity gradient. The water

circulation is anti-clockwise, with an eastward transport along the Estonian coast and

a westward transport along the Finnish coast (HELCOM 1996). The bottom topography

favours formation of mesoscale circulation patterns with transition zones (ALENIUS et

al. 1997, MOISANDER et al. 1997). The theoretical residence time of water is about

three years in the whole Gulf and one year in the easternmost part (PITKÄNEN et al.

2003). The Gulf is covered with ice every winter (ALENIUS et al. 1997).

The Gulf of Finland is the most polluted sub-region of the Baltic Sea (HELCOM

2002). The nutrient inputs to the surface layer are 2-3 times higher the average

Fig. 5.1. The Gulf of Finland sub-region.

56

Gulf of Finland

amount of inputs to the whole Baltic (PITKÄNEN et al. 1997, 2001). The largest loads

of nutrients and organic matter deriver from the River Neva and the town of St.

Petersburg in Russia (~ 5 million inh.; PITKÄNEN et al. 1993, 1997, LEPPÄNEN et al.

1997). The Neva Estuary acts as an effective trap for nutrients and autochthonous

POM, thus regulating the trophic conditions in the whole Gulf (PITKÄNEN 1991,

CONLEY et al. 1997, PANOV et al. 2002). In the beginning of the 20th century, the

Neva Estuary was an oligotrophic system (TELESH et al. 1999). The intensification of

the agriculture started later in the eastern Baltic Sea region and the population density

was low (SCHERNEWSKI & NEUMANN 2005). Although, according to references in

PITKÄNEN et al. (1993), effects of eutrophication were suggested during spring bloom

already in the 1910s.

Besides the St. Petersburg area, other nutrient sources in the Gulf of Finland

are the Helsinki/Helsingfors region (~ 1 million inh.), River Kymijoki/Kymmene älv

and the town of Kotka (60.000 inh.) region in Finland, the town of Vyborg (80.000

inh.) in Russia, and the town of Tallinn (400.000 inh.) and the River Narva, including

the waste water from the town of Narva (80.000 inh.), in Estonia (PITKÄNEN et al.

1993).

On the HELCOM list of environmental hot spots in the Baltic Sea catchment

area in February 2004, the Gulf of Finland is represented with 14 hot spots. The

sources are mainly municipal waste from the Helsinki region and industrial and

municipal pollution from four hot spots in Estonia and nine hot spots in the St.

Petersburg region (HELCOM-HOT SPOTS 2004).

5.1 Secchi depth The water transparency in the Gulf of Finland has decreased linearly with 40 %, from

approximately 8 to 5 m during the period 1905-2004 (LAAMANEN et al. 2004). In the

western Gulf, east of the Hanko peninsula, the average transparency in the early 1990s

was 4 m (KIIRIKKI 1996), which is classified as good (Table 2.1). According to

references in JUMPPANEN & MATTILA (1994), the Secchi depth in the same area in

1914-1939 had an average value of 9 m.

Along the Finnish coast at the end of the 1990s, the Secchi depth in the inner

archipelago was 1-3 m, and 3-5 m in the outer archipelago. In the innermost bay in

Helsinki, the water transparency in summer 1998 varied between 0.1 and 0.5 m.

57

Gulf of Finland

(PELLIKKA & VILJAMAA 1999). The Secchi depth in the Vyborg Bay in 1996 was 1 m

in the inner areas and increased to 2.5-3.5 m further out (LEHVO & BÄCK 2000).

Recordings from the Neva Estuary in 2000 gave a transparency around 2 m

(NIKULINA 2003).

5.2 Salinity The salinity has strong horizontal and vertical gradients in the Gulf of Finland,

forming horizontal density fronts. Fronts are important convergence areas where

material accumulates and affects the redistribution of discharged material in the sea

(ALENIUS et al. 1997).

The surface salinity in the Gulf of Finland varies from 0 to 6.5 psu from east to

west (PITKÄNEN et al. 1993, ALENIUS et al. 1997). In August 2002, an inward flow of

saline water from the Baltic proper gave salinities of 9-10 psu in the deepest bottoms

in the south-western Gulf. In the shallower northern coast, the bottom salinity was 6-7

psu (PITKÄNEN et al. 2003).

The larger salt water intrusion to the Baltic Sea in the winter 2002/2003 was

still not recognised in the Gulf of Finland in the summer 2004 (SYKE 2005). The

influx in 1993 increased the salinity in the Gulf of Finland during the period 1995-

1998, which coincided with a decrease in oxygen and increase in phosphate

concentrations (KAHRU et al. 2000).

Vertical mixing during autumn storms may break the salinity stratification

down to the bottom in the deep parts of the eastern Gulf (PITKÄNEN et al. 1993, 2003).

5.3 Oxygen The general trend for the deep water oxygen in the period 1970s-1990s was increasing

from late summer concentrations of 3-5 ml l-1 to 7-9 ml l-1 (PITKÄNEN & VÄLIPAKKA

1997). There were no advectional inflows of saline water, which gave a weaker

stratification in the deep layers (ANDERSIN & SANDLER 1991).

The trend ceased in the 1990s, when a strong stratification of the water column

caused reduced conditions in an extended area in the Gulf of Finland. Large quantities

of nutrients, especially phosphorus, were released from the bottoms (PITKÄNEN et al.

2001, LEHTORANTA 2003). In PITKÄNEN et al. (1993) the lowest values (30-50 %)

were measured in the semi-enclosed basins in the Finnish archipelago. In August

58

Gulf of Finland

1998, the oxygen saturation in the waters off Helsinki varied between 75 and 100 %

in the innermost areas and declined further out to 25-50 % (PESONEN 1999). In the

summer of 1996, anoxic bottoms were partly present in the Finnish archipelago (~300

km²). The situation was probably similar in the whole central basin of the eastern

Gulf, in areas deeper than 40 m (~3000 km²) and in the deep parts of the outer Neva

Estuary (PITKÄNEN & VÄLIPAKKA 1997, KAUPPILA & BÄCK 2001).

Surveys of the bottom conditions in the Finnish coastal waters of the Gulf of

Finland show very poor oxygen condition during the latest years. In 2001, 17 % of the

investigated stations had oxidised sediments. After a mixing event in the autumn of

2001, 53 % of the investigated bottoms were oxidised in August 2002. In the summer

of 2003, the situation was again reversed, and 73 % of the bottoms were reduced

(HELCOM 2003b, KANGAS et al. 2003, HAAHTI & KANGAS 2004). The oxygen

conditions have further deteriorated, and in 2004 only 13 % of the investigated

bottoms were oxidised. However, hydrogen sulphide was recognised only in the inlet

to the Gulf of Finland, whereas H2S occurred at all the deep bottoms in 2003 (SYKE

2005).

Variations in bottom water oxygen concentrations are likely to have large

effects on nutrient recycling processes in the Gulf of Finland (CONLEY et al. 1997).

Under good oxygen conditions, the eastern Gulf of Finland can effectively retain land

based nutrient fluxes. Thereby the nutrient input to the rest of the Gulf and the Baltic

proper is reduced (PITKÄNEN et al. 1993, PITKÄNEN & TAMMINEN 1995). The deeper

western Gulf is more sensitive to reduced conditions since it is under direct influence

of the deep saline inflow from the Baltic proper (PITKÄNEN et al. 2003).

5.4 Nutrients The winter concentrations of nutrients in the Gulf of Finland are among the highest in

the whole Baltic Sea area (HELCOM 2002). The nutrient concentrations in the Neva

Estuary are two times higher (tot-N: 600 µg l-1, tot-P: 40 µg l-1) compared to the open

eastern Gulf of Finland. Concentrations of both nutrients decrease from east to west

already in the inner estuary. Dissolved fractions of nutrients are stored below the

mixed surface layer (PITKÄNEN et al. 1993).

In August 2002, the south western part of the Gulf of Finland had high values

of both inorganic nitrogen and phosphorus. The extreme concentrations of ammonium

59

Gulf of Finland

(400-800 µg l-1) were found in the north west, along the Finnish coast, while nitrate

and nitrite were highest (200-400 µg l-1) in the Neva Estuary. Phosphate has the

highest concentrations (400-800 µg l-1) in the coastal zone of the Finnish part of the

Gulf (PITKÄNEN et al. 2003). The following summer, 2003, the nitrogen

concentrations were stable or had slightly increased, but the phosphorus values in the

bottom water had further increased. The southern coast of the Gulf had medium high

values, 19-25 µg l-1, and the northern coast high values, 25-31 µg l-1. Since the end of

the 1990s, the increase is about 60 % (HAAHTI & KANGAS 2004, BALTIC SEA PORTAL

2004).

Nitrogen is the limiting factor for primary production in the Gulf of Finland

(PITKÄNEN 1991). The N:P ratio of the benthic nutrient fluxes is low in the coastal

Gulf of Finland (PITKÄNEN et al. 2001). The external load of phosphorus to the Gulf

of Finland has decreased by 30 %, or 3400 tons, in the 1990s (LEHTORANTA 2003).

The interannual change in the phosphate content was 11.000 t in the winter 2001-

2002, whereas the estimated annual net influx of phosphorus is about 8000 t to the

Gulf of Finland (PITKÄNEN et al. 2003). However, the concentration of P in the water

has increased, due to the high degree of internal phosphorus released from the

sediment under reduced conditions. The concentrations in the bottom water are high,

especially in late summer. The P-rich water is generated to the rest of the water

column during the next spring bloom (LEHTORANTA & HEISKANEN 2003). According

to LEHTORANTA & HEISKANEN (2003), the biogeochemical processes of phosphorus

play an important role for the trophic state of the Gulf of Finland, especially for the

summer bloom of cyanobacteria.

The total amount of phosphorus and nitrogen off the coast of Helsinki in the

late 1990s are high to very high (Table 2.1) in the innermost areas and medium high

in the open sea. In the inner areas, the tot-P and tot-N reached levels of 29-110 µg l-1

and 370-1400 µg l-1, respectively. Further out, in the open sea, the tot-P was 22 µg l-1

and the tot-N was 320 µg l-1 (PELLIKKA & VILJAMAA 1999, PESONEN 1999). In the

1970s, the water quality off Helsinki varied between fair to poor in inner areas and

between satisfactory and good in the open sea (PESONEN 1999).

60

Gulf of Finland

5.5 Primary production and chlorophyll a The chlorophyll a concentrations in the coastal waters usually range between 4 and 10

µg l-1, and in the open waters below 4 µg l-1. High chlorophyll a concentrations (> 10

µg l-1) are regularly measured in the inner bay areas off the towns of Helsinki,

Porvoo/Borgå and Vyborg. In the more open and deep Estonian coast, chlorophyll a

concentrations vary around 5 µg l-1 in summer but in shallow bays the concentrations

may be higher (LEPPÄNEN et al. 1997). MOISANDER et al. (1997) found that changes

in the concentration of chlorophyll a coincided with the salinity fronts in a transect

from Helsinki and westwards to the Baltic proper.

In 2004, the content of chlorophyll a in the open Gulf was 2-4 µg l-1 and in the

Finnish archipelago 4-6 µg l-1 (BALTIC SEA PORTAL 2004), which is classified as

medium high to very high (Table 2.1). The concentrations were higher compared to

year 2003 (SYKE 2005). An increasing gradient from the eastern Gulf of Finland

towards the Neva Estuary is seen, with values from 5.3 to 14.7 µg l-1 (PITKÄNEN et al.

1993, TELESH et al. 1999). Chlorophyll a concentrations in Neva Bay were lowest in

the north eastern part and highest in the southern part, averaging at 9.7 (±5.2) µg l-1

for the whole Neva Bay. In the southern Neva Bay, the chlorophyll a concentration

has increased with 40 % since the early 1980s (TELESH et al. 1999). Increased values

were measured also from the Kymijoki Estuary and some other loaded inner bays

(PITKÄNEN et al. 1993).

The primary production of plankton in the Neva Bay in 1982-1996 was

0.7±0.2 g C m-2d-1, in the eastern Gulf of Finland 0.3-0.7 g C m-2d-1. In general,

primary production of plankton in the Neva Bay in 1996 was 2.7 times higher than in

the 1980s (TELESH et al. 1999). Even if the planktonic primary production is almost

doubled in the inner than in the outer Estuary, the rate of decomposition of organic

matter is the same (GOLUBKOV et al. 2003). In the open Gulf of Finland, the level of

in vitro phytoplankton primary productivity varied from 5 to 8 mg C m-3h-1 in the late

summer 1990. Elevated values, generally from 10-20 mg C m-3h-1, were measured in

the Neva Estuary and in the Finnish archipelago (PITKÄNEN et al. 1993).

5.6 Phytoplankton and algal blooms Cyanobacteria are favoured in stratified waters (KANOSHINA et al. 2003). In the Gulf

of Finland, the dominant phytoplankton groups are cyanobacteria, dinoflagellates and

61

Gulf of Finland

flagellates (HELCOM 1990, 1996). The horizontal salinity gradient determines the

distribution of the species, but the marine species are few with low biomasses even in

the open Gulf (KAUPPILA et al. 1995). Aphanizomenon spp and Nodularia spumigena

are the most common species of cyanobacteria in the western part of the Gulf

(MOISANDER et al. 1997), while the limnic Planktothrix agardhii dominates in the

eastern part (KAUPPILA et al. 1995, HELCOM 1996). Blooms are formed in late

summer and autumn, but in the Neva Estuary dense phytoplankton aggregates are

found during the entire growth season (MOISANDER et al. 1997). In the Neva Bay,

most of the phytoplankton species have their origin in Lake Ladoga (KAUPPILA et al.

1995).

Russian monitoring data indicated an increase of cyanobacterial blooms in the

eastern Gulf of Finland (excluding the Neva Bay) in the late 1980s. Especially the

filamentous, non-heterocystic Planktothrix spp increased (KAUPPILA et al. 1995). The

cyanobacterial blooms were exceptionally intense in the Gulf of Finland in the

summers of 1997 and 1999. Due to the wind directions and upwelling, the northern

Gulf of Finland suffered the most in 1997, while the largest algal aggregates occurred

in the southern Gulf in 1999 (KANOSHINA et al. 2003). In 2002, a strong bloom of A.

flos-aquae occurred in the inner Neva Estuary (NIKULINA 2003). The algal biomass in

the summer 2004 was over the long-term average, the bloom was most intense in the

inlet to the Gulf of Finland (BALTIC SEA PORTAL 2004, SYKE 2005).

5.7 Macrovegetation The eutrophicated water in Tallinn Bay has caused changes in the species composition

of the bottom vegetation. Compared to the 1970s, the share of green algae has

increased two to three times. The share of the epiphytic brown algae Pilayella

littoralis has increased as well. In the same area the amount of Fucus vesiculosus has

declined. The red algae make up less than 35 % of the total amount of algae. Their

share decrease towards the east, due to the salinity. The percentage of both brown and

green algae increases eastwards (KUKK et al. 1997).

F. vesiculosus has a salinity range between 3 and 6 psu from the Virolahti Bay

in the east to the Hanko peninsula in the west in the Gulf of Finland. The optimum

depth for Fucus was 2 m. The maximum depth, 6.5 m, was reached outside the Hanko

peninsula in the western Gulf (BÄCK & RUUSKANEN 2000). The eelgrass, Zostera

62

Gulf of Finland

marina, occurs at its limit of distribution (5-7 psu) in the western Gulf of Finland. The

species creates patches of 1-10 m in diameter outside the Hanko peninsula, where Z.

marina is the dominating macrophyte since the Fucus-belt has declined during the

latest decades (BOSTRÖM et al. 2002).

In the coastal waters east of Helsinki, the amount of eutrophication favoured

algal species has increased from 1984 to 1997, but the total number of species has

been reduced, probably due to the eutrophication process in the eastern Gulf of

Finland (SAARNIO 1998, LEHVO & BÄCK 2001). The species, which are referred to, are

Polysiphonia violacea, Furcellaria lumbricalis, Chorda filum, Dictyosiphon

chordaria and Cladophora rupestris (SAARNIO 1998).

5.8 Ephemeral algae and algal mats In the summer of 1996 small patches - maximum size 10 m², 2-30 cm thick - of

drifting algae were found at about 30 localities at 1-10 m depth between Helsinki and

eastwards to the Vironlahti Bay in the coastal area in the northern Gulf of Finland.

(LEHVO & BÄCK 2001).

In the Neva Bay, decaying macrophytes have accumulated on the shores after

the construction of a storm-surge barrier in the early 1990s, which changed the

hydrodynamics in the bay. Cladophora glomerata is the dominant filamentous algae

in the Neva Estuary. Excessive amounts (recordings of 2 tons wet weight per 100 m

shoreline) of decaying C. glomerata spoiled on the beaches, cause serious recreational

problems in the Resort District of St. Petersburg. These algal mats enhance the growth

of macrophyte beds on the beaches (PANOV et al. 2002, ref. in GOLUBKOV et al.

2003).

5.9 Zoobenthos The benthic communities at a shallow (3 m) and a deeper (35 m) station off the Hanko

peninsula in the western Gulf of Finland were studied by Segerstråle in 1928 and

revisited in 2000 (LAINE et al. 2003). In 70 years, the snapshots of the benthos show

more similarity between the shallow and deep station in 2000 than in the 1920s. The

shallow station has changed from a Corophium volutator-Macoma balthica-

Chironomidae dominating community to a community only dominated by M.

balthica. The total abundance has declined, and of 14 species recorded in 1928, only

63

Gulf of Finland

three was found in 2000. The total species number in 2000 was 8. At the deeper

station, the Monoporeia affinis community had changed to a dominance of M.

balthica. In 1928, 90 % of the abundance consisted of M. affinis, compared to less

than 5 % in 2000. Of 7 species found in 1928 and 5 in 2000, 4 of them were common

(LAINE et al. 2003).

Except for the central part of the Gulf of Finland, in the 1960s and 1970s the

faunal abundances in the Gulf of Finland were generally low. The benthic

communities were dominated by Pontoporeia femorata and M. balthica. Below 70 m

the bottoms were more or less devoided of fauna (ANDERSIN et al. 1978, LAINE et al.

1997). The conditions for the zoobenthos improved considerably during the period

1987-1994, and P. femorata and M. affinis dominated the communities at 60-80 m. In

1996-1997, the fauna collapsed due to hypoxia (LAINE et al. 1997, LAINE &

ANDERSIN 1998). In the end of the 1990s, 7 species were found in the deep bottoms of

the Gulf (LAINE 2003).

In August 2001, most of the investigated stations at the Finnish-Russian

boarder in the Gulf of Finland lacked macrofauna (PITKÄNEN et al. 2003). M. affinis

and P. femorata occurred in small numbers compared to the densities in the early

1990s, found in LAINE et al. (1997). According to references in PITKÄNEN et al.

(2003), the benthos recolonisated the deeper areas of the central and eastern Gulf of

Finland in 2002. However, in a survey along the Finnish coast in the summer 2004

only 5 of 45 stations have rich and diverse benthic communities (SYKE 2005). The

fauna associated in Zostera marina meadows in the north western Gulf of Finland has

increased three times in abundance compared to the 1970s, which also indicate a

serious degree of nutrient over-enrichment in the Gulf (BOSTRÖM et al. 2002).

In the Neva Estuary, oligochaetes and chrionomides dominate in the inner

estuary, and the crustaceans M. affinis and Saduria entomon in the outer (GOLUBKOV

et al. 2003).

5.10 Ichthyofauna In the eastern part of the Gulf of Finland, 53 species of fishes and cyclostomes are

recorded, 19 of which are marine and only 5 are considered commercial (KUDERSKY

1997). The catches of herring (Clupea harengus membras) in the Gulf of Finland have

decreased in the beginning of the 2000s. After a peak in the sprat (Clupea sprattus)

64

Gulf of Finland

catches in 1999, they have decreased too (HAAHTI & KANGAS 2004). Declining

salinity, which influences abundances of marine zooplankton and cod, affects the

herring stock in the Gulf of Finland (RÖNKKÖNEN et al. 2004).

The stocks of whitefish (Coregonus lavaretus), pike (Esox lucius) and

flounder (Platichthys flesus) have also decreased, especially in the other archipelago

areas (LEHTONEN et al. 2000, LAPPALAINEN et al. 2001, PITKÄNEN 2004). On the

contrary, the roach (Rutilus rutilus) stock has increased in the western Gulf of Finland

during the last 20 years. Roach and perch (Perca fluviatilis) are the dominating fresh

water species in the inner archipelago. The abundance of perch has been stable since

the 1970s, while roach has strongly increased in the outer archipelago (LAPPALAINEN

et al. 2001).

5.11 The present situation and suggestions for measurements The main problems and consequences of the eutrophication process in the Gulf of

Finland are shown in Fig. 5.2. The high external and internal nutrient loads give a

high phytoplankton production and a high rate of sedimentation. The transparency and

oxygen condition in the bottom water are harmed, which leads to consequences for

macrovegation, invertebrates and fish. The availability for harmful algal blooms is

also enhancing. In the spring of 1992, toxic cyanobacteria are thought to have caused

mass extinctions in seabirds in the eastern Gulf of Finland (KAUPPI 1993). The decline

of the zoobenthos has continued since the assessment made in 2001 (RÖNNBERG

2001).

The load of nutrients to the Gulf of Finland is 2-3 times higher than in the

entire Baltic Sea. In the easternmost Gulf, the factor is 5 (PITKÄNEN et al. 1997,

2001). The annual external nutrient load to the Gulf of Finland is 6400 tons of total

phosphorus and 120.000 tons of total nitrogen. About 70 % of the phosphorus and 50

% of the nitrogen are derivered from the easternmost part of the Gulf (KIIRIKKI et al.

2003). However, the nutrient loads to the Gulf of Finland have decreased with 30 %

and the load of organic material with 50 % since the 1980s (PITKÄNEN et al. 1997).

Political and economic changes in the 1990s in Russia and Estonia decreased

the agricultural and industrial production and thereby the amounts of nutrients. The

use of fertilizers and amount of animals in livestock breeding decreased abruptly.

Also in Finland the use of fertilizers has decreased since the early 1990s (PITKÄNEN et

65

Gulf of Finland

al. 1997, KIIRIKKI et al. 2003). In the late 1990s, the main anthropogenic source for

nutrients (36 % of P, 20 % of N) in the Gulf of Finland was derivered from

municipalities (PITKÄNEN et al. 1997). The further reduction in Estonia is mostly due

to the improvement of the waste water treatment plant in Tallinn (PITKÄNEN 2004).

According to LÄÄNE et al. (2002), only a small part of the measures was due to an

active water protection. A historic view of the development of the wastewater

treatment in Helsinki is found in LAAKKONEN (2001) and LAAKKONEN & LEHTONEN

(1999).

Since the late 1980s, the loads to the Gulf of Finland from Finland are stable

(~800 t P, ~30.000 t N in 2000). The Russian and Estonian loads are decreasing (for

Russia: ~5000 t P, ~75.000 t N, for Estonia: ~500 t P, 10.000 t N in 2000). In the

Fig. 5.2. The Gulf of Finland is most serious eutrophied of the sub-regions of the Baltic Sea (see Table 2.1 and Fig. 2.2 for explainations).

66

Gulf of Finland

period 1997-2000, the load from Russia was stable, while loads from Estonia

decreased and the Finnish loads increased slightly. The atmospheric load decreased

slightly since the 1980s, and was ~17.000 tons in 2000 (KIIRIKKI et al. 2003). St.

Petersburg is still the largest source for bioavailable phosphorus and the River Neva

the largest source for bioavailable nitrogen in the Gulf (PITKÄNEN et al. 2001).

Models that examine ecosystem processes for the Helsinki sea area is presented in

KORPINEN et al. (2004), and for the Gulf of Finland in KIIRIKKI et al. (2001, 2003).

The nutrients from the Neva River and St. Petersburg area are mainly

derivered from municipalities and industries. The town of St Petersburg had three

major waste water treatment plants partly in function in 2000. The main industrial

sectors are metal and machine building. Only 11 % of the rural area outside St.

Petersburg is used for farming (KIIRIKKI et al. 2003). The resort area in the shallow

northern part of the Neva Estuary and but also hydraulic constructions have a heavy

anthropogenic impact of the state of the environment (NIKULINA 2003). According to

PANOV et al. (2002), the nuisance growth of Cladophora glomerata is an appropriate

indicator for the effectiveness of the management action in the Neva Estuary.

Together with the eutrophication, oil spills and ecosystem effects of

introduced species are other urgent problems for the Gulf of Finland (TELESH et al.

1999, PANOV et al. 2002). A significant increase in oil pollution after the construction

of new ports can also be a very serious problem, The capacities of the ports in the

Neva Estuary will increase in the near future and are estimated to exceed 100 million

ton yr-1 by 2010 (ref. in PANOV et al. 2002). In 2003, 40 oil spills were detected.

However, the sea traffic is increasing and the oil transportation probably will double

until 2010 (HAAHTI & KANGAS 2004).

5.12 Multiple use of monitoring data; case studies from the Gulf

of Finland The nutrient over-enrichment and its effects are well-studied in the Gulf of Finland.

However, much of the research has been concentrated either to the open sea area (e.g.

PERTTILÄ et al. 1995, CONLEY et al. 1997) or to the easternmost part and the Neva

Estuary (e.g. TELESH et al. 1999, PITKÄNEN et al. 2001, LEHTORANTA et al. 2004).

The eutrophication in small estuaries along the Finnish coastline is described and

analysed using limnological models in MEEUWIG et al. (2000) and KAUPPILA et al.

(2003). The archipelago area, which is an important transition zone between the coast

67

Gulf of Finland

and the open sea, has been studied less thoroughly. The topography, circulation

patterns and local anthropogenic inputs affect the archipelago ecosystem directly on a

local scale (BONSDORFF et al. 1997a). Therefore, the coastal zone is not comparable

with the open sea, but its functions are vital for a complete understanding of the

aquatic ecosystem.

Multivariate techniques have been used to detect long-term trends and overall

assessments of environmental data in e.g. ZITKO (1994) and PARK & PARK (2000).

These methods identify patterns in long-term water quality data in a way where actual

relationships among independent variables appear (ZITKO 1994). By following the

principles of APPELGREN & MATTILA (2002), data from the environmental monitoring

programme from the Finnish coast to the Gulf of Finland is used in paper IV in order

to detect local and regional patterns in the eutrophication process. Long-term (1980-

2002) data from 23 sampling localities, which cover an area from the town of

Hamina/Fredrikshamn (60º15Ń 27º15É) to the archipelago west of the Hanko

peninsula (59º52Ń 22º52É; paper IV), are studied. The aim was to see how the

physico-chemical and biological parameters in the waterbody formed zonation

patterns in space and time.

The results indicate that the eutrophication has been an accelerating process in

the study area during the entire period studied. Only a few exceptions, where point-

source nutrient reductions from land have been effective, were noted. Both in time

and space, the innermost areas of the Finnish coast to the Gulf of Finland are more

negatively affected to eutrophication than the outer archipelago areas. The oxygen

concentration is the driving element in the current analyses. A clear zonation pattern

from the coastal regions towards the open sea is evident, whereas differences between

the eastern and western coastal areas of the Finnish part of the Gulf of Finland were

non-existent.

Proposeful environmental monitoring is an essential tool to determine status,

detect trends and take accurate measures against threats to the aquatic environment.

However, it is important to remember that heterogeneity and variability in the data

material may mask changes in both trends and analyses (BOESCH 2000).

Environmental data has some characteristic properties that make it difficult for

analysing and modelling. Monitoring programmes usually consist of heterogeneous

and large data sets with a range of uncertainty. By using a fuzzy logic approach

68

Gulf of Finland

uncertain knowledge is allowed. Fuzzy rules allow a combination of linguistic

statements and numerical data (CHEN & MYNETT 2003, ADRIAENSSENS et al. 2004).

An attempt to use a fuzzy logic rule-based approach to interpret monitoring

data from the Gulf of Finland is currently under calibration. This test is a co-operation

between marine biologists and computer scientists. By fuzzy-clustering, the data set is

placed within a number of clusters according to degree of membership (BEXDEK

1974). Briefly: the clusters are based on rules of the type: if (input variables), then

(fuzzy conclusion; SUGENO & YASUKAWA 1993). In this case the input criteria are a

number of physico-chemical water quality data (total phosphorus and nitrogen in the

surface water, tot-P in the bottom water, oxygen concentration in the bottom water,

turbidity and chlorophyll a). We have primarily tested bottom water oxygen as the

output variable, but chlorophyll a is another alternative. We have used the input

values from year x-1 and the output value from year x, i.e. the output value is

measured from the year after the input values. The data is finally defuzzyfied

(membership function computed from the rule bases) to deliver an output signal,

which can be compared with the true value, in this example the oxygen concentration

a given year. Based on this model, the defuzzification can be used for any water

quality data. The output is equivalent to a prediction value.

Our experience from testing the data set from paper IV indicates that precise

prediction values are difficult to obtain. But in a managemental opinion the certainty

with exact predictions on a 0.1 decimal level are not of interest. Rather, an appropriate

classification of the output values seem to be of priority. In predicting the oxygen

situation for the forthcoming summer, classes such as 0-2 ml O2 l-1, 2-4 ml l-1, 4-6

ml l-1, and > 6 ml l-1 could be suitable for management. If the model can give

successful predictions in a short-term perspective, it would be a simple tool for

decision-makers to forecast the water quality situation the following summer; i.e. to

provide a service to local inhabitants.

Ecological data are complicated to utilise. It is impossible to consider all

factors that influence e.g. the oxygen condition. Besides the input variables given

above, currents, water circulation, precipitation and discharges from land, as well as

wind and temperature conditions are examples of aspects that may govern oxygen

conditions. Predictions always include a degree of uncertainty, and the decision-

makers sometimes have to rely on the best educated guesses of the experts (ELLIOTT

2002a). But it is better to be aware of the uncertainty than to believe the real world is

69

Gulf of Finland

artificially precise (ENEA & SALEMI 2001). For successful results, we need large data

sets with high variability in the material. If the model can deliver reliable estimates, it

could be an additional tool in MARE´S NEST (MARE 2005).

70

Gulf of Riga

6. THE GULF OF RIGA The Gulf of Riga (57°00´-59°00´N, 22°00´-24°30´E) is semi-enclosed, situated inside

the Estonian Saaremaa island, and connected to the Baltic proper by the Muhu (Suur

Sound) and Irbe Sounds (Fig. 6.1). The Irbe Sound is wide and about 20 m deep,

while the Muhu Sound is narrower. The volume of the Gulf is 424 km³, which

corresponds to 2.1 % - the smallest sub-basin - of the Baltic Sea. The Gulf of Riga has

an open coastline and few islands and banks (BERZINSH 1995). The mean depth is 26

m, and maximum depth is > 60 m (BERZINSH 1995, HELCOM 1996, WASSMANN &

TAMMINEN 1999a). The annual inflow of fresh-water to the Gulf is estimated at 36.2

km3 yr-1 (LAZNIK et al. 1999). The River Daugava flows into the Gulf through the

town of Riga. Other rivers discharging to the Gulf of Riga are Lielupe, Gauja, Pärnu

and Salaca (BERZINSH 1995, LAZNIK et al. 1999, WASSMANN & TAMMINEN 1999a).

The water from the Baltic proper enters cyclonically into the Gulf along the

southern side of the Irbe Sound and flows along the eastern coast towards the north.

During summer the situation may be the opposite (BERZINSH 1995, ANON. 1999b).

The small water volume and the restricted water exchange make the Gulf of Riga

sensitive to pollution (STÅLNACKE & WASSMANN 2000).

Fig. 6.1. The Gulf of Riga sub-region.

71

Gulf of Riga

There is good long-term data from the Gulf of Riga, beginning with Russian

hydrochemical investigations in 1908 (SUURSAAR 1995). In 1993-1997, the Gulf of

Riga was thoroughly studied within the Gulf of Riga Project, which is published in

WASSMANN & TAMMINEN (1999b).

Urbanisation and intense industrial and agricultural activities in the catchment

area started in the 1950s, and have caused an elevated anthropogenic loading

(OJAVEER 1995, YURKOVSKIS 1998). Recent studies have shown that the nutrient

loads to the Gulf of Riga have been relatively stable during the 1990s. The political

changes in the countries surrounding the Gulf of Riga have altered and reduced both

land use and industrial production. The Gulf of Riga has also a large buffering

capacity. The flat landscape and the large wetland areas together with a long retention

time in soil and groundwater prevent the nutrients from directly entering the Baltic

(STÅLNACKE & WASSMANN 2000, STÅLNACKE et al. 2000). In HELCOM-HOT SPOTS

(2004), 10 hot spots are found in the catchment area of the Gulf and two on the

western Estonian coast, north of the Muhu Sound. All the hot spots are either derived

from industries or municipalities (HELCOM-HOT SPOTS 2004).

6.1 Secchi depth The water transparency in the Gulf of Riga has decreased significantly from 1963 to

1990. In 1963, the average Secchi depth in August was over 5 m in the open part of

the Gulf and declined to 4 m in 1990 (BERZINSH 1995). According to BERZINSH

(1995), maximum transparencies are found in the centre of the Gulf, in the deep-water

zone in the northern region and in the Irbe Sound. In the shallow zone of the southern

Gulf and in the inner Pärnu Bay, the Secchi depth values are lowest. In May 2000, the

open Gulf of Riga had an average transparency of 3.7 m. Near the river mouths, the

visibility was only 1 m (UNIVERSITY OF LATVIA 2003).

6.2 Salinity The Gulf of Riga has no halocline and the vertical salinity distribution in the water

column is usually uniform (OJAVEER 1995). A slow outflow of surface water along

the northern part of the Irbe Sound and a deep inflow of Baltic proper surface water

along the southern part of the sound create a slow current system. This system is

72

Gulf of Riga

separated by a salinity front, also called the Irbe Front, which makes the mean salinity

1.5-2.0 psu lower than the surface water in the Baltic proper (BERZINSH 1995).

The amplitude of the annual salinity in the Gulf varies from 5.2 psu in the end

of the 1920s to 6.4 psu in 1977 (BERZINSH 1995). According to KOTTA et al. (2004),

the average salinity in Pärnu Bay has gradually decreased from 6.3 psu 1972 to 5.5

psu in 1989. In 1994-1997 the salinity ranged from 4.8 to 5.7 psu (OJAVEER et al.

1999), and was around 5 psu in 2000 (UNIVERSITY OF LATVIA 2003).

Changes in the salinity influence the Gulf both directly and indirectly. If the

salinity is high enough for presence of e.g. cod (Gadus morhua), the interactions

between species may change the structure of the whole ecosystem (OJAVEER 1995).

6.3 Oxygen Strong currents, regular water exchange with adjacent sea areas, and weak

stratification in the Gulf of Riga, have guaranteed good aeration of the bottom water

layers. The environment in the Gulf has generally not suffered from anoxic

conditions, which has granted possibilities for the adaptation of species requiring high

oxygen concentrations. During the recent decades, under anthropogenic influence,

oxygen content in the bottom layers has shown a clear decreasing tendency in the

autumn, but the formation of H2S has not been observed (BERZINSH 1995, OJAVEER

1995).

In the deepest part of the Gulf, the oxygen decline was significant in 1963-

1984; the mean decrease during summer was about 0.07 ml l-1 yr-1 (HELCOM 1990).

The oxygen concentration in the deepest layer, at 30-50 m, had a value of about 6.7

ml l-1 in 1964 and about 4.3 ml l-1 in 1990 (BERZINSH 1995). Both in August 1999 and

May 2000, the oxygen content in the bottom waters in the northern Gulf of Riga was

above the long-term average (UNIVERSITY OF LATVIA 2003).

6.4 Nutrients The trophic level of the Gulf of Riga has increased in general during the last 20 years,

and the consequences of anthropogenic eutrophication are evident (SUURSAAR 1995).

From 1974 to 1990, the tot-P increased almost 1.4-fold and the trend for

nitrate was even steeper, with a two-fold increase (YURKOVSKIS 1998). The nitrate

content in the 1950s was about 56 µg l-1 and has since then increased to about 210-

73

Gulf of Riga

280 µg l-1 in the 1990s (SUURSAAR 1995). A decline in nitrate concentrations took

place 1990-1996, followed by a new enrichment phase in 1996-2000. Phosphate has

constantly increased from 1975 to 2000 (YURKOVSKIS & MÜLLER-KARULIS 2003).

In July 1994, the average tot-P value was 18.3 µg l-1 and tot-N 320 µg l-1

(TAMMINEN & SEPPÄLÄ 1999), which are considered as low values (Table 2.1). The

highest values were found in the southern part of the Gulf (TAMMINEN & SEPPÄLÄ

1999). In spring 2000, the nutrient concentrations were slight above the long-term

average in the open Gulf of Riga, from 22-84 µg l-1 tot-P, low to very high values, and

434-588 µg l-1 tot-N, medium high to high values (Table 2.1, UNIVERSITY OF LATVIA

2003).

The Gulf of Riga has a high rate of denitrification. Therefore, the stored

nitrogen has decreased together with the decline in nutrient inputs during the 1990s.

Phosphorus is only lost by export to the Baltic proper (YURKOVSKIS & MÜLLER-

KARULIS 2003). The pool of nutrients has a seasonal variation in the Gulf of Riga, but

the water mass is mainly nitrogen-limited (WASSMANN 2004).

6.5 Chlorophyll a and primary production The average chlorophyll a concentration in the surface layer of the Gulf varied

broadly each season, during the period 1972-1991: 4.6-24.6 µg l-1 in spring, 0.9-5.2

µg l-1 in summer, and 1.1-15.2 µg l-1 in autumn (JANSONE 1995). According to ANON.

(1999a), values exceeding 5 µg l-1 in August are considered very high. From 1992 the

chlorophyll concentrations began to decrease (HELCOM 1996, YURKOVSKIS 1998,

KOTTA et al. 2004). The chlorophyll a levels in the surface area of River Daugava has

remained high (TAMMINEN & SEPPÄLÄ 1999). The summer chlorophyll a

concentration started to increase again in 1997, and had a mean value of 5 µg l-1 in

2000 (LATVIAN ENVIRONMENT AGENCY 2004). In August 2002, the central Gulf of

Riga had a chlorophyll a content of 4 µg l-1, which corresponded to the average for

the period 1992-2001 (HELCOM 2003a).

The Gulf of Riga has a high primary production, 350 g C m-2yr-1, due to an

efficient regeneration of nutrients in a relatively shallow area with weak vertical

stratification (ANON. 1999b). The productivity is highest near the mouths of the

Rivers Daugava and Pärnu, and near the coast of Saaremaa Island, where waters of

different salinities meet (OJAVEER 1995, SUURSAAR 1995). The daily plankton

74

Gulf of Riga

productivity had an average of 1353 mg C m-2d-1 during July-August in the southern

and central Gulf of Riga in 1991-1997 (WASMUND et al. 2001).

6.6 Phytoplankton and algal blooms “Blooming of water” was described for the first time in the Gulf of Riga in 1847

(Eichwald 1847, 1852 in KUKK 1995). Cyanobacteria blooms have became frequent

in the Gulf of Riga since the end of the 1980s (KAHRU et al. 1994, TENSON 1995,

YURKOVSKIS 1998). The N2-fixation by cyanobacteria is about 6900 tons per year

(ANDRUSHAITIS et al. 1995) and the phytoplankton biomass has increased from 8.2 %

in 1979-1990 to 20.3 % in 1991-1996 (YURKOVSKIS 1998). However, the renewal is

fast in the free water masses in the Gulf and the algal blooms are therefore more

limited compared with other eutrophicated areas (STÅLNACKE & WASSMANN 2000).

The community of cyanobacteria in the Gulf is dominated by the potentially

toxic Aphanizomenon flos-aquae and to a lesser degree of Nodularia spumigena

(TENSON 1995, YURKOVSKIS 1998). References in TENSON (1995) mention that

already in the 1910s, A. flos-aquae occurred in amounts which resembled of “green

porridge”, but still in the 1960s the species was not frequent in the area. The

dinoflagellate Dinophysis acuminata and the diatom Actinocyclus octonarius are

examples of other dominant phytoplankton species in the Gulf of Riga (HELCOM

1996, YURKOVSKIS et al. 1999).

The long-term changes in species structure of phytoplankton in summer seem

to be mostly determined by nutrient availability. When the concentration of inorganic

nitrogen declined in the early 1990s, the abundance of N2-fixing cyanobacteria rose

(YURKOVSKIS et al. 1999). According to YURKOVSKIS (1998), the cyanobacteria

blooms are either a cause or a consequence of high summer values of phosphorus in

the surface layer. In 2002, the occurrences of cyanobacteria in the Gulf of Riga were

low compared to other regions in the Baltic (HELCOM 2003a).

6.7 Macrovegetation The southern Gulf of Riga has poorer vegetation than the northern area, partly due to

the sandy bottom substrate (KUKK 1995, MARTIN 1999). Except for the Pärnu Bay

and areas in the southern Gulf of Riga, the phytobenthic community is in good

condition (KAUTSKY et al. 1999). Furcellaria lumbricalis is the most frequent red

75

Gulf of Riga

algae in the Gulf, and among brown and green algae, Pilayella littoralis and

Cladophora glomerata are dominating (KUKK 1995, HELCOM 1996, KOTTA & ORAV

2001).

However, several species have disappeared when conditions in the 1980s were

compared to investigations in the 1920s, and even in the early 1970s (KUKK 1995,

HELCOM 1996, ANON. 1999b). For example in the northern part of the Pärnu Bay, the

number of phytobenthos taxa has decreased with 11 species since the 1950s (KUKK

1995, HELCOM 1996, MARTIN 2000). In 2001, no perennial vegetation was found in

the Pärnu Bay. In the southern coast of Saaremaa Island, the Fucus-population has

started to recover again (MARTIN et al. 2003).

In the west Estonian archipelago, north of the Gulf of Riga, Fucus vesiculosus

and Zostera marina have naturally sparse occurrences (MARTIN & TORN 2004).

6.8 Ephemeral algae and drifting algal mats Filamentous green algae have increased intensively in the Gulf of Riga, and the

overgrowing of algae by epiphytes has increased as well (KUKK 1995, HELCOM 1996,

MARTIN 2000). The occurrences are mainly found in areas with high industrial and

municipal pollution load (KUKK 1995). Close to the River Daugava, KAUTSKY et al.

(1999), have only observed annual plants. During the 1990s, Fucus vesiculosus has

been replaced by filamentous brown algae in the northern Gulf (MARTIN 2000).

Drifting algal mats, in patches of 10-20 cm in diamter, are observed at least in

1997 and 2000 in the west Estonian archipelago after mass growth of Pilayella

littoralis (MARTIN et al. 2003).

North of the Gulf of Riga, in the Kassari Bay (the southern Hiiumaa Island),

unique, loose-lying communities of the red algae Furcellaria lumbricalis and

Coccotylus truncatus occur on shallow soft-bottoms since the early 1960s (MARTIN &

KUKK 1999, MARTIN & PAALME 2003). In the latest years, the loose-lying algae have

formed a secondary substrate for filamentous algae, such as P. littoralis (MARTIN &

KUKK 1999, KOTTA & ORAV 2001).

6.9 Zoobenthos The abundance of benthos in the Gulf of Riga had increased four times in 10-15 years,

when the period 1958-1963 and the 1970s were compared. Especially worms and

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Gulf of Riga

crustaceans increased (ANDRUSHAITIS et al. 1995, CEDERWALL et al. 1999). In the

central Gulf, there has been a sharp decline in abundance during the latest decade,

from 8207 ind. m-2 in 1989 to 6 ind. m-2 in in 2000. For the coastal areas, the trend is

opposite: 546 ind. m-2 in 1991 and 3249 ind. m-2 in 2000 (LATVIAN ENVIRONMENT

AGENCY 2004). According to KOTTA et al. (2003), 6 species of zoobenthos were

found in the Pärnu Bay in 2001.

The increase in the biomass is a consequence of the increase of Monoporeia

affinis, Macoma balthica and Marenzelleria viridis. M. affinis was the dominating

zoobenthos species during the 1980s, but has since then decreased in abundance. The

non-indigenous species M. viridis was first found in 1988 and became the most

important species during the 1990s (ANDRUSHAITIS et al. 1995, CEDERWALL et al.

1999). The Gulf of Riga has generally a lower benthic biomass compared to the Baltic

proper, as the bivalve Mytilus edulis is lacking. Close to River Daugava, the low

animal biomass is dominated by omnivores (KAUTSKY et al. 1999).

Changes in the faunal communities seem, at least partly, to follow prevailing

oxygen concentrations. The observed increase in biomass during 1950-1980 is likely

an effect of eutrophication (ANDRUSHAITIS et al. 1995, CEDERWALL et al. 1999).

Together with the nutrient enrichment, the climate may also influence the benthos in

the Gulf of Riga (DIPPNER & IKAUNIECE 2001).

6.10 Ichthyofauna The Gulf of Riga has been one of the most important fishing areas in the Baltic Sea.

The surface area constitutes only about 4 % of the whole Baltic Sea, but in the 1960s

the fish catches were 12 % (50.000-90.000 tons) of the total catches in the Baltic. In

the 1990s the share had decreased to 3 % (20.000-30.000 tons). Altogether more than

70 fish and cyclostome species have been found in the Gulf of Riga. About 35 species

are abundant, 17 are rare (OJAVEER, E. 1995, 2002, OJAVEER & GAUMIGA 1995).

The Baltic spring spawning herring (Clupea harengus membras) is the best

adapted marine fish in the Gulf of Riga. However, due to the eutrophication, the

herring has started to spawn at lower depths, probably because of higher turbidity and

lower oxygen concentrations in the bottom water, which has lead to changes in

structure and decrease in the vegetation that serve as substrates for the eggs (OJAVEER

& GAUMIGA 1995, GAUMIGA et al. 1997, OJAVEER, E. 2002).

77

Gulf of Riga

Further examples where the deterioration of oxygen and other pollution effects

have affected the fish populations are eelpout (Zoarches viviparus) and smelt

(Osmerus eperlanus). The reproduction and nursery areas of the eelpout have been

disturbed since the 1980s, and the fishery was forbidden. Since the 1990s, a limited

annual catch quota is allowed. Toxic pollution of a Soviet air base in the River Pärnu

resulted in mortality in the smelt population. After the closure of the base the

abundance of smelt larvae has increased (OJAVEER & GAUMIGA 1995, GAUMIGA et al.

1997, OJAVEER, E. 2002).

The abundance and catches of the adaptable and relatively pollution tolerant

perch (Perca fluviatilis), have increased (OJAVEER & GAUMIGA 1995, GAUMIGA et al.

1997). However, the stocks of pikeperch (Sander lucioperca) and perch have recently

been overexploited, and an increase in abundance of their prey organisms are seen

(OJAVEER, E. 2002).

Besides eutrophication, the coastal fishery influences the state of the ichthyo-

fauna in the Gulf of Riga (ATIS et al. 2003). According to ATIS et al. (2003), changes

in age structure of commercially important species are more a result of the fishery

than of eutrophication. An increase in cormorants (Phalacrocorax carbo) in the

northern Gulf of Riga is also a potential pressure for the fish stocks (OJAVEER, E.

2002).

6.11 The present situation and suggestions for measurements The Gulf of Riga has been regarded as one of the most eutrophic sub-basins in the

Baltic Sea. After the “Gulf of Riga Project” in 1993-1997, the statement seems to be

false. The environmental status in the Gulf of Riga is not worse than in the Baltic Sea

in general (STÅLNACKE & WASSMANN 2000, WASSMANN 2004). This is indicated by

comparisons of the nutrient concentrations in the Gulf of Riga and other regions in the

Baltic (LAZNIK et al. 1999, TAMMINEN & SEPPÄLÄ 1999). The buffering capacity

limits occurrences of excessive cyanobacterial blooms (WASSMANN 2004).

The present situation in the Gulf of Riga in relation to the eutrophication is

exemplified in Fig. 6.2. Based on my assessment of recent published data, the state of

the Gulf of Riga has improved since the end of the 1990s. Better oxygen conditions as

well as recoveries in macrovegetation and fish stocks are evident.

78

Gulf of Riga

However, the nutrient loads from the rivers, mainly Daugava and Lielupe, still

contribute to an overall eutrophication in the area (WASSMANN 2004). LAZNIK et al.

(1999) estimated the average annual riverine load of nutrients to the Gulf of Riga in

1977-1995 to 113.300 tons of N and 2050 tons of P, which corresponds to 80 % N

and 68 % P of the total load of respective substance. According to LATVIAN

ENVIRONMENT AGENCY (2004), approximately 12 % of the nitrogen and 8 % of the

phosphorus that enter the Gulf of Riga reach the Baltic proper.

Fig. 6.2. The conceptual model of the Gulf of Riga. In the recent years, improvment in the oxygen condition and the state for the fishes have occurred (see Table 2.1 and Fig. 2.2 for explainations).

About 40 % of the drainage area is used for agricultural production, with the

centre in the rural areas around the Lielupe and Pärnu Rivers (LAZNIK et al. 1999).

The agricultural production and the use of fertilizers had a sharp decrease in the

drainage area after the collapse of the Soviet era in 1989 (HELCOM 1996, LÄÄNE et al.

79

Gulf of Riga

2002). According to STÅLNACKE & WASSMANN (2000), the breakdown and

reconstruction of the agriculture in Estonia, Latvia, Lithuania and Russia is the most

remarkable in the modern history of Europe. These countries have achieved a point-

source reduction between 40 and 80 % from 1980 to 1995 (LÄÄNE et al. 2002).

Besides the reduction in fertilizers, declines in the use of manure, pesticides, and milk

and meat production are seen in the agriculture. The development in the industry is

similar. On the other hand, the maritime traffic and the numbers of cars have

increased (BERNET 2000b, JUHNA & KLAVINS 2001). For example in Latvia, the

numbers of cars increased from 66 per 1000 inhabitants in 1980 to 214 in 1999

(JUHNA & KLAVINS 2001).

Improvements and extensions of the biological and chemical treatment in the

wastewater purification plant in Riga were ready in 1991 (JUHNA & KLAVINS 2001).

In BERNET (2000b), only 25 % of the wastewater in the county of Pärnu is treated

according to standard.

Except for Russia, the drainage area of the Gulf of Riga is now under the

legislations of the European Union. The same practises for an environmentally

sustainable development should be followed as in the rest of the EU. However, there

are also worries that the agriculture and forestry will start to increase again, which

will lead to higher loads of nitrogen to the Gulf of Riga (BERNET 2000a, STÅLNACKE

& WASSMANN 2000). A drastic numerical experiment, run for 15 years with a 50 %

reduction in terrestrial nutrients loads to the Gulf of Riga, showed that the primary

production would decrease by only 20 % in 12 years. Reduction of only phosphorus

resulted in a 6 % decrease in primary production. On the other hand, a reduction in P

would lead to a decline in cyanobacteria blooms and a export of phosphorus to the

Baltic proper (SAVCHUK 2002).

80

Gulf of Gdansk

7. THE GULF OF GDANSK REGION, including the Curonian and

Vistula Lagoons The Gulf of Gdansk is an open bay in the southern Baltic Sea. The lagoons of Vistula

and Curonia are connected to the Gulf of Gdansk by narrow and shallow straits

(54°20´-55°40´N, 18°30´-21°15´E) (Fig. 7.1).

In the western part of the Gulf of Gdansk, the Puck Bay (surface area: 104

km², mean depth: 3 m, max. depth: 9 m) is situated (CISZEWSKI et al. 1991, 1992a).

The shallow bay is sheltered from the open Baltic by a sand spit, the Hel peninsula.

Within the Puck Bay a sandbar and its shallow, underwater extension create the Puck

Lagoon. The Lagoon has a permanent connection with the rest of the Bay. The well-

mixed Vistula Lagoon (surface area: 838 km², mean depth: 2.6 m) is found in the

eastern part of the Gulf of Gdansk, on the Polish-Russian boarder (ANDRULEWICZ

1996, KWIATKOWSKI et al. 1997).

The Curonian Lagoon (surface area: 1610 km², mean depth is 3.8 m), north of

the Vistula Lagoon, is situated on the boarder between Lithuania and Russia. The

Lagoon is limnic except for the northernmost part, which is affected by water from the

Baltic during stormy inflows through the Klaipeda Strait (OLENINA & KAVOLYTE

1997, SHTUKOVA & JASHINSKAITE 1997). The Klaipeda Strait is also the outlet of the

River Nemunas/Njemen, which accounts for 98 % of the river water discharge to the

Curonian Lagoon (OLENIN et al. 1999).

Fig. 7.1. The Gulf of Gdansk sub-region with the most impor-tant geographic names.

81

Gulf of Gdansk

The Gulf of Gdansk drainage basin covers an area of 323.200 km² and is the

catchment areas of several rivers, mainly Vistula/Wisła and Pregel (ANDRULEWICZ

1996). The River Vistula has the second largest drainage basin among the Baltic

rivers. The majority of the catchment area lies in Poland, the rest in Belarus, the

Slovak Republic and Ukraine (KWIATKOWSKI et al. 1997). The River Vistula

transports approximately 75 % of the pollutants that enter the Baltic Sea from Poland

(ANDRULEWICZ 1996). The biggest cities round the Gulf of Gdansk are Gdansk

(462.000 inhabitants) and Gdynia (255.000 inh.) in Poland and the Russian

Kaliningrad (435.000 inh.). The drainage area to the Gulf of Gdansk is heavily

industrialised, which the number of HELCOM hot spots gives evidence of. A total of 43

hot spots of industrial and municipal wastes and agricultural runoff are listed from the

Nemunas and Vistula river basins and the coasts of Lithuania, Kaliningrad and Poland

(HELCOM-HOT SPOTS 2004). Together with waste from municipalities and industries,

mineral fertilizers used in agriculture are the main problem in the Kaliningrad area.

From the Polish coast there is no agricultural land draining into the Gulf of Gdansk,

even if 60 % of the area is used for agriculture (RUDLICKA et al. 2003). The Gulf of

Gdansk region is thus regarded as a highly eutrophicated area, with the large-scale

growth of filamentous brown algae and high concentrations of chlorophyll a as main

indicators (KRUK-DOWGIALLO 1996, OLENINA & KAVOLYTE 1997).

7.1 Secchi depth The water transparency in the Gulf of Gdansk and the Curonian Lagoon in the late

1990s can be classified as low to very low (Table 2.1), with variations between 0.6

and 2.5 m (OCHOCKI et al. 1995, OLENIN et al. 1999). The water is most turbid in the

Curonian Lagoon during summer algal blooms (OLENIN et al. 1999). The Puck

Lagoon has the most transparent water in the region, even if it is influenced by

discharges from the River Vistula (MATCIAK 1997). In the period 1971-1987, the

Puck Lagoon had an average Secchi depth of 5.5 m, which means sight down to the

bottom (CISZEWSKI et al. 1991, RENK et al. 1991).

7.2 Salinity Trends for long-term salinity in the Gulf of Gdansk and the Gdansk Deep show an

increase in salinity from the beginning of the 20th century to the end of the 1970s.

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Gulf of Gdansk

After the 1970s, periods with decline in salinity and density are due to long-lasting

stagnations. Since the 1990s, the salinity has been unstable due to, on one hand, the

oceanic inflows in 1993 and 2003 and, on the other, freshwater inflows and increased

precipitation (CYBERSKA & KRZYMINSKI 1996, HELCOM 2003b, RUDLICKA et al.

2003). The salinity in the surface layer in the Gdansk Basin varies between 4 and 7.5

psu, with small or no changes since the 1980s (ANDRULEWICZ 1996, CYBERSKA &

KRZYMINSKI 1996).

In the Curonian Lagoon the salinity is more unstable, depending on wind

speed and direction, intensity of the outflow from the Lagoon and water level

fluctuations. The value can change rapidly, from > 6.5 to < 0.5 psu (reference in

OLENIN et al. 1999), and the average annual salinity is 3.6 psu (OLENINA &

KAVOLYTE 1997, OLENIN et al. 1999).

7.3 Oxygen The Gulf of Gdansk suffers from oxygen deficiency especially in late summer-early

autumn. During the stagnation period in 1977-1992 the halocline weakened and

atmospheric oxygen penetrated into the deep water layers, which improved the

oxygen situation (RUDLICKA et al. 2003). In 1989-1993 the oxygen content ranged

from 10 % in shallow waters of the Gulf and increased further out to the Gdansk

Deep. The oxygen concentration in the intermediate water layer decreased from 80-90

% in the end of the 1970s to 40-50 % in the early 1990s (TRZOSINSKA & ŁYSIAK-

PASTUSZAK 1996). The Gdansk Deep was free from hydrogen sulphide in 1989-1994.

During the 1990s, the oxygen conditions in the surface water of the Gulf of Gdansk

were considered good (CISZEWSKI et al. 1991, RUDLICKA et al. 2003). In the bottom

water layers in late summer the oxygen concentration has been alarmingly poor since

the 1970s. From the 1990s onwards the oxygen trend has been significantly negative

(ŁYSIAK-PASTUSZAK et al. 2004). According to OLENIN et al. (1999) the Klaipeda

Strait is generally normoxic, but oxygen deficiency may occur during calm and warm

days in the harbour inlet.

During the period 1998-2001, strong oxygen depletion was recorded annually

in all regions of the Polish coastal zone. The minimum oxygen concentration had a

mean value around 2 ml l-1 in the Vistula Lagoon and 1.5 ml l-1 in the Gulf of Gdansk

(RUDLICKA et al. 2003). Extremely warm water with a little content of oxygen in the

83

Gulf of Gdansk

summer of 2002 removed all hydrogen sulphide from the Gdansk Basin within some

months. Consequences of the saline inflows in the beginning of year 2003 were

detectable in the Gdansk Deep during the spring of 2003. However, severe oxygen

deficiency occurred in the bottom waters in the inner Gulf of Gdansk in August 2003

(HELCOM 2003b and references therein).

7.4 Nutrients In late 1980s, the concentrations of nutrients in the Puck Lagoon were rather low

compared to data from the rest of the Gulf of Gdansk or the open Baltic Sea.

CISZEWSKI et al. (1991) gives the values of 189 µg l-1 for tot-N and 30 µg l-1 for tot-P.

According to Table 2.1, the tot-N can be considered as normal, but the concentration

for tot-P is high. Outside the Curonian Lagoon, the concentrations of tot-N varied

from 140 to 2950 µg l-1 in late summer during the early 1990s (OLENINA &

KAVOLYTE 1997). Overall, the total phosphorus in the Gulf of Gdansk reached a peak

in 1991 with 50 µg l-1, but has thereafter decreased drastically and is relatively stable.

The summer concentration in 2000 was 16 µg l-1 on the contrary, the total nitrogen

has increased since the beginning of the 1990s and was 280 µg l-1 in 2000 (RUDLICKA

et al. 2003). A decreasing trend in winter nutrient concentrations is also detectable

(ŁYSIAK-PASTUSZAK et al. 2004).

According to budget calculations for the Gulf of Gdansk in WITEK et al.

(2003), the water residence time in the area is only 15 days, which makes the system

insensitive to long-term trends and variations in river loads. The removal of tot-N and

–P was thus estimated to 20 and 35 %, respectively (WITEK et al. 2003).

7.5 Chlorophyll a and primary production The Gulf of Gdansk is a productive area with an annual primary production of 200

g C m-2 or higher (LORENZ et al. 1991, OCHOCKI et al. 1995, KWIATKOWSKI et al.

1997, WITEK et al. 1999). The chlorophyll a concentrations have decreased

considerably since the end of the 1980s (LATALA 1997). The highly polluted Vistula

Estuary has got the highest chlorophyll a concentration in the Gulf of Gdansk with

values between 60 and 120 µg l-1 (KWIATKOWSKI et al. 1997). The semi-enclosed

inner Puck Bay is isolated from the rest of the Gulf, and has thus the lowest

chlorophyll a levels (LATALA 1997). The chlorophyll a concentration in the Curonian

84

Gulf of Gdansk

Lagoon varies between 10 and 100 µg l-1 (OLENINA & KAVOLYTE 1997).

Concentrations over 5 µg l-1 are considered as very high (Table 2.1). The mean annual

chorophyll a concentration in the high sea of the southern Baltic was only 1.8 µg l-1 in

1999, and 2 µg l-1 in 2000. In 2001 a further increase was recorded (RUDLICKA et al.

2003).

7.6 Phytoplankton and algal blooms The growing season of spring phytoplankton species in the Gulf of Gdansk begins in

March (HELCOM 1996, TRZOSINSKA & ŁYSIAK-PASTUSZAK 1996). Dominating

cyanobacteria species are Aphanothece clathrata, Nodularia spumigena and

Aphanizomenon flos-aquae. In the northern part of the Gulf of Gdansk region, the

species composition is very similar to that found in the Gdansk Deep (CISZEWSKI et

al. 1992b, WRZOLEK 1996). The summer 2003 had strong occurrences of

cyanobacterial blooms in vast areas of the Baltic proper, and especially the Gulf of

Gdansk (HELCOM 2003b).

In the Curonian Lagoon, including the Klaipeda harbour area, algal blooms is

a regular phenomenon. The peak in phytoplankton biomass was recorded in October

1999 with 252 mg l-1 in the open waters of the lagoon, and nearly 2000 mg l-1 as

extreme value (OLENINA & OLENIN 2002). Usually the blooms begin in May and

consist of diatoms. In June-July the cyanobacteria start to bloom, the most abundant

species is A. flos-aquae and the bloom may last to the end of October (OLENIN et al.

1999). Blooms of Nodularia spumigena were exceptional in the summers of 2001 and

2002. During the twenty latest summers, the biomass of A. flos-aquae reached

hyperbloom condition (> 100 mg l-1) seven times in the Curonian Lagoon. These

events co-incided with intense use of fertlisers and the sharp drop in the use of

fertlisers in post-Soviet time (OLENINA & OLENIN 2002).

6.7 Macrovegetation The Gulf of Gdansk, and especially the Puck Lagoon with its geomorphological and

hydrological conditions, is a unique biotope for macrophytes (CISZEWSKI et al.

1992a). The number of plant taxa in the Puck Lagoon in the 1950s was 80, with the

brown algae Fucus vesiculosus and Furcellaria lumbricalis as dominants (CISZEWSKI

et al. 1991, 1992b, KRUK-DOWGIALLO & PEMPKOWIAK 1997). According to KRUK-

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Gulf of Gdansk

DOWGIALLO (1996), between 1957 and 1971, 60 % of the biomass in the bottom

vegetation consisted of these two species and the eelgrass Zostera marina. However,

increasing discharges from land resulted in a continuous deterioration of the bottom

vegetation, and in the end of the 1970s the former dominating species started to be

replaced by filamentous brown algae. In the period 1977-1992, the number of taxa

decreased to 28, and species of Ectocarpus and Pilayella covered 70 % of the bottom

(CISZEWSKI et al. 1991, 1992b, KRUK-DOWGIALLO 1996, KRUK-DOWGIALLO &

PEMPKOWIAK 1997, and references therein).

In 1996-2000, 36 species of macrovegetation were noted in the Gulf of

Gdansk, 16 species of which were Chlorophytes, 5 Phaeophytes, 7 Rhodophytes and 8

vascular plants. The dominant species was Ectocarpus siliquosus followed by

Cladophora species and P. littoralis. The occurrence of F. vesiculosus and the red

algae Polysiphonia violacea increased during the time period (PLIŃSKI & JÓŹWIAK

2004). In the Puck Bay, 19 plant species was present in 2001. Rhodophyceae and

Chlorophyceae formed the largest groups, four species of Fucophyceae were found

(RUDLICKA et al. 2003).

The meadows of vascular plants have also changed significantly, the very best

example being Zostera marina. In 1950s Z. marina formed meadows in the Puck Bay

down to 10 m. In the period 1969-1971 the meadows were still dense, but in the 1980s

the coverage was largely reduced (CISZEWSKI et al. 1992b). Today Z. marina still

prevails in a small area in the northern Puck Bay (KRUK-DOWGIALLO 1996 and

references therein). It is not only the species composition that has changed, also the

total phytobenthos biomass has decreased, and in 1979-1987 the reduction was 15 %

(KRUK-DOWGIALLO 1996 and references therein).

A project, lead by professor Lena Kautsky, Stockholm University, tried to

reintroduce the F. vesiculosus vegetation in Puck Bay faster by transplantation. Poor

transparency, dense growth of filamentous algae and turbulence in the water were

factors that made the project not succeeded. Only 10 % of 750 transplanted plants in

1999-2000 remained one year later (BERGER 2001).

Occurrences of drifting algal mats have not been reported from the Gulf of

Gdansk region so far.

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Gulf of Gdansk

7.8 Zoobenthos According to a recent report, the muddy bottoms of the Gulf of Gdansk have a

diversity of 3-14 species of macrozoobenthos (RUDLICKA et al. 2003). Irregular

variations in species number are mainly due to the state of oxygen and presence of

hydrogen sulphide (JANAS et al. 2004).

In the mid 1990s, the sediments were inhabited by 10-18 species of fauna in

the coastal zone down to 30 m despite the presence of H2S. Bivalves were the general

dominants (OSOWIECKI 1991, OSOWIECKI & WASZOCHA 1996, JANAS et al. 2004). In

the end of the 1990s the abundance and biomass decreased both at shallow and deep

stations. Bivalves, such as Macoma balthica and oligochaetes, the polychaete Nereis

diversicolor, snails of Hydrobia spp and the crustacean Diastylis rathkei are found

annually since the beginning of the 1980s, even if the abundance of M. balthica is

heavily reduced. Saduria entomon and Corophium volutator appear very infrequently

and only in small numbers (RUDLICKI et al. 2003, JANAS et al. 2004).

In the northern and central hydrodynamically active parts of the Curonian

Lagoon, the fauna is both abundant and diverse according to OLENIN (1996). The area

is structured of Dreissena polymorpha and Valvata spp communities. In the southern

stagnated area of the Lagoon an impoverished macrofauna community was found

between 1980 and 1992, represented by deposit feeders, primarily chironomids and

oligochaetes (OLENIN 1996).

7.9 Ichthyofauna In the mid-1960s, eel (Anguilla anguilla) represented 45 % of the fish caught in the

Puck Bay. Pike (Esox lucius) was the second most caught fish during that time.

During the latest years of the 1960s, the catches of those species decreased, and today

they correspond only to some percent of the total sum (CISZEWSKI et al. 1992b).

Roach (Rutilus rutilus) was the substitute, and had an enormous increase in the end of

the 1970s when it contributed to over 90 % of the catches (CISZEWSKI et al. 1992b).

The number of three-spined stickleback (Gasterosteus aculeatus) also reached mass

occurrences (CISZEWSKI et al. 1991).

The changes in the fish communities can be seen as a consequence of the

reduction in the underwater meadows in the Puck Bay. In the 1980s, mass mortalities

of eel and flatfish took place (CISZEWSKI et al. 1991, 1992b). However, during the last

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Gulf of Gdansk

years, small improvements in the ichthyofauna have been recorded in the Gulf of

Gdansk, most probably due to the construction of new wastewater treatment plants.

Species as mackerel (Scomber scombrus), sword fish (Xiphias gladius) and sole

(Solea solea) that have been disappeared are found more frequently. Even sturgeon

(Acipenser sturio) has been observed in Latvian waters after a break of over 50 years

(RUDLICKA et al. 2003).

The sandy bottoms in the Puck Lagoon and the open coastline of the Gulf of

Gdansk are ideal habitats for gobiid species. A new gobiid for the Baltic Sea, the

round goby Neogobius melanostomus, was first recorded near the Hel peninsula in

1990. The species has its origin in the Caspian Sea, and has spread to the entire Gulf

of Gdansk (HORACKIEWICZ & SKORA 1998). The goby feeds on herring spawn, which

may become a threat to the herring stock in the area (RUDLICKA et al. 2003). In the

summer of 2003, the round goby was the dominant species together with perch (Perca

fluviatilis) in the Puck Bay. Outside Hel, flounder (Platichthys flesus) dominated the

biomass (ALMQVIST et al. 2003).

7.10 The present situation and suggestions for measurements The state of the eutrophication situation for the Gulf of Gdansk is presented in the

model in Fig. 7.2. The Vistula River, Vistula Lagoon, wastewater treatment plants,

industries in Gdansk and Gdynia, and atmospheric deposition are the main sources of

nutrient pollution in the Gulf of Gdansk (ANDRULEWICZ & WITEK 2002). In 2001, the

annual load of total nitrogen from the Odra, Vistula and other coastal river basins to

the Baltic Sea was 183.171 tons, and 12.281 tons for phosphorus. A decline of 12 %

and 21 %, respectively, has taken place since 1997. Hence, a slight improvement of

the water quality in the Polish rivers is seen, even if the low river runoff also is a

reason for the decrease. The discharges of heavy metals, chlorides and sulphates have

also decreased in comparison with the loads in 1990 (RUDLICKA et al. 2003).

According to ANDRULEWICZ et al. (2004a), the main actions for a further

improvement of the water quality are found in an overall reduction of nutrients loads

and effective sewage treatment in the Gulf of Gdansk drainage area, as well as

restoration of the underwater meadows and reintroduction of lost macrovegetation in

the Puck Lagoon.

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Gulf of Gdansk

The reappearance of many fish species is an indication for an improvement of

the environment. Also the sanitary conditions on Polish beaches are better. In 2000-

2001 all beaches at the coast of central and western Poland were open. Despite the

positive signs in the water quality in the Gulf of Gdansk and overall the Polish coastal

waters, the concentrations of nutrients are still high. The most serious is the situation

in the lagoons and bays. Poland is one of the countries with the highest output of

mineral resources and fossil fuel. The hard coal mining industry is the single most

important polluter of the surface water (RUDLICKA et al. 2003). In the mid-1990s, still

860 towns in Poland lacked a sewage treatment plant and only 100 towns had

mechanical treatment. The situation for the industries was similar. The poor quality of

Fig. 7.2. The conceptual model for the Gulf of Gdansk. Hypoxia and damages to macrovegetation are the main problems for the area (see Table 2.1 and Fig. 2.2 for explainations).

89

Gulf of Gdansk

the water in Poland is mainly a result of the untreated water from the industry

(POLAND´S MINISTRY OF ENVIRONMENT, 1997). However, today 80 % of the beaches

of the Polish coast to the Gulf of Gdansk are open for bathing, compared to none in

the 1980s (ANDRULEWICZ & WITEK 2002).

In the beginning of the 1990s, as a consequence of the end of the communistic

era, the Polish agriculture collapsed, which has lead to clear reductions in the input of

nutrients to the Baltic Sea (BERNET 2000a). The use of energy, management of water

and raw material, and reductions in emissions has improved since the 1990s

(RUDLICKA et al. 2003). When the agricultural and forestry production will increase, it

is important to apply environmentally sustainable practises of measures in

legalisations, technology and economy (BERNET 2000a, RUDLICKA et al. 2003). Since

Poland is now a member of the European Union, the Polish legislation system has to

be standardised with the EU. Achievements for reducing the outlets to the Baltic Sea

are of special priority (BERNET 2000b). The engine and vehicle fuel consumption have

on the other hand increased (RUDLICKA et al. 2003).

The Kaliningrad area is densely populated and highly urbanized (BERNET

2000b). Since the beginning of the 1990s there has been a decline in the agricultural

production and the use of fertlisers, the wastewater load decreased with 60 %, and

total phosphorus with 70 % from 1994 to 1999 (BERNET 2000b, OLENINA & OLENIN

2002). However, the total phytoplankton abundance within the last two decades in the

central part of the Curonian Lagoon has increased (OLENINA & OLENIN 2002).

Introduction of biological treatment in the municipal treatment systems, clean

technology in pulp and paper industries and improved methods for wastewater,

manure and cattle burial grounds in the agriculture are examples of measures to be

taken (BERNET 2000b).

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Swedish East-Coast

8. THE SWEDISH EAST-COAST; from the Himmerfjärden Bay

to the Hanö Bay The coastal region along the Swedish coast to the central Baltic Sea is here separated

to a region of its own. The assessment of the Swedish east-coast is mainly based on

data from the Himmerfjärden Bay, south of Stockholm, and the Hanö Bay inside the

Bornholm Basin. The St. Anna´s Archipelago and areas of the Kalmar Sound also

represent this region (Fig. 8.1).

The Himmerfjärden (area: 174 km², mean depth: 17 m) is well studied due to

its closeness to the nearby field station Askö laboratory, belonging to Stockholm

Marine Research Centre, and a sewage treatment plant, in work since 1976 (ELMGREN

& LARSSON 1997). The Hanö Bay has an archipelagic northern part and an open coast

in the south. The area is important as reproduction and nursery grounds for many fish

species such as cod, herring and flounder (ANDERSSON 1997).

The only HELCOM-hot spot found in this region belongs to the Swedish

agricultural programme with runoff to the Hanö Bay (HELCOM-HOT SPOTS 2004).

Fig. 8.1. The sub-region for the Baltic coast of Sweden.

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Swedish East-Coast

8.1 Secchi depth The Secchi depth measurements in the outermost Himmerfjärden Bay increased from

the 1970s to the end of the 1980s, with 6 m as minimum value. In 1996-1997 the

transparency had the same value as 1976-1977; 8 m (LARSSON et al. 1998, 1999). In

August 2004, Secchi depths of 3.7 and 5.5 m were recorded at two stations outside the

Himmerfjärden (STOCKHOLM UNIVERSITY 2004b). In the Kvädöfjärden Bay, south of

St. Anna’s archipelago, the transparency was 3.5 m in summer 2000 (ÅDJERS et al.

2001). In the Kalmar Sound, the transparency increased from 4.8 m in 1995 to 8 m in

1999 (ANDERSSON, J. et al. 2000). The Secchi depths in the Hanö Bay have been

stable during the latest years, from 4 to 8 m (TOBIASSON et al. 2002, 2003). The

transparency in the region may therefore be classified as medium good to very good

(Table 2.1).

8.2 Salinity In the northern parts of the region, the salinity has fluctuated during the period 1975

to 2004 (SJÖBERG & LARSSON 1996, STOCKHOLM UNIVERSITY 2004a). In 1979, the

salinity was about 7.1 psu (SJÖBERG & LARSSON 1996), in August 2004 5.8 psu

(STOCKHOLM UNIVERSITY 2004a). The Kalmar Sound has a relatively stable salinity

of 6.5 psu (ANDERSSON, J. et al. 2000, MALM et al. 2004).

The salinity has an economically importance for the Hanö Bay area, since cod

is the most valuable fish species in the area. Cod requires a salinity of at least 11-12

psu for a successful reproduction (ANDERSSON 1997). In the coastal parts of the Hanö

Bay, the salinity is about 7 psu (TOBIASSON et al. 2002, 2003).

8.3 Oxygen In the end of the 1970s, hypoxia was a regular phenomenon in the Himmerfjärden

Bay. In the end of the 1980s, windy summers improved the turbulence of the water

and oxygen deficiency has been avoided since (ELMGREN & LARSSON 1997). In

August 2004, the oxygen conditions in the outer Himmerfjärden ranged between 2.0-

4.5 ml l-1 (STOCKHOLM UNIVERSITY 2004b). In the St. Anna’s Archipelago, the

number of hypoxia and anoxia suffering bottoms has increased since the 1950s to the

mid 1980s, which indicates a long-term eutrophication process. Better wind

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Swedish East-Coast

turbulence during the 1990s seems to have improved the bottom situation in the area

(OLSSON 1999). In the Kalmar Sound, no oxygen concentrations below 8 ml l-1 were

found in 2001 (MALM et al. 2004).

The summer 2001 shows a more stable oxygen situation for the Hanö Bay

compared to the period 1991-2000. Minimum value in 2001 was 3.2 ml l-1, but most

stations had values > 6 ml l-1 (TOBIASSON et al. 2002). However, the oxygen situation

in the Hanö Bay is more related to stagnation of bottom water than a consequence of

eutrophication (ANDERSSON, L. et al. 2000). After the saline influx in January 2003,

the mean oxygen concentration in August 2003 was 7.9 ml l-1, or extremely good

(TOBIASSON et al. 2003).

8.4 Nutrients Long-term nutrient trends for the eastern coast of Sweden show that the increasing

nutrient curve was broken in the end of the 1980s (LARSSON et al. 1998). In the

Himmerfjärden area, the tot-N concentration has increased significantly from 250 to

300 µg l-1 during the period 1977-1990. After that, the concentrations have slightly

decreased, being relatively stable around 280 µg l-1 since the beginning of the 1990s

(LARSSON 1994, ELMGREN & LARSSON 1997, HAJDU et al. 2000, STOCKHOLM

UNIVERSITY 2004b), which is classified as low in Table 2.1. The tot-P has gradually

increased from 20 to 25 µg l-1 in 1977-1992 (ELMGREN & LARSSON 1997). The peak

was reached in 1989, 31 µg l-1 and very high (Table 2.1). Since then the value has

declined again, and in June 2004 the amount was 20 µg l-1 (STOCKHOLM UNIVERSITY

2004b).

In the St. Anna’s Archipelago, the nutrient concentrations have increased from

the 1970s to the mid 1980s, and then declined slowly again (OLSSON 1999).

Considering the criteria in Table 2.1, the nutrient values in 1993-1995 were medium

to very high. In 1996-1998 the situation had changed to the classification low (Table

2.1). In shallow bays in the Kalmar Sound area in 1999-2000, the total phosphorus

varied between 29 and 45 µg l-1 and total nitrogen between 200 and 360 µg l-1

(DAHLGREN & KAUTSKY 2004). This is low-medium high for nitrogen and high-very

high for phosphorus (Table 2.1).

In the Hanö Bay, a decreasing trend for phosphorus is seen from 1996 to 2001.

For nitrogen a similar trend started in 1990 (TOBIASSON et al. 2002). In August 2003

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Swedish East-Coast

the tot-P was 23 µg l-1 and tot-N 330 µg l-1 (TOBIASSON et al. 2003), both medium

high (Table 2.1).

8.5 Chlorophyll a and primary production There is no clear long-term tendency for increasing chlorophyll a concentration at the

eastern coast of Sweden during the period 1977-1997 (LARSSON et al. 1998). In the

Himmerfjärden area the chlorophyll a content shows a linear relationship with the

Secchi depth and the concentration of tot-N (ELMGREN & LARSSON 1997). The

chlorophyll had a low value in August 2004, 2.5 µg l-1 (STOCKHOLM UNIVERSITY

2004b).

The chlorophyll a increased to > 4 µg l-1 in the Kalmar Sound in the beginning

of the 1990s, but decreased to a concentration of 3.5 µg l-1 in 1999 (ANDERSSON, J. et

al. 2000). In 2000, the value varied from 2 to 7 µg l-1 in shallow bays in the area

(DAHLGREN & KAUTSKY 2004). In the Hanö Bay the chlorophyll trend is decreasing

since 1993 (SOUTH COAST´S WATER CONSERVATION ASSOCIATION, 2002). The

concentrations in late summer are generally low, 0.8 µg l-1 in 2003 (TOBIASSON et al.

2003).

8.6 Phytoplankton and algal blooms According to ELMGREN & LARSSON (1997), the Prymnesiophycae Chrysochromulina

spp has occurred regularly in the Himmerfjärden area. During the 1990s, the

dinoflagellate Heterocapsa triquetra has been abundant in the coastal waters of

Sweden and caused dense and lengthy blooms. The reason for this seems to be found

in hydrographical and meteorological situations due to annual variations, and not as a

consequence of eutrophication (LARSSON et al. 1999). In the summer of 2002,

cyanobacteria occurred at the Swedish east coast, but no mass occurrences were

reported (SOUTH COAST’S WATER CONSERVATION ASSOCIATION, 2002).

8.7 Macrovegetation Outside the Himmerfjärden, the plant biomass decreased radically between 1974 and

1990. The main reason was the disappearance of Fucus vesiculosus. In 1993, the

biomass increased again, but the new dominators were ephemeral algae such as

Furcellaria lumbricalis and Pilayella littoralis (WULFF & HALLIN 1994). The mean

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Swedish East-Coast

growth depth of F. vesiculosus decreased with 1 m from the 1970s to the 1990s.

Between the years 1995 and 1996, a decline of 0.5 m was recorded. This change is

eutrophication related, due to decreased Secchi depths (KAUTSKY 1999).

The Fucus-belts increased in the Kalmar Sound in the 1980s, but declined in

the end of the 1990s due to intense grazing of the isopod Idothea balthica. A recovery

phase was observed in 1999-2000, but the growth depth has decreased (JOHANSSON

1999, ENGKVIST et al. 2002, NILSSON et al. 2004a).

In the Hanö Bay region, 90 % of the F. vesiculosus, Fucus serratus and F.

lumbricalis cover are lost on exposed localities since 1990. The amount of epiphytes

has increased and the filamentous P. littoralis is now dominating. The changes seem

not to be in relation to the location of point-sources, since no changes or recovery of

perennial macrophytes are seen at sheltered places (ENGKVIST et al. 2002, TOBIASSON

et al. 2002, 2003, NILSSON et al. 2004a). The biomass and the number of shoots for

Zostera marina have slightly decreased, according to SOUTH COAST’S WATER

CONSERVATION ASSOCIATION (2002).

8.8 Ephemeral algae and drifting algal mats An increase of perennial and ephemeral algae is seen outside the Himmerfjärden area

compared with the 1970s, most probably due to eutrophication (KAUTSKY 1999).

According to KAUTSKY (1999), drifting algal mats at thicknesses of some dm´s have

been found in the area with occurrences of hypoxia below them.

During the 1990s, large quantities of drifting red algae, mainly Polysiphonia

fucoides, have been an increasing problem along the coasts of the Öland Island

(MALM & ENGKVIST 2001, MALM et al. 2004). Filamentous algae such as Cladophora

glomerata, has a dominating role in small bays in the Kalmar Sound area (DAHLGREN

& KAUTSKY 2004). High occurrences of attached and loose lying filamentous algae

are also observed in the Hanö Bay (SOUTH COAST’S WATER CONSERVATION

ASSOCIATION 2002).

8.9 Zoobenthos In the inner part of the Himmerfjärden, the biomass of macrobenthos increased

rapidly from 1972 until the end of the decade, when all animals died due to hypoxia.

The recolonisation crashed again in 1985, and a new recovery process started. In the

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Swedish East-Coast

middle and outer areas of the bay, the biomass has increased since the beginning of

the 1970s, and the fauna has only temporarily been reduced during shorter periods of

hypoxia. Macoma balthica and Harmothoë sarsi are dominating, while the number of

oxygen-sensitive species such as Monoporeia affinis and Pontoporeia femorata has

diminished. Outside the Himmerfjärden Bay, the increased organic enrichment has

changed the composition of the benthic fauna. The total biomass increased from 1971

to 1991, due to the increased abundance of M. balthica, while the total abundance

decreased during the same period (CEDERWALL 1996, 2003, ELMGREN & LARSSON

1997, SAVAGE et al. 2002). The lower depth limit for macroscopic life changed from

125 m in 1996 to 71 m in 2001 (CEDERWALL 2003).

The zoobenthos in the St. Anna’s archipelago has been affected by expansion

of laminated sediments since the 1960s. As a consequence of the improved oxygen

conditions in the 1990s, a recolonisation of macrofauna is seen (PERSSON & JONSSON

2000, JONSSON 2003). In the Kalmar Sound, the abundance and extension of

chironomidae and the biomass of M. balthica have increased since the end of the

1980s, related to the increased eutrophication in the area (ANON. 1996, 1998). M.

balthica and Nereis diversicolor were most abundant in 2001 (MALM et al. 2004).

In the Hanö Bay, only slight changes in the zoobenthic communities are

observed during the last 5 years. M. balthica is dominating and the abundances of

oligochaets have increased. The number of Pygospio elegans and M. affinis has

decreased (TOBIASSON et al. 2002, 2003).

8.10 Ichthyofauna The lack of cyanobacteria in the Himmerfjärden has lead to larger amounts of fish

inside the bay than in areas nearby. This is the situation especially for the young of the

year herring (Clupea harengus membras; ELMGREN & LARSSON 1997). In the

Kvädöfjärden Bay, south of the St. Anna’s archipelago, perch (Perca fluviatilis) and

roach (Rutilus rutilus) dominate. Perch has increased significantly in the inner

archipelago, but roach has decreased in the outer areas in Kvädöfjärden. Marine

species are only caught in small numbers (ÅDJERS et al. 2001).

In the Kalmar Sound, high mortalities of 4-5 year old perch have been a

problem since the mid 1990s. The stock of stickleback (Gasterosteus aculeatus) is

also reduced and pike (Esox lucius) has suffered from recruitment problems. The

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Swedish East-Coast

probable reason is a combination of pollutants, algal toxins, eutrophication, and

predation by the increasing stock of cormorants (Phalacrocorax carbo) in the Kalmar

Sound area (KARÅS 1998, JOHANSSON 1999, ANDERSSON, J. et al. 2000, SAULAMO et

al. 2001, NILSSON et al. 2004b).

An inventory of the fish communities in the northern Hanö Bay in 2001

showed a dominance of perch and roach on shallow bottoms. The catches were

overall small compared to the mid-1990s, mainly due to lower temperatures. In 1995,

ruffe (Gymnocephalus cernuus), flounder (Platithythys flesus) and cod (Gadus

morhua) were the most abundant species. In 2001, no cod was found in the area

(ANDERSSON 2001).


The Swedish east coast is a region in the Baltic Sea where the problems with

eutrophication is evident, but not serious. Changes are mainly seen in reduced

communities of Fucus vesiculosus and diminished living condition for zoobenthos on

deeper bottoms (Fig. 8.2).

The reasons to the eutrophication in this region are mainly found in the

agriculture, and somewhat in municipal sewage plants, industries, and outlets from

scattered dwellings (JANSSON 1998). For example, in the Hanö Bay the discharges of

nutrients were 4787 t nitrogen and 135 t phosphorus in 2001 (TOBIASSON et al. 2002).

The leakage of phosphorus from agriculture has decreased with a stricter use of

fertilizers. Reductions in nitrogen are not as significant (JANSSON 1998). According to

ELMGREN & LARSSON (1997), the best solution for the eutrophication problem in the

Himmerfjärden is reductions of nitrogen discharges together with regulations of

phosphorus discharges in order to get a balanced nutrient situation in the innermost

part of the bay.

Other pressures on the state of the environment, such as heavy metals and

toxins from point source industries, have damaged the local survival and conditions of

fish in this area (ANDERSSON, T. et al. 1988, KARÅS et al. 1991).

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Swedish East-Coast

Fig. 8.2. The conceptual model for the east coast of Sweden (see Table 2.1 and Fig. 2.2 for explainations).

98

Central Baltic

9. THE CENTRAL BALTIC SEA The Central Baltic Sea, the Baltic proper, is the largest sub-region (51 %) of the Baltic

Sea. The surface area is 211.069 km2 and the volume 13.045 km3. The Landsort Deep

in the Northern Baltic proper is the deepest point in the Baltic Sea at 459 m. The

Central Baltic is the open sea area from the Darss Sill in the south west to the Åland

Sea, the Archipelago Sea and the Gulf of Finland in the north (Fig. 9.1). Topographic

features subdivide the Central Baltic into six basins: the Arkona, Bornholm and

Gdansk Basins, the Eastern and Western Gotland Basins and the Northern Baltic

proper (HELCOM 1996). The basins with volumes and maximum depths are presented

in Table 9.1.

Fig. 9.1. The sub-region of the Central Bal-tic. This region covers the open Baltic Sea area.

A halocline at 60 m stratifies the Baltic Sea. The upper part is well-mixed in

winter, but during summer a termocline is formed (ENGEL et al. 2002). The Central

Baltic, as well as the whole Baltic Sea, is dependent on inflows of saline and

oxygenated water from the North Sea. The topography with narrow channels and

shallow sills restricts the circulation. Water stagnation for periods of several years in

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Central Baltic

the central deeps are common (MATTHÄUS & SCHINKE 1999). Hence, oxygen

depletion in the deep basins is both a natural phenomenon and a consequence of

eutrophication.

After a long stagnation period with no larger inflows since the winter 1993-

1994, 200 km3 highly saline water entered the Baltic Sea in January 2003. The cold

inflow was mixed with the local water in the Arkona and Bornholm Basins, and the

mixed water spread further to the Gdansk Basin. In May 2003 it reached the Gotland

Deep, which was detected by a decrease in temperature and increases in salinity and

oxygen concentrations (NAUSCH et al. 2003, PIECHURA & BESZCZYŃSKA-MÖLLER

2003).

Table 9.1. The names, volumes and maximum depths for the basins in the Central Baltic (after HELCOM 1996). Basin Boundary Volume (km3) Deepest point (m) Arkona east of 12°00É 36 53; Arkona Deep west of 14°20É Bornholm east of 14°20É 306 105; Bornholm Deep west of 17°00É Gdansk east of 17°00É 426 108; Gdansk Deep south of 55°50É Eastern Gotland north of 55°50É 1195 249; Gotland Deep south of 57°50É Western Gotland west of 19°00É 657 Northern Gotland east of 19°00É 863 459; Landsort Deep west of 22°00É north of 57°50É

9.1 Secchi depth According to RENK et al. (1991), the mean annual Secchi depth is 8.2 m in the

Gdansk Deep, 9.6 m in the Bornholm Deep, and 9.9 m in the southern part of the

Gotland Deep. In the Gdansk Deep, the most extreme variations in transparency are 3

m in summer and 15 m in winter (RENK et al. 1991). The annual mean transparency in

the Landsort Deep shows a declining curve from 10.5 m in 1990 to about 7.8 m in

1997 (LARSSON et al. 1998).

Long-term trends for the Secchi depth during the last century are decreasing in

all sub-regions of the Baltic Sea. In twenty years, from 1980 to 2000, the transparency

decreased drastically in the Central Baltic. The reason is probably found in the

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Central Baltic

increase in algal biomass and occurrence of cyanobaterial blooms (LAAMANEN et al.

2004).

9.2 Salinity The average annual salinity in the bottom water of the Arkona Basin in 1960-1980

was 16 psu (OJAVEER & ELKEN 1997). After marine influxes, the salinity exceeds 20

psu in the Arkona Basin, like in spring 2003 (FEISTEL et al. 2003). The flow of saline

water from the Bornholm Basin northwards is limited due to the narrow Slupsk Sill

(HELCOM 1996). According to CYBERSKA & KRZYMINSKI (1996), the bottom salinity

in the Gdansk Deep was lower than 10.5 psu in 1989-1992. In 2002 it was measured

to 11 psu and in surface layer to 7 psu (FEISTEL et al. 2003).

The salinity in the Gotland Deep in the beginning of the 2000s was recorded to

12 psu (FONSELIUS & VALDERAMA 2003, NAUSCH et al. 2003). Earlier short-term

trends in salinity from the Eastern Gotland Basin are found in NEHRING (1991), LAINE

et al. (1997), LAINE & ZORITA (1999) and ZORITA & LAINE (2000). The salinity in the

Northern Baltic proper has been decreasing since 1990. The bottom water salinity in

the Landsort Deep is about 10 psu (LAINE et al. 1997, FONSELIUS & VALDERAMA

2003).

Long-term trends from the 1960s to 2001 for the deeps in Bornholm and

Eastern Gotland Basins and the Northern Baltic proper are found at STOCKHOLM

UNIVERSITY (2004a).

9.3 Oxygen In Unverzagt (2001) the areas with hypoxic and anoxic bottoms in the Baltic Sea are

estimated to 77.000 and 14.000 km2, respectively.

The Darss Sill had oxygen concentrations of 1-4 ml l-1 in the summer of 2002.

In the Bornholm Basin, no anoxia was recorded in winter-spring of 2003 (FEISTEL et

al. 2003). However, fluctuations may temporarily be strong and the concentrations

may quickly increase from anoxic levels to ~ 8 ml l-1 (CEDERWALL & SJÖBERG 1995).

The mean oxygen content in the Gdansk Deep is about 2 ml l-1. In the deep water,

oxygen varies from ~ 8 ml l-1 to anoxia and presence of hydrogen sulphide

(CYBERSKA & KRZYMINSKI 1996, HELCOM 1996). In the whole productive season of

2002, H2S was detected below 85 m in the Gdansk Deep (FEISTEL et al. 2003).

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Central Baltic

During the period 1981-1993, oxygen was totally lacking and the

concentrations of hydrogen sulphide were high in the Gotland Deep (NEHRING 1991,

LAINE et al. 1997, MATTHÄUS & SCHINKE 1999, STOCKHOLM UNIVERSITY 2004a). A

reoxygenation took place in the mid-1990s, and oxygen concentrations of 3 ml l-1

– the highest since the 1930s – were measured. During that period, all the bottom

water in the whole Baltic Sea was oxygenated (CEDERWALL & SJÖBERG 1995,

STOCKHOLM UNIVERSITY 2004a). In 1998-2002, the oxygen trend has again been

decreasing with anoxia and presence of H2S, but in 2003 the concentration increased

to 4 ml l-1 (FONSELIUS & VALDERAMA 2003, NAUSCH et al. 2003). The western part of

the Eastern Gotland Basin was still anoxic in 2003 (HELCOM 2003b).

Anoxic conditions prevailed in the deep layer of the Northern Baltic proper in

periods during the 1980s. In the beginning of the 1990s, a decreased vertical

stratification facilitated a more intense mixing and even the deep layers became oxic

(HELCOM 1996). Thereafter the trend is again decreasing and hydrogen sulphide was

recorded in 1999 (ANDERSSON, L. et al. 2000, FONSELIUS & VALDERAMA 2003).

Long-term trends from the 1960s to 2001 for the deeps in Bornholm and

Eastern Gotland Basins and the Northern Baltic proper are found at STOCKHOLM

UNIVERSITY (2004a).

9.4 Nutrients The trends for phosphate and inorganic nitrate have been increasing since the 1960s in

the Central Baltic, either in the bottom water or in the entire water column

(FONSELIUS & VALDERAMA 2003). In the beginning of the 1990s, the concentration of

tot-P in the surface layer in the Central Baltic decreased with 20 % since late 1980s.

At the same time, the concentration of tot-N first started to increase, but declined to

the same level as in the late 1980s (LARSSON & ANDERSSON 1999). According to

LARSSON et al. (2001) the average concentration of tot-N increased with about 28

µg l-1 during the summer months in all the basins of Central Baltic, except for the

Bornholm Basin, where the increase was even higher; ~ 45 µg l-1. The co-incidents

with H2S and the increased amounts of carbon and phosphorus in the sediments are

evident for the Eastern Gotland Basin. In the Arkona and Bornholm Basins, the larger

fluxes of the deep-water have caused less burial of nutrients in the sediments (EMEIS

et al. 2000).

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The total amounts of nitrogen in the whole water column have been relatively

stable for all sub-basins of the Central Baltic since 2000. In the Bornholm, Gdansk

and Eastern Gotland Basins phosphorus has decreased, while in the Western Gotland

Basin and Northern Baltic proper increases have taken place (SMHI 2004). However,

since the year 2000, the concentration of nitrogen is decreasing in the surface water,

while the phosphorus amount has started to increase. These changes are caused by

internal biogeochemical processes and the supply of oxygen in the deep water, not by

the discharges from land. During the winter 2004, the highest phosphate

concentrations in the surface water were recorded since the end of the 1950s. On the

contrary, the winter concentrations of nitrate were halved and are now back at the

same level as in the 1970s. As an example, the Landsort Deep has a surface value of

28 µg l-1 for phosphate and 42 µg l-1 for nitrate (LARSSON & ANDERSSON 2004).

According to Table 2.1, the concentrations are high for both nutrients.

9.5 Chlorophyll a and primary production Comparing the chlorophyll a concentrations in different regions of the Central Baltic

from 1979 to 1998, a slight increase is seen (SCHULZ et al. 1997, HELCOM 2002). In

the Arkona and Gotland Basins the average summer chlorophyll a concentrations has

increased from 2 to 2.8 µg l-1 since the 1980s (HELCOM 1996, ENGEL et al. 2002,

WASMUND &UHLIG 2003), which is regarded as medium-high levels (Table 2.1). In

the Bornholm Basin, the concentration decreased from over 3 µg l-1 in 1980 to 1 µg l-1

in 1989, and then again increased significantly until 1997. A similar trend is seen in

the values from Eastern Gotland Basin (HELCOM 1996, WASMUND & UHLIG 2003). In

the Landsort Deep, the chlorophyll a varied between 1.4 and 2.3 µg l-1 in the 1990s

(LARSSON & HAJDU 1997), values that can be classified as low (Table 2.1).

On average of all the annual measurements for primary production in HELCOM

(1996) between 1979 and 1993, a slight increase is seen. The mean primary

production in the Arkona Basin had a summer peak at 10-20 mg C m-3h-1. The

Bornholm Basin had lower spring and autumn concentrations but the summer

productivity was about the same as in the Arkona Basin. In the Eastern Gotland Basin,

summer and autumn productivity ranged between 15 and 25 mg C m-3h-1 in 1979-

1993. Data of primary production in the Northern Gotland Basin is too scarce for

trend lines or to give a clear average picture (HELCOM 1996).

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9.6 Phytoplankton and algal blooms According to LARSSON et al. (2001), the total fixation in the Central Baltic Sea is

estimated to 180.000-430.000 tons N annually, 30-90 % of which correspond to the

pelagic net community production during the summer months. This corresponds to

25-50 % of the annual nitrogen load via rivers and the atmosphere (HAJDU et al.

2003). Strong algal blooms are often caused by high concentrations of phosphorus

during the winter-spring and high temperatures in the spring and summer (HAJDU et

al. 2003, LARSSON & ANDERSSON 2004).

The most abundant phytoplankton in the Central Baltic Sea is filamentous

cyanobacteria, diatoms and dinoflagellates (ENGEL et al. 2003). Of the cyanobacteria

Aphanizomenon flos-aquae, Nodularia spp and Anabaena spp are dominating in all

basins of the Central Baltic. Aphanizomenon spp occurs throughout the growth

season, while the others appear only in the warmer months (KONONEN & LEPPÄNEN

1997, LARSSON et al. 1998, 2001).

The cyanobacterial blooms were intense in the Central Baltic in the summers

of 1997-1999. In the summer of 2000, the situation was better, but the surface

occurrences covered the entire Baltic Sea, except for the Bothnian Bay. The situation

in August 2002 was serious compared to the mean value for the period 1992-2001

(HELCOM 2003a). Nodularia spp occurred in the highest concentrations in the areas

around the Gotland Island. The Northern Baltic proper had the longest duration of the

bloom, because of high water temperatures and calm weather conditions (HAJDU et al.

2003). The spring bloom in 2003 was intense – lasted for nearly 60 days in the

Northern Baltic proper. The cyanobacterial bloom in the summer of 2003 was strong

in the Central Baltic, except for the Northern Baltic proper (HELCOM 2003b).

The BALTIC SEA PORTAL (2004) gives accurate information of the algal

situation in different parts of the Baltic Sea. Predictions for the intensity of

cyanobacterial blooms are published in e.g. KIIRIKKI et al. (2003) for the Central

Baltic and the Gulf of Finland.

9.7 Macrovegetation In the southern Bornholm Basin, on the Slupsk Bank, 16 macroalgal species were

found attached to the stones in 1999 between 8.5 and 16.5 m depth (ANDRULEWICZ et

al. 2004b). The red algae Delesseria sanguinea was reported from this area already in

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1929, and was still present in 1999 (HELCOM 1996, ANDRULEWICZ et al. 2004b).

Furcellaria lumbricalis had the highest biomass, but Pilayella littoralis and

Ectocarpus spp were also common. The filamentous brown algae were attached on

shallower parts, but occurred free-floating deeper down (ANDRULEWICZ et al. 2004b).

South of the Gotland Island, at the Hoburg and Midsjö banks, attached brown

and red algae have been found from 8-29 m depth. Sphacelaria arctica and Ceramium

tenuicorne were most common at the Hoburg bank, while P. littoralis and Ectocarpus

spp dominated at the Midsjö bank. A photic zone down to 29 m indicates a good

water quality. Fucus vesiculosus and F. serratus were absent, most likely because of

unsuitable substrate and disturbance from waves. The undisturbed habitat at these

locations is unique in the Baltic Sea and has an important protection value (ALUTOIN

et al. 2001).

9.8 Zoobenthos Since the mid 1990s, the depth limit of macrofauna has risen with about 30 m in the

Central Baltic Sea and in the Gulf of Finland. In the Landsort area even with 45 m.

This corresponds to an area of 38.000 km2 of so called dead bottoms in 15 years and

an amount of 1-1.5 million tons wet weight of killed benthos (ANDERSIN &

CEDERWALL 2003).

In the end of the 1990s, from 48 to 236 m, 19 species of infauna and epifauna

were found in the Central Baltic and the Gulfs of Bothnia and Finland. Harmothoë

sarsi, Saduria entomon, Monoporeia affinis and Macoma balthica were most

abundant. Halicryptus spinulosus, Diastylis rathkei and Scloplos armiger had

important roles more locally. In the Eastern Gotland Basin, anoxia eliminated the

macrofauna at some of the sampling stations (LAINE 2003).

The Arkona and Bornholm Basins have faster turn-over rate of deep water

compared to the northern basins of the Central Baltic. Therefore, the oxygen is faster

consumed, which has influence for the zoobenthos. Two years after the saline flux in

1993-1994, no benthic life was found in the Bornholm Basin, while the same process

took four years in the Western Gotland Basin and three years in the Northern Baltic

proper (CEDERWALL 2003).

In the Arkona Basin reductions in macrofauna have occurred in periods. The

abundance and biomass increased in the mid 1990s, when the dominance switched

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Central Baltic

from crustaceans to polychaetes (KUBE et al. 1996). The biomass decreased below the

halocline in the Bornholm Basin from 1950 to 1980. Alterations between lamellibrach

and polychaete dominating communities, in relation to the present oxygen situation

and variations between stagnant and influx periods are seen both in the Bornholm and

Gdansk Basins (TULKKI 1965, LEPPÄKOSKI 1969, 1971, ANDERSIN et al. 1978,

ROSENBERG 1980). In the Bornholm Deep, re-establishments of benthos were

observed in 1997 and 2001 (CEDERWALL 2003). Fluctuations in zoobenthos have

occurred above 60 m in the Gdansk Basin since the 1970. In the period 1989-1993, no

fauna was recorded at all (OSOWIECKI 1991, OSOWIECKI & WASZOCHZ 1996).

The fauna in the Eastern Gotland Basin and the Northern Baltic proper was

fairly diverse and dense below 100 m in the beginning of 1950s (SJÖBLOM 1955). In

the southern part of the Eastern Gotland Basin, the fauna has continuously decreased

since the late 1960s, and the community has become polychaete dominated. Above

the halocline, recolonisation is periodically allowed due the salt water flushes. In the

Northern Baltic proper, only sporadic observations of macrofauna are reported below

100 m during the period 1965-1994 (ANDERSIN et al. 1978, HELCOM 1996, 2002,

LAINE et al. 1997).

9.9 Ichthyofauna The economically most important fish species in the open sea in the Baltic is cod

(Gadus morhua), herring (Clupea harengus membras), sprat (Clupea sprattus),

salmon (Salmo salar) and species of flatfish. Changes in the fish stocks are a

combination of climatic variations, eutrophication, pollution and over-fishing

(OJAVEER, H. 2002). Top catches of the eastern Baltic cod were reached in the mid

1980s. The combination of changes in the physical environment and a high fishing

pressure has lead to drastically decimated stocks. In the end of the 1990s, the catches

were five times lower compared with 1985 (RADTKE 2003).

The open-sea system of the Central Baltic has change from a cod- to a clupeid-

dominated system (MÖLLMANN & KÖSTER 1999, OJAVEER & LEHTONEN 2001). The

stocks of herring and sprat are favoured by the eutrophication, but a strong over-

fishing has reduced the stocks to a minimum level in the Central Baltic. The catches

have decreased considerably since 1980, although the diminished cod populations

have positive effects for the production of herring and sprat (HANSSON 2001). The

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Central Baltic

competition for food between sprat and herring has also increased (KORNILOVS et al.

2001). Decreased surface salinity has forced the big marine copepods southwards. As

they are the basic food item for herring, the stock in the Northern Baltic proper is

negatively affected (JANSSON & DAHLBERG 1999).

Some recovery in the cod stock has been detected in the southern and central

parts of the Baltic. North of the islands of Gotland and Öland, the catches are still

poor and show no signs of improvement (ANDERSSON et al. 1997). Hypoxia and

anoxia in the bottom water prevent the reproduction of cod (JANSSON & DAHLBERG

1999).


The total amount of nitrogen and phosphorus to the Central Baltic in 2003 is

estimated to 3.000.000 t and 400.000 t, respectively. Compared to the previous years,

a slight decrease is seen for both of the nutrients (LARSSON & ANDERSSON 2004).

The high concentration and frequency of phytoplankton and cyanobacterial

blooms, oxygen deficiency in the bottom water and poorer living conditions for

zoobenthos and fish may be regarded as the main problems for the deep water and

open sea areas in the Baltic Sea (Fig. 9.2). The problems are not only eutrophication-

related, but a combination with frequency of saline influxes and other direct human

influences, such as fishing and toxic pollution. All the effects of the last marine inflow

in 2003 have not yet been described.

Studies have shown that the amount of nutrients in the Central Baltic may

fluctuate strongly, even if the external land- and air-based loads are stable. The state

of the Central Baltic depends on legislation and reductions in emissions in other

regions before improvements can be seen. The international shipping also contributes

considerably to the airborne emissions of sulphur and nitrogen oxides at the surface

layer of the Baltic Sea (ANON. 2001b). However, the only solution is further

reductions of the discharges both via water and air, for example by natural uptake in

rivers and wetlands (LARSSON & ANDERSSON 2004). The central areas of the Baltic

Sea adapt slowly to changes in external loads, because reductions in phosphorus are

only depended on sedimentation. Nitrogen, on the other hand, has a faster adaptation,

as N-fixation and denitrification play roles together with the sedimentation

(SCHERNEWSKI & NEUMANN 2005).

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Central Baltic

Fig. 9.2. In the Central Baltic, cyanobacterial blooms, oxygen deficiency and decreases or lack of zoobenthos are the most serious eutrophication-related problems (see Table 2.1 and Fig. 2.2 for explainations).

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Belt Sea region

10. THE BELT SEA REGION, including Great Belt, Little Belt

and the Sound, Fehmarn Belt, and the Bights of Kiel,

Mecklenburg and Pomerania The narrow Great Belt, Little Belt and the Sound connect the Kattegat to the southern

Belt Sea, where Kiel Bight, Fehmarn Belt, and the Bights of Mecklenburg and

Pomerania are situated (Fig. 10.1). The area of the Belt Sea Region is about 24.000

km2 and holds a volume of 335 km3 (HELCOM 1996, FRANKOWSKI et al. 2002). The

water transport from Kattegat to the Baltic has a relationship of 7:3:1 for Great Belt,

Sound and Little Belt (RASK et al. 1999a). The maximum depth varies from 53 m in

the Sound to 80 m in Little Belt (MAGAARD & RHEINHEIMER 1974, GRANÉLI 1984).

The Kiel Bight is situated in the southern part of the Belt Sea and is connected to the

Mecklenburg Bight by the Fehmarn Belt. Kiel Bight has a mean depth of 17 m,

maximum depths in the channels and basins extend to about 30 m (BREUER &

SCHRAMM 1988, BABENERD 1991).

East of the Rügen Island, the Greifswalder Bodden and the Pomeranian Bight

are found. The Greifswalder Bodden is the largest inland sea area (514 km2) in

Germany (MESSNER & V. OERTZEN 1991) with a mean depth of 6 m (SCHIEWER

2002). The Pomeranian Bight is less than 20 m deep and the bottom consist mainly of

sandy sediments (MESSNER & V. OERTZEN 1991). The Szczecin Lagoon/Oderhaff,

which is situated south of the Pomeranian Bay, as well as other boddens or lagoons in

this region act as sediment traps for pollutants. The discharges from the River Odra/

Oder, on the border between Germany and Poland, are filtered and buffered before

entry to the Baltic Sea.

The river catchments of Odra and Vistula were early under intensified

anthropogenic influence, and the nutrient concentrations were high already in the

early 20th century (SCHERNEWSKI & NEUMANN 2005). Today, Odra is one of the most

polluted rivers in central Europe (PASTUSZAK et al. 1996, FRANKOWSKI et al. 2002).

About 35 % of the river water and 40 % of the waste water in Poland flow through

lagoons and coastal lakes before entering the sea, which reduces the direct pollution

load to the Baltic (PASTUSZAK et al. 1996). In the years 1996-2000, the annual load of

total P discharges from the Odra has decreased by 10 % (RUDLICKA et al. 2003).

However, an overloading of pollution may restrict the buffering capacity (MEYER-

REIL & KÖSTER 2000).

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Belt Sea region

Fig. 10.1. The Belt Sea region.

According to HELCOM (2003a), Denmark (1862 kg km-2) and Germany (581

kg km-2) have the highest country specific loads for nitrogen in this region. For

phosphorus, the highest country specific loads originated from Denmark (83 kg km-2),

Poland (43 kg km-2) and Germany (35 kg km-2). The nitrogen load is mainly related to

agriculture, whereas the phosphorus in the first place is linked to high population

densities and industrial activity (HELCOM 2003a). At present, there are 16 HELCOM

hot spots in this area. Many point-sources from Poland and Germany have been

deleted from the list during the latest years (HELCOM-HOT SPOTS 2004). Reductions in

nutrients discharges from wastewater and cultivated land have taken place in

Denmark between 1989 and 2002 (ANDERSEN et al. 2004b), but the agricultural

runoff programme to the Sound and Belt Sea is still a hot spot for Denmark (HELCOM-

HOT SPOTS 2004).

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Belt Sea region

10.1 Secchi depth In Great Belt, the transparency has increased from 5 m in 1977 to 6-10 m in 2002

(RASK et al. 1999a, FYNS AMT 2004a). The Secchi depth has increased in both fjords

and open waters in Denmark since the mid-1980s (ÆRTEBJERG et al. 2003).

The Pomeranian Bight had a transparency of 3-7 m in the period 1993-1997,

the Secchi depth was lower in the plume of River Swina (WASMUND et al. 2001). In

Greifswalder Bodden, Secchi depth during the summer algal bloom has decreased

from 2.5 m in 1938 to 1-1.5 m in 1955 and further to 0.2 m in 1988. This gives an

indication on the advancing eutrophication process during the past 50 years (MESSNER

& V. OERTZEN 1991).

10.2 Salinity The supply of freshwater into the Baltic Sea makes the sea level in the Southern Baltic

a few centimetres higher than the level in Kattegat, and the low salinity surface water

is therefore forced to the Kattegat through the Belt Sea (HELCOM 1996). Water masses

of different origin meet in the Belt Sea area; generally a surface layer of low-saline

Baltic water (10-15 psu) flows northwards to the Kattegat. A compensating stream of

high-saline Skagerrak water (25-33 psu) flows near the bottom into the Baltic Sea

(HELCOM 1996, RASK et al. 1999a). Due to turbulence, considerable mixing takes

place in the Great Belt, and the salinity of the surface water varies between 16 and 32

psu (HELCOM 1996, ÆRTEBJERG et al. 1998, RASK et al. 1999a). The Sound has a

strong halocline at about 15 m depth, and the total salinity range lies between 8 and 30

psu (GRANÉLI 1984, HELCOM 1996, ÆRTEBJERG et al. 1998).

The salinity in the Fehmarn Belt and the Mecklenburg Bight varies between

11 and 15 psu in the surface water (HELCOM 1990, PRENA 1994). In the Pomeranian

Bight and the Greifswalder Bodden, the salinity has a range of 6-12 psu (e.g.

MESSNER & V. OERTZEN 1991, FEUERPFEIL et al. 2004).

10.3 Oxygen Since 1981, incidents of oxygen depletion in the bottom water have been observed

almost annually in the southern Kattegat, the Sound and the Belt Sea. At least in Kiel

Bight, the decrease seems to have started in the late 1950s (HELCOM 1996). Overall,

the oxygen concentrations have declined in the Belt Sea area since the beginning of

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Belt Sea region

the 1970s (ANON. 1991, AGGER & ÆRTEBJERG 1996). The oxygen concentrations in

the Great Belt fluctuated between 2 and 13 ml l-1 during 1976-1989 (ANON. 1991). In

the summers of 1997 and 1998, the oxygen varied between total anoxia and 7.3 ml l-1,

showing that the concentrations vary strongly among the sampling stations (RASK et

al. 1998, 1999a, b). The oxygen situation in the water around the Funen Island has

improved in the latest years compared with the total average for the period 1976-

1997. In 1997-1998 the oxygen concentration in the Little Belt varied between 0.6 and

8.6 ml l-1 (RASK et al. 1998, 1999a). According to AGGER & ÆRTEBJERG (1996), the

oxygen situation in the Sound has changed from > 3.5 ml l-1 in 1974 to < 2.5 ml l-1 in

1992.

The southern Belt Sea suffered from serious oxygen deficiency and formation

of hydrogen sulphide in 1981 and 1986 (MARKAGER et al. 1999), but the worst

oxygen depletion ever recorded in Danish coastal water took place in September

2002. Hypoxia and even anoxia occurred from the southern Kattegat to the Belt Sea

and the Pomeranian Bight. The reason is probably a combination of high nutrient

inputs and high precipitation in combination with warm water temperatures and lack

of wind (ANDERSEN et al. 2004b). The event is thoroughly described in ÆRTEBJERG et

al. (2003). Also in August 2003, serious oxygen depletion was observed in the same

area (ÆRTEBJERG et al. 2003). In the summer 2004, only the Belt Seas suffered from

serious and intense anoxia from July to September. The reason is found in a large

phytoplankton bloom in early summer, together with weak winds and repercussions of

the anoxic event in 2002 (NERI 2004).

In the other areas of the Belt Sea region a decline in the oxygen situation is

seen in the long-term perspective as well. The oxygen concentration in Kiel Bight was

measured to 3 ml l-1 in 1979 and to 2 ml l-1 in 1990 (HELCOM 1990). For the Fehmarn

Belt there is a decline from 4.5 ml l-1 in 1974 to < 3 ml l-1 in 1992 (AGGER &

ÆRTEBJERG 1996). The oxygen concentration in the Mecklenburg Bight had a

saturation of 75-110 % in 1977, and acute hypoxia was for the first time observed in

the southernmost Bight in the late 1980s. In 1990, the oxygen saturation had

decreased to a level of 45-75 % (PRENA 1994).

After 1990, a negative trend in the late summer oxygen concentration is

detectable in the Pomeranian Bight (ŁYSIAK-PASTUSZAK et al. 2004). The minimum

oxygen concentration in the bottom water in the period 1998-2001 was 2.7 ml l-1 in

Pomerania Bight and 2.9 ml l-1 in the Szczecin Lagoon (RUDLICKA et al. 2003).

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Belt Sea region

10.4 Nutrients The water in the Belt Sea, as well as in Kattegat, has a short residence time; only one

or two months for the surface water. Together with strong horizontal gradients,

changes in nutrient concentrations are difficult to detect (HELCOM 1996). The

increased use of commercial fertilizers in Danish agriculture has resulted in high

losses of nitrogen since the 1960s.

The amount of tot-N in the Great Belt decreased after a peak of nearly

600 µg l-1 in 1977 to a relatively stable mean value of about 300 µg l-1 in 1988-1998

(ANON. 1991, RASK et al. 1999a). The trend for tot-P has been more fluctuating

during the same period, and the range is between 25-40 µg l-1 (ANON. 1991, RASK et

al. 1999a). In August 2003, tot-N had a value of 250 µg/l and tot-P 10 µg/l in the

waters around the island of Funen. These concentrations lay far below the mean

values during the period 1989-2002 (FYNS AMT 2004a). Both ANDERSEN et al.

(2004b) and FYNS AMT (2004a) show that the summer and winter mean

concentrations of total nitrogen are decreasing since 1994. For total phosphorus, the

mean summer values are decreasing, while the winter values have increased slightly

since 1997 (ANDERSEN et al. 2004b, FYNS AMT 2004a). Comparing these values in

Table 2.1 both tot-N and -P has decreased from a very high to a low level.

The nutrient conditions in the Pomeranian Bight are governed by the dynamics

in the Bight, the Szczecin Lagoon, as well as by the discharges from River Odra

(PASTUSZAK et al. 1996). The winter concentrations of phosphate increased rapidly in

the Pomeranian Bay during 1960s and 1970s. In the late 1980s, accumulation of

phosphate took place in the surface water. During the period from 1960 to 1990,

nitrite and nitrate increased in the surface layer. Since the 1990 a decrease in the

winter accumulation of nitrogen is observed in the Pomeranian Bight (ŁYSIAK-

PASTUSZAK et al. 2004). The summer concentration of tot-P in the surface water of the

Pomeranian Bight has variated from 39 µg l-1 in 1990 to 50 µg l-1 in 2000. For tot-N,

the summer concentrations fluctuated between 250 µg l-1 in 1990 and 350 µg l-1 in

2000 (RUDLICKA et al 2003). To summarise, the nitrogen values in the Pomeranian

Bight are considered medium high; the phosphorus values are very high, in

accordance with Table 2.1.

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Belt Sea region

10.5 Chlorophyll a and primary production (N.B: The unity for primary production is given in mg C m-2 for the Danish waters and in mg C m-3 for

the rest of the areas).

The trends for chlorophyll a and primary production are decreasing, both in

Danish coastal and open waters. From the mid 1980s, the annual concentrations have

declined. However, the primary production has increased slightly since the year 2000

(ANDERSEN et al. 2004b). In Danish coastal waters, the concentrations of chlorophyll

a varied between 1.5-7.7 µg l-1 in 1975-1989 (ANON. 1991). From 1988 to 1997, the

chlorophyll a decreased from ~4 to 2 µg l-1 (ANON. 1991, RASK et al. 1998). In

August 2003 the chlorophyll a in the northern Great Belt reached a level of about 1.7

µg l-1, which lies under the mean value for the period 1989-2002 (FYNS-AMT 2004).

The present chlorophyll a situation in Danish waters can overall be considered as low

(Table 2.1).

The primary production in Great Belt had a summer mean value under 1000

mg C m-2d-1 in 1989-1997 (RASK et al. 1998, RASK et al. 1999a, FYNS-AMT 2004).

After that a rapid increase took place and the peak occurred 1999 with 1500

mg C m-2d-1. The concentration has declined again and was about 1000 mg C m-2d-1 in

August 2003. The trends for Little Belt and the Sound are similar (ANON. 1991, RASK

et al. 1998).

Both in Kiel Bight and Mecklenburg Bight a decrease in chlorophyll a

concentration was observed from 1986 to 1998 (HELCOM 1996, 2002). In both bays

chlorophyll a has increased from 2.5 µg l-1 in 1978 to 4.5 µg l-1 in 1988 (HELCOM

1990, SCHULZ et al. 1992). The overall level of primary production was lower in Kiel

Bight – 8 mg C m-3d-1 in 1978 and 36 mg C m-3d-1 in 1989 – compared to

Mecklenburg Bight – 200 mg C m-3d-1 in 1970 and 400 mg C m-3d-1 in 1988 (HELCOM

1990, SCHULZ et al. 1992). Table 2.1 classifies the increases of chlorophyll a from a

medium to a high level in both Kiel and Mecklenburg Bights.

The maximum chlorophyll a concentration, 87 µg l-1, in the Pomeranian Bight

was recorded in the outlet of the Szczecin Lagoon in September 1995 (WASMUND et

al. 2001). In 2001, the chlorophyll a concentration in the Pomeranian Bay was

7.58 µg l-1, which was the highest mean concentration in the coastal zone of Poland

(RUDLICKA et al. 2003). Chlorophyll a correlated significantly with both phosphate

and nitrate in the surface water of the inner parts of Pomeranian Bight (MASLOWSKI

2003). The content of chlorophyll a in the sediment indicates that the Szczecin

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Belt Sea region

Lagoon can be classified as permanently eutrophic and the Pomeranian Bay as

mesotrophic, according to KOWALEWSKA et al. (2004).

10.6 Phytoplankton and algal blooms In Great and Little Belts, mass occurrences of cyanobacteria have been sparse, while

other toxic or potentially toxic phytoplankton species have bloomed frequently since

monitoring was initiated in 1976, according to RASK et al. (1999a). In 1988,

Chrysochromulina polylepis (Prymnesiophyceae) was the first toxic algae with mass

occurrence in the Belt Sea. The bloom did not cause direct damage to fish or benthic

fauna, but widespread oxygen depletion was detected in the coastal waters and this

was most likely related to the algal growth (ANON. 1991). Larger quantities of the

cyanobacteria Nodularia spumigena were first recorded in 1992 and 1995 in the

southern Belt Sea and the Sound (MARKAGER et al. 1999). In 1997 mass occurrences

of the toxic dinophyte Gyrodinium aureolum – also called “marine snow” – were

found both in Little and Great Belt, and caused extensive mortality of benthos in

waters north of Funen Island (RASK et al. 1998). In 2001, the average phytoplankton

biomass in Danish waters was lower than the long-term averages (ÆRTEBJERG et al.

2003).

In the open Kattegat and Belt Sea, the diatom biomass has decreased

significantly over the past 20 years and is now 50 % of that in 1980. In estuaries and

coastal areas no long-term trend in diatom biomass was found (ÆRTEBJERG et al.

2003). In the Greifswalder Bodden, the diatoms still represent 40 % of the

phytoplankton abundance (SCHIEWER 2002). Long-term data from the Odra estuary

indicate a shift and decrease in the diatom composition during the last century

(ANDRÉN 1999). In the end of the 1990s dinophytes were the dominat group in late

spring and early autumn, while cyanobacteria as Aphanizomenon flos-aquae and N.

spumigena associated with colony forming species were the most abundant species

during summer. The phytoplankton biomass was significantly lower outside the

Rügen Island than in the Pomeranian Bay (FEUERPFEIL et al. 2004).

10.7 Macrovegetation The Belt Sea region has suffered from changes in benthic macrovegetation since the

1970s. The enrichment of nitrogen is inversely related to the water transparency, and

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Belt Sea region

hence to the depth distribution of macroalgae, and the typical development from

perennial to fast growing annual species are seen (HELCOM 1996, 1998).

For the Belt Sea, most of the macrophytobenthos investigations are made on

eelgrass, Zostera marina. The studies of Z. marina from Danish waters are unique,

starting from the beginning of the 20th century and give a good overview of the

situation before the eutrophication process seriously started (ÆRTEBJERG et al. 1998).

In 1901, Danish eelgrass meadows covered almost 7000 km2, or 15 % of the Danish

marine waters (Petersen 1901, 1914 in FREDERIKSEN et al. 2004a). The reduction in

the Zostera-stock began with the “eelgrass disease” in the beginning of the 1930s,

when about 90 % of the whole North European stock disappeared (ANON 1991, RASK

et al. 1998, LOMSTEIN 1999). Only 8 % of the level in 1901 was left in 1941 (Lund

1941 in FREDERIKSEN et al. 2004a). Increased nutrient loading has reduced the

vertical recovery and the depth distribution is therefore much shallower than before

(ANON. 1991, HELCOM 1996, LOMSTEIN 1999).

Decreases in depth distribution are seen in fjord areas from 6-8 m in 1900 to 2-

4 m in 1989-2002, and in the open areas from 9-10 m in 1900 to 4-6 m in 1989-2002

(ANON. 1991, RASK et al. 1998, LOMSTEIN 1999, RASK & BONDGAARD 2000,

ANDERSEN et al. 2004b). New data of Z. marina in Danish water is found in e.g.

NIELSEN et al. (2002a, b), BOSTRÖM et al. (2003), and FREDERIKSEN et al. (2004 a, b).

In the Kiel Bight, in the beginning of the 1960s, the red algae Furcellaria

lumbricalis was the dominant species at all depths. Laminaria saccharina and

Phycodrys rubens were important components of the vegetation below 10 m.

Shallower, at 6 m depth, Fucus serratus and Z. marina were dominating. About 30

years later, by the end of the 1980s, a combination of the epilithic Phyllophora

truncata and the epiphytic P. rubens dominated at all depths. L. saccharina was

sparsely found below 14 m, F. serratus and Z. marina were reduced to less than 2 m

depth, and Polysiphonia nigrescens was instead common in shallower areas. There

were, however, no significant changes observed for species such as Ceramium spp,

Rhodomela confervoides and Delesseria sanguinea. All these changes are

eutrophication related, primarily caused by reductions in light conditions (BREUER &

SCHRAMM 1988, HELCOM 1996).

Greifswalder Bodden in the Pomeranian Bight was mostly covered of red

algae in the 1930s, with zones of Z. marina and Potamogeton pectinatus along the

coasts. In 1955 only small patches of red algae were left. In 1988, almost all red algae

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Belt Sea region

had disappeared, and the amounts of Zostera were also drastically reduced (MESSNER

& V. OERTZEN 1991). Even since the mid-1990s, the macrophyte coverage in the

Greifwalder Bodden decreased from 75 % to 15 %. The diversity of the vegetation is

still high, but the transparency is reduced (SCHIEWER 2002).

10.8 Ephemeral algae and drifting algal mats In the Great and Little Belts, the filamentous algae constitute the highest phytobenthic

biomass in early summer. In July they start to decompose and form drifting algal

mats, and Zostera marina regains its dominating role in the community. The

ephemeral algae have seasonal variations in Danish waters; in spring and early

summer Cladophora sericea and Pilayella littoralis are dominating, C. sericea is later

replaced by Ectocarpus siliculosus in deeper waters (3-6 m). Chaetomorpha linum is

the most frequent filamentous algae in late summer (ANON. 1991). According to

ANDERSEN et al. (2004b), the coverage of eutrophication-related algae has remained

stable during the period 1993-2002.

In Kiel Bight, larger amounts of ephemeral algae were reported for the first

time in 1977, when agglomerated Furcellaria lumbricalis drifted down to the

sublittoral slopes. Of all Furcellaria found, 40 % was loose lying, while Phyllophora

spp was mostly attached (BREUER & SCHRAMM 1988).

10.9 Zoobenthos The composition of the zoobenthos in the Great Belt and the Sound has changed since

the 1960s, even if typical species for polluted areas – Nereis diversicolor, Capitella

capitata and Scoloplos armiger – were dominating in the Sound at that time. Probably

because of changes in food supply, the crustaceans have decreased since the 1980s,

whereas the polychaetes, molluscs and echinoderms have increased. According to

investigations from the Little Belt in 1910-1933, areas deeper than 30 m had sparse

fauna. In 1973-1988 it was evident that areas less than 20 m were affected too.

Species tolerant to hypoxia have increased by a factor of 2-5 in this area (HENRIKSSON

1969, ÆRTEBJERG et al. 1998, MARKAGER et al. 1999). The biomass and abundance

of zoobenthos had extremely low values during the period 1998-2001 (ÆRTEBJERG et

al. 2003), and the events with serious oxygen depletion in 2002 and 2003 have

resulted in mass mortality of benthos in the affected areas (ANDERSEN et al. 2004b,

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Belt Sea region

FYNS-AMT 2004). Bivalves used to be the dominating class in the Great Belt.

However, in the summer 2003, the bristleworm Scalibregma inflatum dominated the

abundance in the water north of Funen Island, and C. capitata was most abundant

south of Funen (FYNS-AMT 2004).

Macrofauna tolerant to organic pollution in the Kiel Bight have increased

since 1981. In the late summer of 1981, hypoxia below the halocline (> 20 m) caused

mass mortality among the 60 recorded faunal species in the Bight. The fauna

recolonised rapidly after the event, but a new similar crash occurred in 1983

(WEIGELT & RUMOHR 1986, WEIGELT 1990). The sandy bottoms of Greifswalder

Bodden are dominated by molluscs and ostracods (SCHIEWER 2002).

In the innermost part of the Pomeranian Bight, near the mouth of Swina River,

the macrofauna biomass consists to 80 % of the bivalves Mya arenaria and Macoma

balthica (MASLOWSKI 2003). Further out in the Bight, Mytilus edulis dominates,

followed by Corophium volutator, Marenzelleria viridis and Nereis sp (FEUERPFEIL et

al. 2004).

10.10 Ichthyofauna The Belt Sea region is an important fishery area especially for flatfish, eel (Anguilla

anguilla), herring (Clupea harengus) and cod (Gadus morhua). After a period with

large catches and an increased fishery activity in the 1960s, the general tendency has

been diminishing catches of herring and plaice (Pleuronectes platessa), particularly

since the beginning of the 1980s (ANON. 1991). From 1990 to 1997, the catches of

cod, flounder (Platichthys flesus) and plaice increased, but thereafter especially the

cod catches started to decrease. It is hard to assess the real situation for the fish stocks

from the size of the catches, but the trends give at least some hints (ANON. 2001c).

The poor oxygen concentrations in the Belt Sea may have a negative impact

on fish spawning, and have lead to restrictions in commercial trawl fishery, allowing

it only in the period from December to April. Cod, plaice, and flounder spawn from

February to April in this area, when the oxygen conditions normally are better.

Therefore, hypoxia-related problems do not affect the survival of the eggs and larvae

of these species (HELCOM 1990). Altered patterns in phytoplankton spring bloom due

to eutrophication has, however, had a negative impact on the food sources for fish,

especially for survival of the young spring-spawning species such as cod (references

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Belt Sea region

in ANON. 1991). The worst occasions with fish kills happened in 1981 and 2002. The

direction of the wind forced the upwelling of hypoxic or H2S-containing anoxic

bottom water to the coastal zone where the fishes get trapped (ÆRTEBJERG et al.

2003).

In the Boddens on the German coast and in the Szczecin Lagoon, the annual

catches of freshwater species have doubled during the last century, which partly

seems to be due to eutrophication. The stocks of herring, pike (Esox lucius) and

pikeperch (Sander lucioperca) are eutrophication-favoured. Pike has been negatively

affected during the latest years due to the declines in macrophytes, which are needed

for a successful reproduction. The catches of roach (Rutilus rutilus) and bream

(Abramis brama) have declined during the 1990s. Blooms of the Chrysophyte

Prymnesium spp have caused local fish mortalities along the German and Polish

coasts (WINKLER 2002).


The Belt Sea region is a widespread area with coastlines to four different countries,

and therefore the assessment of the eutrophication situation differs within the region.

The state of the Secchi depth and the oxygen situation in Fig. 10.2 is assessed for the

Danish waters, whereas the situation for zoobenthos in the Polish and German areas

has affected the result in Fig. 10.2. The oxygen situation and its concequences, and

the state of macrovegetation are, however, the main problems in the whole area.

In Poland a clear reduction in pollution load is seen as a consequence of the

switch to a market economy in the 1990s. There was a decrease in agricultural

production and a modernisation or closing of factories. Since the mid 1990s, most of

the towns around the Szczecin Lagoon have got wastewater treatment plants

(WEILGAT 2002). Szczecin with over 400.000 inhabitants is, according to WEILGAT

2002, still an exception. Despite the nutrient reductions, any improvement in the water

quality in the coastal zone of Germany and Poland is not seen, due to the external load

of phosphorus (MEYER-REIL & KÖSTER 2000).

The water quality planning and environmental legislation system are relatively

well developed in both Denmark and Germany. Until 2002, the discharges of nitrogen

and phosphorus in Denmark have been reduced by 77 % and 91 %, respectively

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Belt Sea region

Fig. 10.2. The conceptual model for the Belt Sea region. Hypoxia and damages to macrovegetation are the main problems for the area (see Table 2.1 and Fig. 2.2 for explainations).

(ANDERSEN et al. 2004b). Great reductions in nutrient loads in Denmark are

succeeded by the “Action Plan and Aquatic National Monitoring Programme”, started

in 1988 (RASK et al. 1999a, CONLEY et al. 2000, 2002, ÆRTEBJERG et al. 2003). The

Action Plan has shown positive results in the Danish waters already after a few years

(RASK et al. 1999a, CONLEY et al. 2002). Denmark is a specialized and highly

productive agricultural country. Animal farms dominate in western Denmark and

cereal production in the eastern part. Legislations for point source inputs and changes

in agricultural practices together with an effective sewage treatment have reduced the

nutrient load to Danish waters during the last decade (BERNET 2000b, CONLEY et al.

2000).

The county of Schleswig-Holstein in Germany has restrictions in the

agriculture in the use of fertilizers, collection of manure, and avoiding of direct run-

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Belt Sea region

off. The industries have emission standards, and only 10 % of the households are

outside the central wastewater treatment system (BERNET 2000b). The external load

from both manure and fertilizers in the coastal area in Germany has decreased

(MEYER-REIL & KÖSTER 2000). For both Denmark and Germany further reductions in

nutrient loads seem to be the remaining problem. The fields of concern are the

agriculture, scattered dwellings and deprivations of streams, lakes and wetland

meadows (BERNET 2000a, ANDERSEN et al. 2004b). New facilities for aquaculture are

not permitted in the coastal waters of Denmark (BERNET 2000b).

Information of status of the aquatic environment is continuous and of high

quality in Denmark. The National Environmental Research Institute of Denmark

(NERI 2004) together with environmental research in the Danish counties (e.g. FYNS

AMT 2004b) give accurate information on the situation and trends in the Danish

coastal waters.

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Kattegat

11. THE KATTEGAT The Kattegat is located in the transition area between the proper Baltic Sea and the

Skagerrak (North Sea) (Fig. 11.1). The northern border, in accordance with the Baltic

Sea drainage area, is parallel to the point of Skagen, northern Denmark. The surface

area of Kattegat is 22.387 km2, and the volume is recorded to 421 km3 (HELCOM

1996). The shallow Kattegat has a mean depth of only 23 m (BADEN et al. 1990,

LINDAHL et al. 1998).

At the Swedish west coast, the Laholm Bay in the southern Kattegat and the

Gullmar Fjord on the border to the Skagerrak are well-studied areas. The Laholm Bay

is an area highly influenced by the inputs from farmlands by increased use of

fertilizers and leakage from forest land (BADEN et al. 1990, ROSENBERG 1992). The

Gullmar Fjord, however, has no major sources of water pollution from sewage or

industry any longer. In the Danish part of the Kattegat, the Hevring and Aalborg

Bights are found, where the fjords of Randers, Mariager and Limfjord discharges.

The first signs of eutrophication appeared in the Kattegat in the mid 1970s

when large quantities of filamentous algae were washed ashore, after increasing loads

of nutrients since the 1950s (ROSENBERG 1985, ROSENBERG et al. 1990). There are

Fig. 11.1. The sub-region no. 9: the Kattegat.

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Kattegat

three HELCOM hot spots in the Kattegat catchment area, derived from agricultural

runoff programmes in both Sweden and Denmark, and municipal wastes from the

town of Gothenburg, Sweden (HELCOM-HOT SPOTS 2004).

11.1 Secchi depth According to SMHI (2001), the Secchi depth east of the Anholt island (56°40´N,

12°07´E) in the southern Kattegat varied between 6 and 17 m in August during the

period 1985-1999. In the summer of 2003, the Aalborg Bight had a mean transparency

of 9.5 m, which is significantly higher than the mean value for the period 1985-2003

(ANON. 2004). Similar trend is seen in the Aarhus Bight in the northern Belt Sea

(AARHUS AMT 2004).

11.2 Salinity In the Kattegat, outflowing surface water from the Baltic Sea meets high saline ocean

water from the Skagerrak, which forms the so called Kattegat-Skagerrak front (BADEN

et al. 1990, JAKOBSEN 1997, LINDAHL et al. 1998). The water is therefore strongly

stratified in two generally persistent layers, separated of a halocline at 15 m depth.

Mixing between the layers is restricted to strong winds. Since the tidal amplitude is

less than 20 cm, the water exchange occurs slowly (PIHL et al. 1995, JAKOBSEN 1997).

The Kattegat surface water increases from 8-14 psu near the sills to 25-27 psu in the

border to the Skagerrak. The deep-water salinity varies between 31 and 34 psu

(RASMUSSEN & GUSTAFSSON 2003).

11.3 Oxygen The Kattegat is regarded as an eutrophication-sensitive area; the strong halocline

enhances the primary production and makes the sedimentation of organic material

slower, causing hypoxia in the bottom waters (BADEN et al. 1990). The oxygen

concentrations have been decreasing in the Kattegat since 1970. During the

productive period, an estimated decreasing trend of 0.05-0.1 µg l-1 yr-1 has occurred in

the Kattegat deep water during the period 1970-1995. In the southern Kattegat, the

oxygen saturation in 1973 was 60 %, in 1995 it had decreased to 45-50 % (AGGER &

ÆRTEBJERG 1996, HELCOM 1996).

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Kattegat

Hypoxia was observed for the first time in the south eastern Kattegat in 1981,

and re-occurred every summer and autumn during the rest of the 1980s (BADEN et al.

1990, AGGER & ÆRTEBJERG 1996, HELCOM 1996, ÆRTEBJERG et al. 1998). Serious

and extended oxygen deficiency periods took place in the Kattegat and the Belt Sea

both in 2001 and 2002, which is described in ÆRTEBJERG et al. (2003). The

development of the depletion started already in June, and in 2002 it continued until

October. Especially the open sea areas and the southern Kattegat were affected and in

September 2002 an area of 16.000 km2 suffered from hypoxia, of which 5500 km2 had

concentrations below 2 ml l-1 (ÆRTEBJERG et al. 2003). In the Hevring Bight, oxygen

concentrations < 4 ml l-1 have occurred in totally 9 years since 1989, for Aarhus Bight

three of the last 13 years. Warm water temperature in the summer 2003 caused serious

oxygen depletion in both Bights in August-September (ANON. 2004). Except for the

Aarhus Bight, the oxygen situation was relatively good along the Kattegat coast in

August-September 2004 (AARHUS AMT 2004).

11.4 Nutrients Changes in nutrient concentrations in the Kattegat are hard to detect, due to the strong

horizontal gradients and the short residence time of the surface water (1-2 months)

(HELCOM 1996). Dense nutrient-rich water from the Skagerrak causes net transport of

dissolved inorganic nutrients towards the Baltic Sea during the summer period. The

transport may reverse during winter when the surface concentrations of DIN and DIP

increase (RASMUSSEN & GUSTAFSSON 2003). A remarkable increase in inputs of

nutrients to the Kattegat took place during the 20th century. Nitrogen has increased by

a factor of 6, and phosphorus by a factor > 10 (ROSENBERG 1992, ROSENBERG et al.

1992). According to RASMUSSEN & GUSTAFSSON (2003), the pools of phosphorus

(both DIP and tot-P) have decreased since the 1980s, while nitrogen pools have been

more variable. In the 1990s the transport between the Kattegat and the Skagerrak

decreased, but the Belt Sea continued to export DIN to the Kattegat (RASMUSSEN &

GUSTAFSSON 2003).

The inorganic forms of nitrogen and phosphorus increased from 1968 until a

peak was reached during the period 1979-1988 (HELCOM 1990, 1996, STOCKHOLM

UNIVERSITY 2004a). Since the beginning of the 1990s, both total nitrogen and

phosphorus started to decrease. In the western part of the Kattegat the declines are

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Kattegat

significant. Tot-N has changed from a annual mean concentrations of 600 µg l-1 in

1991 to 200 µg l-1 in 2003, for tot-P the annual mean concentrations of 35 µg l-1 in

1993 can be compared to 15 µg l-1 in 2003 (ANON. 2004). According to the

assessment in Table 2.1, both nitrogen and phosphorus have changed from a very high

to a very low class during the last 15 years.

Better wastewater treatment has lowered the output of phosphorus, and low

precipitation and runoff in the beginning of the 1990s have lead to reductions in the

output of nitrogen (AGGER & ÆRTEBJERG 1996). The total amount of phosphorus in

the Kattegat has been stable in the last four years; 6000-7000 tons in the summer

period. For nitrogen the corresponding value lies between 100.000-120.000 tons

(SMHI 2004). According to ROSENBERG et al. (1990), the best way to reverse the

eutrophication in the Kattegat is reducing the supply of nitrogen.

11.5 Chlorophyll a and primary production The chlorophyll a in Kattegat has increased from a mean value of 1 µg l-1 in 1970 to 2

µg l-1 in 1988 (SCHULZ et al. 1992, JOSEFSON et al. 1993). Maximum values around

20 µg l-1 may be measured during summers (HELCOM 1996). From 1989 to 2002, the

growing season mean concentration of chlorophyll a seems to be around 2 µg l-1 in

offshore areas of the eastern part of the Kattegat, while Swedish inshore waters are

around 2.5 µg l-1. Even higher concentrations are observed near the mouth of

freshwater sources, as the Göta River. The northern Kattegat also seems to have

higher levels than the offshore areas in the central and southern regions (HÅKANSSON

2003, CARSTENSEN et al. 2004). The chlorophyll a recording from the Danish Bights

of the Kattegat in 2003 was higher than the mean for 1989-2002 in February, June and

December, but lower for the rest of the months (ANON. 2004).

The mean annual primary production for the Kattegat during the years 1981-

1985 was 144 g C m-2, and indicated an increase with 25-30 % compared with the

1970s (ROSENBERG 1992 and references therein). In the Gullmar Fjord, the primary

production has undergone three phases: a “background” level of 100 g C m-2a-1 before

1950, an increasing period to values > 200 g C m-2a-1 from 1960s to mid-1980s due to

the eutrophication, and since then a slight increase (close to 250 g C m-2a-1) due to

climate forcing (strong positive NAO index; RICHARDSON & HEILEMANN 1995,

HÅKANSSON 2003). In relation to other nutrient-rich areas, the primary production in

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Kattegat

the Kattegat is not high, according to BADEN et al. (1990). The annual primary

production for 2003 in the Aalborg Bight was 140 g C m-2, which lies under the mean

production of 157-224 g C m-2 for the period 1998-2002 (ANON. 2004).

11.6 Phytoplankton and algal blooms Phytoplankton blooms in the Kattegat are generally caused by mass occurrences of

dinoflagellates, chrysophyceans and prymnesiophyceans, while blooms of

cyanobacteria are a rarely occurring phenomenon (ROSENBERG et al. 1990, SCHULZ et

al. 1992). The first mass occurrences of cyanobacteria in Danish waters were

recordings of Nodularia spp in Aarhus Bight in 1975 (ÆRTEBJERG et al. 1998).

The highest bloom frequencies during the 1990s were observed along the

Danish coast and in the southern Kattegat that is influenced by the outflow from the

Baltic Sea. The blooms are generally local in nature (CARSTENSEN et al. 2004). The

dinoflagellates have increased in abundance (SCHULZ et al. 1992), for example the

potentially toxic Gyrodinium mikimotoi has been observed as blooms since 1960, and

is now a common species in the Kattegat (KAAS et al. 1999). In 1988, dense blooms

of the Prymnesiophyceae Chrysochromulina polylepis caused problems in the

Kattegat-Skagerrak area (see Chapter 10: Belt Sea region; ROSENBERG et al. 1990,

KAAS et al. 1999). The Raphidophyceae Chattonella spp formed blooms in the spring

of 1998 and 2001. The algae may cause damage on fish gills and the occurrence in the

Kattegat-Skagerrak is believed to be eutrophication-related (HÅKANSSON 2003).

Overall, non-harmful diatoms as the genus Rhizosolenia spp and Ceratium spp

represented more than half of the bloom observations according to CARSTENSEN et al.

(2004). Toxic algae caused no damages in Kattegat in 2003 (ANON. 2004).

11.7 Macrovegetation The salinity gradient plays a major role in the distribution patterns of benthic

macroalgae in the Kattegat, where the eulittoral vegetation is more similar to the

Skagerrak and the North Sea. The flora and fauna have higher species richness due to

the higher salinity, and no single algal species may dominate as in the Baltic Sea. The

number of registered macroalgal species in the northern Kattegat is 325 (HELCOM

1996, DAHLGREN & KAUTSKY 2002).

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Kattegat

Long-term measurements show a decline in Fucus species since the end of the

19th century, especially in the central and southern Kattegat (ROSENBERG et al. 1990,

NIELSEN & DAHL 1992, HELCOM 1996). Changes in species composition, new depth

distributions and narrower ranges are observed both on the Swedish and Danish coasts

(ROSENBERG et al. 1990, ÆRTEBJERG et al. 1998, HÅKANSSON 2003). The depth

distribution of Zostera marina in the Danish Kattegat water was 8.5-11 m in 1900

(ÆRTEBJERG et al. 1998). In 2001-2003 the mean depth was 3 m and maximum depth

5 m for Z. marina in the Hevring Bight (ANON. 2004). Along the Swedish Skagerrak

coast a decline in eelgrass up to 60 % is observed in the last 10-15 years (BADEN et al.

2003).

11.8 Ephemeral algae and drifting algal mats Macroalgae favoured by the increase in nutrient supply in the Kattegat are species

such as the Rhodophyceae Erythrotrichia spp, the Phaeophyceae Ectocarpus spp and

Pilayella spp and the Chlorophyceae Chaetomorpha spp, Cladophora spp and Ulva

spp (HÅKANSSON 2003). The species composition of the ephemeral algae may differ

(PIHL et al. 1995). The filamentous green algae have increased in dominance since the

1970s along the Swedish Kattegat coast (BADEN et al. 1990, ROSENBERG et al. 1990,

HELCOM 1996). According to estimations in the mid-1990s, 30-40 % of all the soft-

bottoms along the Swedish Kattegat coast were covered by filamentous algae and

over 90 % of the algal biomass was found drifting (PIHL et al. 1995, 1999). Drifting

algae are also reported from stone reefs in the southern Kattegat, but the relation to

eutrophication is not clear (NIELSEN & DAHL 1992).

A European Commission supported project in 1996-2000 studied harvesting of

filamentous and drifting algae in shallow bays. The experimental areas were the

Swedish Kattegat-Skagerrak coast and the Åland archipelago in Finland. Results are

presented at COUNTY ADMINISTRATION VÄSTRA GÖTALAND, SWEDEN (2001).

11.9 Zoobenthos The macrofauna in the open Kattegat has undergone clear reductions when benthic

biomasses from 1911-1912 and 1984 are compared. The former dominating groups,

echinoderms and molluscs, have been replaced by ophiuroids and polychaetes. Also

the size of the species has diminished (JOSEFSSON & JENSEN 1992). However,

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Kattegat

ROSENBERG et al. (2004), classified the environmental quality in the northern and

offshore Kattegat to a high level in 1990 according to the benthic status. Along the

Swedish coast some stations were classified as poor (ROSENBERG et al. 2004).

Hypoxia has caused severe reductions in the benthic fauna in the Laholm Bay,

where especially crustaceans, e.g. Norway lobster, Nephrops norvegicus, have been

affected (BADEN et al. 1990, PEARSON & ROSENBERG 1992). A recovery of fauna was

seen in the period 1993-1998, but a new decline was reported in 1999-2001

(GÖRANSSON 1999, HÅKANSSON 2003). Monitoring along the Swedish west coast in

2002 showed no signs of oxygen depletion. The ophiuroids Amphiura filiformis and

A. chiajei dominated at both shallow and deep bottoms. At bottoms deeper than 50 m,

the polychaets Heteromastus filiformis and Scalibregma inflatum were also abundant,

probably due to high nutrient levels. The bivalve Mysella bidentata was a common

species at shallow localities (AGRENIUS 2003).

Events of oxygen depletion at the Danish Kattegat-coast have caused crashes

of the zoobenthic community, but the periods of recolonisations seem to be relatively

short (FELLESEN 1992, KAAS et al. 1996). The hypoxic-anoxic period in 2002 caused a

40 % reduction in species number in 2003 in the Danish Hevrings Bight and for

example the former common echinoderms totally disappeared. Also the abundances

were reduced, for the polychaets with almost 90 %. The stable stocks of the sea urchin

Echinocardium cordatum in 1998-2001 give a sign of good oxygen condition before

the collapse in 2002. In 1991 and 1998, the toxic dinoflagellate Gyrodinium aureolum

reduced the zoobenthos (ANON. 2004). For the summer 2004, a period with oxygen

deficiency took place in Aarhus Bight, but AARHUS AMT (2004) reports no changes in

the stock of E. cordatum.

11.10 Ichthyofauna On the Swedish west coast, over 50 fish species are recorded on shallow bottoms

(PIHL & WENNHAGE 2002). Flatfish and cod (Gadus morhua) have been the

dominating demersal species in the Kattegat. The total catches of these species in the

open sea have decreased by 40 % since the 1970s and by 70 % since the 1960s

(BADEN et al. 1990). According to HÅKANSSON (2003), there is no clear link to a

deterioration in the environment. However, close to the coast, a positive correlation

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Kattegat

between increased catch yield and eutrophication during the 1990s is seen, but not

proven (SANDSTRÖM 2000).

Cod usually spawn close to the shore and the spawning area in the Kattegat

has been concentrated to the south eastern part until the 1980s, when the spawning

stock was reduced in biomass and spread to the whole southern Kattegat (BADEN et

al. 1990). Hypoxic bottom cause this escape reaction, and has happened irregularly in

the Laholm Bay since 1959 (BADEN et al. 1990, HELCOM 1990). The areas with

hypoxia and the amounts of suffering fishes have extended during the past two

decades (BADEN et al. 1990) and fish mortality due to hypoxia is reported

(ROSENBERG et al. 1990). Filamentous algae in shallow areas have also impact on the

recruitment of fish larvae (SANDSTRÖM 2000). Therefore, the spawning areas in the

North Sea have become more important for the fish recruitment in the Kattegat-

Skagerrak area (HÅKANSSON 2003). The Laholm Bay has remained as the main

nursery area for flatfish in the Kattegat (BADEN et al. 1990, PIHL et al. 1995).


The anthropogenic loads have started to diminish already in the 1970s (HELCOM

1996), but the improvements are slow. The Kattegat is still affected of the over-

enrichness of nutrients. However, the input of phosphorus and nitrogen has decreased

since the late 1990s, which is indicated in Fig. 11.2. Excessive growth of filamentous

algae, occurrences of hypoxia and anoxia, as well as elimination and changes in

habitats and conditions for zoobenthos and fish, are still nuisances of special concern

in the Kattegat region. Toxic cyanobacterial blooms are not a problem for the region,

but algae species producing neuron toxins may cause for example ASP (amnesic

shellfish poisoning; ANON. 2004).

The nutrient loads are mainly derivered from the agriculture and the

atmosphere. The atmospheric deposition stands for > 30 % of the nitrogen load

(HELCOM 2003a). The deposition of total nitrogen is reduced by 12 % in 1996-2000

compared to 1991-1995 (HELCOM 2003a). Measurements in the reduction of direct

nutrient loads are the most important way to diminish the eutrophication problem

even further (BERNET 2000a, KARLSON et al. 2002). According to CARSTENSEN et al.

(2003), a 50 % reduction in the external nitrogen inputs to the Kattegat would

correspond to a 20 % reduction in the net primary production.

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Kattegat

Fig. 11.2. The conceptual model for the Kattegat. Hypoxia and its consequences for zoobenthos and fish, togehter with excess growth of filamentous algae are the main problems for the area (see Table 2.1 and Fig. 2.2 for explainations).

The measures taken in Sweden include restrictions in agricultural methods,

e.g. limits on animal densities, manure management and taxes on fertilizers. Single

households outside the sewage treatment net have nowadays infiltration and other

more effective methods than the former sludge-reduction (BERNET 2000b). The action

programme in Denmark and its progress is already discussed in Chapter 10: The Belt

Sea Region.

130

Conclusions

12. CONCLUSIONS

Eutrophication is a serious problem in the whole Baltic Sea area. The same physico-

chemical drivers are involved in the process of the nutrient over-enrichness, but the

degree of change varies within and between sub-regions, as well as the primary and

secondary effects and consequences.

In Fig. 12.1 the conceptual models (Fig. 3-11.2) from the Baltic sub-regions

are summarised. Based on the Fig. 12.1, the sub-areas may be divided into three

groups: the Gulf of Bothnia and the Swedish coastline towards the Baltic proper are

the healthiest parts of the Baltic Sea. The Archipelago region, the Gulf of Riga and the

Belt Sea region are all severely affected by eutrophication, but the situation also

shows some improvement, compared to the late 1990s. However, a national

assessment of the state of the Finnish coastal waters, published in January 2005,

indicates a deterioration of the water quality in the inner archipelago of the

Archipelago Sea, compared to the situation in 1993-1997 (SYKE 2005). The positive

changes in fish stocks in the Gulf of Riga and the increased Secchi depth in the Belt

Fig. 12.1. A summary of the conceptual models in Fig. 3-11.2 for the nine sub-regions of the Baltic Sea. The changes in relation to eutrophication are explained with: white dot – no changes, green – small-moderate changes, yellow – severe changes, and red – very serious changes. A combination of two alternatives is expressed with a two-coloured dot. The black circles indicate parameters where the assessment of change differs from RÖNNBERG (2001). The state in 1990s-2001 is presented in the first dot; the second shows the present status, i.e. in almost all cases a slight improvement for the most recent period.

131

Conclusions

Sea area can clearly be seen as positive consequences of measures taken to combat the

eutrophication.

The Gulf of Finland, the Gulf of Gdansk, the Central Baltic and the Kattegat

are the most seriously eutrophied areas in the Baltic Sea region. The Gulfs of

Finland and Gdansk are both extensions of the Baltic proper and are not buffered by

archipelago areas. The annually occurring events of harmful algal blooms at the

surface and the areas with oxygen deficiency in the bottom water may be considered

as the largest problems. In the Gulf of Gdansk positive signs in both the input of

nutrients and the state of the fish community have been seen in the recent years. The

Gulf of Finland is the only sub-region where the eutrophication process has been

aggravated since my previous assessment in RÖNNBERG (2001). The hypoxic bottom

conditions have even further damaged the benthic fauna in the Gulf of Finland. This is

in accordance with the assessment of the water quality in the Finnish aquatic areas,

where the Gulf of Finland has changed from good status in the open sea and

satisfactory in the coastal areas in 1993-1997, to an overall satisfactory level in 2000-

2003 (SYKE 2005).

In the Kattegat the situation is somewhat different. As the Kattegat, together

with the Skagerrak, forms a transition area between the Baltic Sea and the North Sea,

the natural conditions influence the sensitivity to eutrophication. The large-scale water

current systems cause strong stratification in the water column, which reduces the

vertical transport of oxygen (ÆRTEBJERG et al. 2003). Therefore, periods with oxygen

depletion are almost an annual occurring phenomenon.

Large-scale, basin wide calculations and predictions for the Baltic Sea is an

important framework for reducing the eutrophication problem. For example, the

decision-support system presented in the MARE’S NEST (MARE 2005; the NEST

programme, including manual, is available on the Internet), shows how reductions in

one sub-region may influence the entire Baltic Sea system. The programme can also

provide cost-effective strategies to counteract the Baltic eutrophication.

On the other hand, the differences and degree of effects in the various sub-

regions indicate the need for area-specific measurements to reach positive reductions

both spatially and temporally in a small-scale perspective. The knowledge achieved at

the small-scale level is a valuable background for large-scale modelling. Adequate

and integrated monitoring programmes are important sources of data information. For

example, long-term monitoring data of physcio-chemical parameters provides

132

Conclusions

background trends and patterns of eutrophication, both in time and space. The same

data may also be used in attempts to roughly predict the oxygen situation on a year to

year basis, by using fuzzy logical methods.

To achieve a sustainable aquatic environment and suppress the eutrophication

process, the physico-chemical and ecological effects must link directly to socio-

economy. The human demands are an extremely important aspect, when the carrying

capacity of an ecosystem is discussed (DUNLAP 1993). All factors that control and

influence the effects and consequences of the eutrophication-process modify the

ecosystem. Regardless of whether this happens on a small- or large-scale level,

permanently or episodically, the process is complex. A conceptual model is a useful

tool to explain the network of events. An essential aim for science is to reach out to

the public, managers and politicians. To quote ELLIOTT & DE JONGE (1996): “if

scientists do not help to provide the answers then the managers and politicians will

provide them anyway”. A challenge for science is to learn to be better at explaining

threats, solutions and predictions for a mutual trust with the general public and a

fruitful interdisciplinary co-operation with other scientists.

The DPSIR-approach is another framework which indicates the

interdisciplinary interrelations of the eutrophication-process. In Fig. 12.2 an attempt to

show how this thesis supports and provides information for the DPSIR thinking is

presented. Beside the five corner-stones in the framework, two more boxes are added

to the model. Institutional barriers may prevent or delay the socio-economic drivers or

the environmental pressure, which are a factor to consider in the process. All

pressures on the environment are not of anthropogenic origin. The external variability

and uncertainty in nature, such as climate change, sea-level rise and storm events, is

another aspect of a dynamic ecosystem (TURNER 2000, MEE 2004).

The eutrophication-related changes must also be considered in relation to other

pressures and threats to the health of the ecosystem. The interaction with other

environmental problems, such as toxic compounds, erosion, over-fishing and invasive

species, influence the state and resilience of an aquatic system. Integration of several

complex problems in an ecosystem will lead to an even further intricate

horrendogram.

This thesis shows that in spite of being a highly complex problem,

eutrophication in the Baltic Sea can be reduced by combining approaches from small-

scale local effects to international large-scale measures.

133

Conclusions

Fig. 12.2. A schematic figure of the DPSIR-framework, where the contribution from this thesis is included in brackets (modified from MEE 2004).

134

Acknowledgement

13. ACKNOWLEDGEMENT

A bunch of thanks goes to my supervisor, Erik Bonsdorff. Without your engagement and inspiration, perhaps this work had never went through its metamorphosis from boring project report to licentiate thesis and further to a doctoral one?!? Throughout the process, you have always guaranteed that we can work every problem out. The discussions on science as well as on life in general have been pleasurable. I send my co-authors, besides Erik, Kati Aarnio, Malin Lönnroth and Mikael von Numers, lots of thanks. It has been interesting working with you, because it seems to be a myriad of ways to study the same eutrophication phenomenon. I am also glad to being a part of the MARE network. The programme is a good multidisciplinary micro-cosm, and I feel fine to be a little cog in the decisions-support machinery. I do not either forget all friendly MARE meetings with excellent food and perfect opportunities to come together with both same level and senior (even real celebrities in biological research!) colleagues. The long and winding road to a PhD exam has also been much lighter and easier in good company. Therefore, I direct a collective thank to the colleagues and students at the Environmental and Marine Biology. Special thanks to Johan Lindholm for help with the maps in this thesis in particular and for assistance with various computer issues in general, and to Malin Lönnroth and Britt-Marie Jakobsson, my first two graduate students. It is something special to supervise and be supervised at the same time. All kind persons almost across the universe that have delivered me with scientific papers, reports and other information are acknowledged. Without you it had been much harder summarising the state of knowledge in the Baltic sub-regions. The warmest thanks to Sif Johansson at Sweden’s Environment Protection Agency and Dan Conley at NERI in Denmark for spending time at hard days´ nights and Easter holiday to review the thesis and for given me the most positive, valuable and constructive criticism. And what is life without a little help from my friends! To the best girls, both “biolog-flickorna” and others, thank you for listening, joy and friendship! From me to You, the deepest thanks to my “biology”-family! My parents Birgitta and Ola have unconsciously, without any pressure, raised me to be what I am today. I should have known better when brother Peter chose the same field of study... Finally, kisses to Mats, my greatest supporter. Thank You for giving me a taste of honey even on a rainy day, being my perfect popular-science referee, and for your important role at the Big Reference Control for this thesis. I look forward to our Magical Mystery Tour in August! I will also remember the Åbo Akademi University and the MARE project that have financed this work. Åbo in April 2005

135

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