Hydrobiologia 138: 9-24, (1986). 9 © Dr W. Junk Publishers, Dordrecht

Succession of phytoplankton in a deep stratifying lake: Mondsee, Austria

Martin Dokulil & Claudia Skolaut Institut fiir Limnologie der Osterreichischen Akademie der Wissenschaften, Gaisberg 116, A-53JO Mondsee, Austria

Keywords: seasonality, phytoplankton, seasonal succession, oligotrophication, lakes, ecology

Abstract

Phytoplankton numbers, biovolume, chlorophyll-a and various physico-chemical characteristics were fol­lowed at weekly intervals in Mondsee, Austria during the year 1982. Secchi-disk transparency varied from 10 m in winter to 2 m in September. Prior to the onset of stratification phosphate-phosphorus concentration was 4 p,g I-I decreasing to undetectable values thereafter. Nitrate-nitrogen dropped from 590 p,g I-I to about 100 p,g 1-1 during the same time. The vernal bloom was dominated by Asterionella formosa Hass. which abruptly declined after silicon depletion. Spring growth ceased in early June, when Tabellaria flocculosa (Lyngb.) Kiitz var. asterionelloides Grun. dominated.

Oscillatoria rubescens D.C. and Microcystis aeruginosa Kiitz. dominated summer and early autumn fol­lowed by the chrysophyte Dinobryon diver gens Imh. and D. sociale Ehr. which formed up to 69070 of total biovolume in October. Thereafter diatoms and Cryptophyceae (Rhodomonas lacustris Pascher and Ruttner, Cryptomonas pusil/a Bach.) became abundant again.

Maximum chlorophyll-a concentration in the epilimnion (16 p,g I-I) was reached during spring growth of the diatoms. During summer higher chlorophyll-a levels were always associated with the metalimnetic layer of Oscillatoria.

Compared with earlier studies, both the total biovolume and the share of Oscillatoria rubescens signifi­cantly decreased because of reduced nutrient loading of the lake and wash-out of Oscillatoria (theor. renewal time of the lake: 1.7 years).

Introduction

Cyclic fluctuations in abundance and species composition constitute a major characteristic of the freshwater phytoplankton. Despite the existence of a large and expanding literature describing sea­sonal succession, understanding of the factors that regulate the wax and wane of algal populations is still far from complete. Recently some progress has been made towards a generalized hypothesis to ac­count for seasonal periodicity (Reynolds, 1980), and successional pathways have been proposed for lakes of different trophic status (Reynolds, 1982). Although based on long-term studies in a variety of

lakes, these valuable generalizations still suffer from limited availability of detailed data and can therefore not be applied universally. More informa­tion is needed on the depth-time distribution of the phytoplankton in various freshwater systems to ex­pand the above-mentioned hypothesis. To accom­plish this, frequent (e.g. weekly) sampling intervals and consideration of the loss processes are impor­tant (Sommer, 1981b; Crumpton & Wetzel, 1982; Reynolds & Wiseman, 1982; Reynolds et al., 1982).

This paper describes the seasonal periodicity of the phytoplankton in a deep stratifying alpine lake, Mondsee, Austria, during the first year of an inten­sive weekly sampling programme.

10

Mondsee is situated in central Austria (Fig. 1). Morphometrical and hydrological data are summa­rized in Table 1. The lake underwent considerable changes due to eutrophication in the late 1960s (Danecker, 1969; Findenegg, 1969). Oscil/atoria rubescens D.C. was first recorded in autumn 1968, and was a nuisance until a sewage treatment plant was put into operation during 1974. Recent investi­gations (Schwarz, 1979, 1981) demonstrated a sub­stantial drop of the Oscil/atoria population and a reduction in total phytoplankton biovolume. The lake has been monitored since 1978 through the Austrian Eutrophication Programme and intensive­ly studied since fall 1981 by the Limnological Insti­tute of the Austrian Academy of Sciences, Dept. Mondsee.

Table 1. Mondsee, morphometrical and hydrological data (from Miiller (1979) and Wurzer (1982».

Altitude above sea level Catchment area Surface area (A) Maximum depth Mean depth Volume (V) Maximum length Shore length Average outflow

Theor . water renewal

Methods

481 m 247 .2 ·1()6 m2

14.2 ·1()6 m2

68.3 m 36.0 m

510 ·1()6 m 3

10800 m 28.3·1()6 m

9.2 m3

S-I

ca 1.7 years

Samples were collected during the year 1982 at weekly intervals (approx. biweekly between Decem­ber and March) in the western part of the lake (z=45 m, Fig. 1) from the surface down to 40 m depth. The depth of the 1070 light level equal to eu-

MONDSEE N

& lkm

Fig. 1. Location and contour map of Mondsee, Austria.

photic zone depth, was located from radiation measurements using an underwater quantum sen­sor (LI-COR, USA). A transmissometer (Schenk, Austria) was used to locate the position of metalim­netic algal layers. Phytoplankton biovolume was es­timated from cell counts and size measurements on an inverted microscope (Lund et al., 1958) using ge­ometric approximations (Rott, 1981). Chlorophyll­a, corrected for degradation products, was meas­ured spectrophotometrically after extraction in cold methanol following the procedure of Holm­Hansen & Riemann (1978). Standard analytical techniques were used for all other chemical quanti­ties (Mackereth et al., 1978).

Results

From early December to the end of April the lake was completely mixed, and partially ice-covered from 14 January to 28 March (Fig. 2). Lowest tem­peratures (below 3°C) occurred throughout the wa­ter column during this period. Stratification be­came progressively apparent in May, developing a clear thermocline between 10 and 15 m in June.

m

5

10

15

20

1982

3

J F M A M J

11

Highest surface temperature (23.3 0c) was recorded on 22 July. Wind mixing at the end of September deepened the epilimnion. As a consequence a strong thermal gradient was observed in October between 12 and 15 m. Turbulence and convectional cooling gradually destroyed stratification during November and re-established isothermal conditions on 23 December.

Secchi-disk transparency varied between 10 m on 2 January and 2 m on 17 September. The depth to which 1070 of surface photosynthetic active radia­tion penetrates is related to secchi-depth by a factor of 2.5. Consequently the limits of the euphotic zone were 25 and 5 m respectively.

On 13 April, before the onset of thermal stratifi­cation, nutrients were distributed more or less uni­formly with depth (P04-P 4 p.g 1-1; NOrN 600 p.g 1-1; NH4-N 10 p.g 1-1). Thereafter, phos­phate-phosphorus concentration in the epilimnion decreases to an undetectable level and mtrate­nitrogen to about 100 p.g 1-1. Ammonium-nitrogen reached the highest concentration of 650 p.g I-I in the hypolimnion on 13 September (Fig. 3, Jagsch & Bruschek, 1984).

Significant increase of numbers and biovolume

J A s a N o Fig. 2. Mondsee, 1982. Secchi-disk depth and depth-distribution of isotherms (0C); arrows indicate sampling dates.

12

m O~----------.---r-------~" 2 5

10

I I I I I I

20 I I I I

30 10

40

50

60

68~~~~~~~~~~-r~~

o~ __ ~ __ ~~ __ ~ ____ ~~~~ ~ (b)

10 500

20

30

40

50

600 500 ) n ~U

60

68~~~ .. ~~~~~~~~~ J FMAMJ JAS ONO

10 5 1

10

n \)0 (d

10

J FMAMJ J ASONO

Fig. 3. Depth-time diagrams of seasonal dissolved nutrient concentrations, all in p,g I-I, of NH.-N (a), NOl-N (b), PO.-p (c) and total-P (d), Mondsee, 1982. (After Jagsch & Bruschek, 1984).

of the phytoplankton assemblage began in March, peaking on 26 April before the onset of thermal stratification (Figs 4 and 5). This period was domi­nated by the diatoms Asterionella formosa Hass., Tabellaria flocculosa (Lyngb.) Klitz. var. asterionel­loides Grun., and Melosira italica (Ehr.) Klitz. (Fig. 9B). Addition~1 species included Fragilaria crotonensis Kitton, Synedra acus Klitz var. an­gustissima Grun., eye/otella comensis Grun., C.

bodanica Eul. and Stephanodiscus astraea (Ehr.) Grun. Comparison of biovolume and chlorophyII-a isopleths (Figs 4, 5) with the depth-time distribu­tion of isotherms (Fig. 2) idicates that the phytoplankton was homogeneously distributed within the water column as long as turbulent, mixed conditions prevailed. Thermal stratification restricted growth to the epilimnion. As a conse­quence dissolved-reactive silicon became depleted

1

80

20

m 5

10 1 1

20

(a)

1982

J F M

13

A M J J A s o N o Fig. 4. Total phytoplankton biomass in Mondsee, 1982: (a) Integrated values (cm3 biovolume per m2) for the total water column (0-40 m; open symbols) and for the euphotic zone (filled symbols) and (b) vertical distribution of total biovolume. Isopleths in mm3

I - I. Depth markings and arrows indicate depth and time at which samples were collected.

14

mg

400

20

10

10 12 1

20

30

(0)

1982

J F M A M J J A s o N o Fig. 5. Chlorophyll-p' concentration in Mondsee 1982: (a) integrated values (mg chl-a m- 2) for the total water column (0-40 m; open symbols) and for the euphotic zone (filled symbols) and (b) vertical distribution of chlorophyll-a concentration. Isopleths in /lg 1-1. Depth markings and arrows indicate depth and time at which samples were collected.

m

4

m

5

10 12 15

30

(a)

1982

(b)

J F

15

M A M J J A s o N o Fig. 6. Depth-time distribution of (a) dissolved reactive silicon concentration (p.g I-I) and (b) biovolume of diatoms per unit volume of water (mml I - I) in Mondsee, 1982. Depths markings and arrows indicate depth and time at which samples were collected.

to concentrations below 500 Jlg I-Ion 19 May (Fig. 6a). A heavy fungal infection during this peri­od resulted in an abrupt decline of the Asterionella population. Spring growth of the diatoms ceased

after 4 June when Tabellaria dominated (Figs 6, 9B). Their growth is reflected in total biovolume (Fig. 4) but not in chlorophyll-a concentration (Fig. 5). Similar results were observed during the autumn

16

growth period of the diatoms (24 September to 23 December; Fig. 6) when Tabellaria was again a ma­jor component of the population (Fig. 9B). The discrepancy is probably due to low pigment content of the Tabellaria cells.

5

10 12 15

20

30

40

m

5

10 12 15

20

J

1982 (a)

1982 (b)

F M A M

0 0.1

J

Cryptophytes and dinoflagellates were other im­portant components of the phytoplankton assem­blage during spring (Fig. 8), but had their main growth period between 26 June and 7 August after the decline of the diatom population (Fig. 7a, b).

0.1

J A s o N o

m

5

10 12 15

20

30

m

5

1~ 15

20

30

1982 ( c)

J F

1982 (d)

17

M A M J J A s o N o

Fig. 7. Depth-time "distribution of the biovolume of algal groups in Mondsee, 1982. Isopleths in mm l I-I: (a) Cryptophyceae; (b) dinoflagellates; (c) Chrysophyceae; (d) blue-green algae. Depths markings and arrows indicate depth and time at which samples were collected.

18

Rhodomonas lacustris Pascher and Ruttner, Cryp­tomonas erosa Ehr. and C. marssonii Skuja were present through the year (Fig. 9B) and represented the main components during winter and spring (Figs 8, 10).

Peridinium cf. willei Huitf.-Kaas and Gym­nodinium helveticum Pen. represented the dino­flagellates between March and May, whereas Ce­ratium hirundinella O.F. Mull. formed the maximum between 9 July and 3 September (Figs 7b, 8 and 9B).

The Chrysophyceae, represented primarily by Dinobryon diver gens Imh. and D. sociale Ehr., first appeared on 1 April. Two small growth phases paralleled the diatom peak in spring (Fig. 7c). The main growth period was between 3 September and 4 November (Figs 7c, 9A), when Chrysophyceae oc­cupied 13 070 to 69% of total phytoplankton bio­volume (Fig. 8).

Blue-green algae were most abundant between 11

1

J F M A M

blue-green algae

J

June and 4 November (Figs 7d, 8). An Oscillatoria rubescens D.C. population reached a metalimnetic maximum at 10 to 15 m (Figs 4, 9A) followed by massive development of Microcystis aeruginosa Kutz. in the epilimnion (12 mm3 I -I at 5 m depth on 3 September, Fig. 9A). Additional species in­cluded Anabaena flos-aquae (Lyngb.) Breb., Chroococcus dispersus (Keissl.) Lemm. and Gom­phosphaeria lacustris Chod. (Fig. 9A). Aphanizomenon flos-aquae (L.) Ralfs was not de­tected before 4 November but became increasingly important afterwards.

Numerous green algal species never contributed significantly to total phytoplankton biovolume. Among the species present throughout the year were Oocystis lacustris Chod., Monoraphidium contortum Kom.-Leg. and Eudorina elegans Ehr. Additional species during July and August includ­ed Pandorina morum (Mull.) Bory, Phacotus len­ticularis (Ehr.) Stein, Planktosphaeria sp. G.M.

J A s o N o diatoms 111111111 11 = dinoflagellates

= Cryptophyceae l1li= green algae

Fig. 8. Relative abundance of algal groups in Mondsee, 1982 (in 070 of total biovolume). Arrows indicate sampling dates.

19

(a )

n

I

/'1'"l (( ) (d) E ::J...

'-D 0

01 0

Q)

E -2

::J

0 > 0

.n 01 0

'J

n R

Fig. 9. Seasonal changes of average biovolume in the euphotic zone, in log·l()6 J.lm3 1- ', for various algal species, Mondsee, 1982. A) Blue-green algae and Chrysophyceae: (a) Oscil/atoria rubescens, (b) Microcystis aeruginosa, (c) Gomphosphaeria lacustris, (d) Aphanothece c/athrata, (e) Dinobryon divergens, (f) Dinobryon sociale. B) Diatoms, dinoflagellates and Cryptophyceae: (a) Asteri­one/la jormosa, (b) Fragilaria crotonensis, (c) Tabel/ariajlocculosa var. asterionel/oides, (d) Melosira italica, (e) Ceratium hirundinel­la, (f) Cryptomonas marssonii, (g) Cryptomonas erosa, (h) Rhodomonas lacustris. -

20

(a) ( b)

A R

(c) (d)

..---!.....,

m E ::L

-D C> - 1

R n R

Ol 0

OJ ( e) (f) E ::l

0 > 0

..Cl

Ol 0

-1

-2 0 n

(h)

n o

Smith and Sphaerocystis schroeteri Chod. The seasonal succession of species contributing

significantly (more than 10070) to total biovolume in the euphotic zone is depicted in Fig. 10. Several phases can be distinguished:

1. Cryptomonas-Rhodomonas phase: Winter plankton with highest biovolumes of Cryptomonas marssonii and Rhodomonas lacustris. Additional species are Cryptomonas erosa and C. pusil/a with minor contributions from Asterionella formosa and Tabellaria flocculosa var. asterionelloides.

2. Spring diatom phase: Asterionella formosa built the first maximum followed by Melosira itali­ca. If this is a persistent sequence the phase might be separated into two individual steps. Gymnodini-

0/0

30 10 I

50

10

50

10

50

10

50 1 H[ A 2 G!l'1 L

10 3 APH C

50 1 DIN D 2 DIN S

10

1 TAB F 50 2 FRA C

3 CRY M

10

21

um helveticum was co-occurring during the Asteri­onella peak.

3. Tabellaria phase: High biovolumes during the onset of stratification.

4. Oscillatoria-Ceratium phase: Summer phase during July and August, followed by Ceratium hirundinella.

5. Blue-green algal phase: Pronounced develop­ment during August and September. Microcystis

'aeruginosa occurred at the beginning of this phase, followed by Gomphosphaeria lacustris and Aphanothece clathrata (Fig. 9A). Microcystis might be separated into a particular phase if the type of occurrence is regular.

6. Dinobryon phase: Autumnal development of

IV

1 AST F 2 MEL I 3 GYM H

TAB F

1 OSC R 2 CER H

Fig. 10. Succession of phytoplankton species in Mondsee, 1982 as average percentage contribution to biovolume in the euphotic zone. Only those species are considered which contributed more than 100J'0 of biovolume at some time. For details see text. Abbreviations of species: APH C: Aphanothece clathrata, AST F: Asterionella formosa, CER H: Ceratium hirundinella, CRY E: Cryptomonas erosa, CRY M: Cryptomonas marssonii, CRY P: Cryptomonas pusilla, DIN D: Dinobryon diver gens, DIN S: Dinobryon sociale, FRA C: Fragilaria crotonensis, GOM L: Gomphosphaeria lacustris, GYM H: Gymnodinium helveticum, MEL I: Melosira italica, MIC A: Microcystis aeruginosa, OSC R: Oscillatoria rubescens, RH L: Rhodomonas lacustris, TAB F: Tabellariaflocculosa var. asteri­onelloides.

22

Dinobryon diver gens and D. sociale (comp. also Fig.9A).

7. Diatom-Cryptomonas phase: The late fall, early winter plankton is dominated by Tabellaria fenestrata and Fragilaria crotonensis. Development of Cryptomonas marssonii by the end of year returns to the winter phase.

Comparison of Figs. 9, 10 and 4 reveals that some of the successional events coincide with total biovolume peaks (e.g. Tabellaria in early June and' mid November) whereas others occur during a peri­.ad of declining biovolumes (e.g. Oscillatoria and Ceratium).

Discussion

Oscillatoria rubescens developed at temperatures between 6 an:d 8°C, below or at the 10/0 light level (Figs 2, 7d). In early September maximum develop­ment was around 12°C, in accordance with obser­vations by Findenegg (1971, 1973). According to Konopka (1981) the vertical stratification of O. rubescens is primarily determined by light intensity, but the position of the layer on the vertical light gradient is affected by nutrient availability (Schmitt & Olive, 1980). Microcystis grew well at tempera­tures between 15 ° and 20°C, as observed by Ganf & Oliver (1982). This is below the temperature opti­mum of isolates reported by Kriiger & Eloff (1978) or Nicklisch & Kohl (1983). Population increase in early August seems to have emerged from the sedi­ment (Reynolds & Rogers, 1976) because biovolume values below 15 m in Fig. 7d are essentially Microcystis.

Maximum growth of the Dinobryon species (Figs 7c, 9A) occurred in most cases in the epilimnion near the lake surface, indicating a preference for high irradiance. 'Thmperature seems to be of minor importance (cf. Findenegg, 1971).

Of the dinoflagellates, Peridium willei and Gym­nodinium helveticum, are associated with low tem­peratures and low light (Figs 2, 6b), whereas Cerati­um hirundinella has a preference for higher temperatures and light (Heaney & Talling, 1980). The seasonal appearance of C hirundinella (bimo­dal in July and August, Figs 9, 10) is generally simi­lar to that repQrted for temperate-zone and subarc­tic populations (Moore, 1981). Cryptophyceae (Fig. 7a) occur all the year round (Fig. 9B) but are more

important during winter (Figs 8, 10). Taking laboratory work (Cloern, 1977; Morgan & Kalff, 1979) into consideration, it seems that Cryptomonas-species have high growth rates and are able to adapt to a wide range of light­temperature conditions (Ramberg, 1979). Already Findenegg (1971) considered the genus as 'euryok'. Various observations in the field on seasonal fluc­tuations of both Cryptomonas- and Rhodomonas­species report high growth rates and persistent oc­currence throughout the year (Ramberg, 1979; Sommer, 1981a, b; Reynolds, 1982, 1984). However, relative winter dominance is probably more a reflection of smaller loss rates and reduced grazing pressure (Sommer, 1981b). Feeding by herbivorous zooplankton also seems to be responsible for the low population densities attained during summer (Fig. 9B).

Spring development and succession of the dia­toms is caused by rapid growth of Asterionella, in­fluence of turbulence on Asterionella and Melosira, and temperature distribution in the water column (Lund, 1955, 1964; Reynolds, 1980). Termi­nation of the growth period of these two species is influenced by silicon depletion (Fig. 6a), fungal in­fection and high loss rates through sinking at the onset of temperature stratification. When the rapid growth of both species ceased they were replaced by Tabellaria which became dominant (Figs 9, 10) de­spite its low division rate. Lower death and sinking rates, rare fungal infection and higher photosyn­thetic rates are possible factors influencing the sea­sonal appearance of Tabellaria (Knoechel & Kalff, 1978). The population increase of Fragilaria crotonensis at the end of the year (Figs 9B, 10) may be related to temperature distribution, turbulence and concentration of soluble reactive silicon.

Comparison of the present observations with previous investigations on the lake (Schwarz, 1979, 1981) indicates considerable changes in phyto­plankton composition. Biovolume of Oscillatoria rubescens significantly decreased since 1978, be­cause of reduced nutrient input to the lake and high loss rates through the outflow (Miiller-Jantsch, 1979). Unlike 1980 (Schwarz, 1981), Aphanizome­non has not been recorded during summer. Instead, Microcystis has become an important component of the phytoplankton assemblage in August and September 1982 (Fig. 9A), possibly because of favourable temperature conditions in the epilim-

nion. As a result nitrate concentrations decreased substantially in the epilimnion (Fig. 3). Larger dia­toms, such as Asterionella, Tabellaria or Fragilaria, dominate the diatom populations as in the period preceeding eutrophication (Findenegg, 1969). These changes in the species composition are also considered responsible for the observed shift of the biovolume peak from spring to early September (Schwarz, 1979, 1981).

Associated with the alteration of species compo­sition, total phytoplankton biomass decreased by about 60070, indicating recovery of the lake after sewage diversion. Unlike several other Oscillatoria­lakes (Edmondson, 1977; Ahlgren, 1978; Faafeng & Nilssen, 1981; Sampl et al., 1981; Wurzer, 1982) the process of oligotrophication in Mondsee is relative­ly quick. Similar observations were made in lakes with short water renewal time (e.g. Ossiacher See) or lakes whose renewal times have been artificially altered (e.g. Klopeiner See, Sampl et al., 1981; Wurzer, 1982), pointing to the importance of water renewal, among other factors, for the recovery of Oscillatoria-Iakes.

Use of the classification of phytoplankton perio­dicity by Reynolds (1982) clearly points to the mesotrophic line including some eutrophic ele­ments. This result is in general agreement with chemical and biological observations in the water and sediment of Mondsee and indicates a certain transitional stage of the lake in 1982, after recovery from eutrophication.

Acknowledgements

We would like to thank Mr R. Niederreiter for his invaluable help in collecting the samples, Doz. Dr. H. Winkler for continuous support in data reduc­tion and computer programming, Mr K. Maier for drawing the figures and Miss I. Gradl for typing the manuscript.

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23

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