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

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

Martin Dokulil & Claudia SkolautInstitut fiir Limnologie der Osterreichischen Akademie der Wissenschaften, Gaisberg 116, A-5310Mondsee, Austria

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

Abstract

Phytoplankton numbers, biovolume, chlorophyll-a and various physico-chemical characteristics were fol-lowed at weekly intervals in Mondsee, Austria during the year 1982. Secchi-disk transparency varied from10 m in winter to 2 m in September. Prior to the onset of stratification phosphate-phosphorus concentrationwas 4 /Ag 1 -' decreasing to undetectable values thereafter. Nitrate-nitrogen dropped from 590 sAg 1-' to about100 tjg 1-' during the same time. The vernal bloom was dominated by Asterionella formosa Hass. whichabruptly 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 fol-lowed by the chrysophyte Dinobryon divergens Imh. and D. sociale Ehr. which formed up to 69% of totalbiovolume in October. Thereafter diatoms and Cryptophyceae (Rhodomonas lacustris Pascher and Ruttner,Cryptomonas pusilla Bach.) became abundant again.

Maximum chlorophyll-a concentration in the epilimnion (16 /ig 1-1) was reached during spring growth ofthe diatoms. During summer higher chlorophyll-a levels were always associated with the metalimnetic layerof Oscillatoria.

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

Introduction

Cyclic fluctuations in abundance and speciescomposition constitute a major characteristic ofthe freshwater phytoplankton. Despite the existenceof a large and expanding literature describing sea-sonal succession, understanding of the factors thatregulate the wax and wane of algal populations isstill far from complete. Recently some progress hasbeen made towards a generalized hypothesis to ac-count for seasonal periodicity (Reynolds, 1980),and successional pathways have been proposed forlakes of different trophic status (Reynolds, 1982).Although based on long-term studies in a variety of

lakes, these valuable generalizations still sufferfrom limited availability of detailed data and cantherefore not be applied universally. More informa-tion is needed on the depth-time distribution of thephytoplankton in various freshwater systems to ex-pand the above-mentioned hypothesis. To accom-plish this, frequent (e.g. weekly) sampling intervalsand consideration of the loss processes are impor-tant (Sommer, 1981b; Crumpton & Wetzel, 1982;Reynolds & Wiseman, 1982; Reynolds et al., 1982).

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

10

Mondsee is situated in central Austria (Fig. 1).Morphometrical and hydrological data are summa-rized in Table 1. The lake underwent considerablechanges due to eutrophication in the late 1960s(Danecker, 1969; Findenegg, 1969). Oscillatoriarubescens D.C. was first recorded in autumn 1968,and was a nuisance until a sewage treatment plantwas put into operation during 1974. Recent investi-gations (Schwarz, 1979, 1981) demonstrated a sub-stantial drop of the Oscillatoria population and areduction in total phytoplankton biovolume. Thelake has been monitored since 1978 through theAustrian Eutrophication Programme and intensive-ly studied since fall 1981 by the Limnological Insti-tute of the Austrian Academy of Sciences, Dept.Mondsee.

Table 1. Mondsee, morphometrical and(from Muller (1979) and Wurzer (1982)).

hydrological data

Altitude above sea level 481 mCatchment area 247.2-106 m2

Surface area (A) 14.2.106 m2

Maximum depth 68.3 mMean depth 36.0 mVolume (V) 510 .106 m3

Maximum length 10800 mShore length 28.3.106 mAverage outflow 9.2 m3

s-I

Theor. water renewal ca 1.7 years

Methods

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

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

11

photic zone depth, was located from radiationmeasurements using an underwater quantum sen-sor (LI-COR, USA). A transmissometer (Schenk,Austria) was used to locate the position of metalim-netic algal layers. Phytoplankton biovolume was es-timated from cell counts and size measurements onan inverted microscope (Lund et al., 1958) using ge-ometric approximations (Rott, 1981). Chlorophyll-a, corrected for degradation products, was meas-ured spectrophotometrically after extraction incold methanol following the procedure of Holm-Hansen & Riemann (1978). Standard analyticaltechniques were used for all other chemical quanti-ties (Mackereth et al., 1978).

Results

From early December to the end of April the lakewas completely mixed, and partially ice-coveredfrom 14 January to 28 March (Fig. 2). Lowest tem-peratures (below 3 °C) occurred throughout the wa-ter column during this period. Stratification be-came progressively apparent in May, developing aclear thermocline between 10 and 15 m in June.

Highest surface temperature (23.3 °C) was recordedon 22 July. Wind mixing at the end of Septemberdeepened the epilimnion. As a consequence astrong thermal gradient was observed in Octoberbetween 12 and 15 m. Turbulence and convectionalcooling gradually destroyed stratification duringNovember and re-established isothermal conditionson 23 December.

Secchi-disk transparency varied between 10 m on2 January and 2 m on 17 September. The depth towhich 1% of surface photosynthetic active radia-tion penetrates is related to secchi-depth by a factorof 2.5. Consequently the limits of the euphoticzone were 25 and 5 m respectively.

On 13 April, before the onset of thermal stratifi-cation, nutrients were distributed more or less uni-formly with depth (PO4-P 4 g 1-1; NO3-N600 g 1-l; NH4 -N 10 g 1-1). Thereafter, phos-phate-phosphorus concentration in the epilimniondecreases to an undetectable level and nitrate-nitrogen to about 100 g 1- . Ammonium-nitrogenreached the highest concentration of 650 g 1- inthe hypolimnion on 13 September (Fig. 3, Jagsch &Bruschek, 1984).

Significant increase of numbers and biovolume

J F M A M J J A S O N DFig. 2. Mondsee, 1982. Secchi-disk depth and depth-distribution of isotherms (C); arrows indicate sampling dates.

J F M A M J J A S O N D

10 10

.~~~ ~

f I1 I' I f I II' I 9 I P P P

J F M A M J J A S O N D

Fig. 3. Depth-time diagrams of seasonal dissolved nutrient concentrations, all in sg l- ', of NH4-N (a), N0 3-N (b), PO4-P (c) andtotal-P (d), Mondsee, 1982. (After Jagsch & Bruschek, 1984).

of the phytoplankton assemblage began in March,peaking on 26 April before the onset of thermalstratification (Figs 4 and 5). This period was domi-nated by the diatoms Asterionella formosa Hass.,Tabellariaflocculosa (Lyngb.) Kiitz. var. asterionel-loides Grun., and Melosira italica (Ehr.) Kutz. (Fig.9B). Additional species included Fragilariacrotonensis Kitton, Synedra acus Kiitz var. an-gustissima Grun., Cyclotella comensis Grun., C.

bodanica Eul. and Stephanodiscus astraea (Ehr.)Grun. Comparison of biovolume and chlorophyll-aisopleths (Figs 4, 5) with the depth-time distribu-tion of isotherms (Fig. 2) idicates that thephytoplankton was homogeneously distributedwithin the water column as long as turbulent,mixed conditions prevailed. Thermal stratificationrestricted growth to the epilimnion. As a conse-quence dissolved-reactive silicon became depleted

12

m

.-

- . Ir ( r

13

cn

40

(a)

1982

\i

J F M A M J J A S O N DFig. 4. Total phytoplankton biomass in Mondsee, 1982: (a) Integrated values (cm 3 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-h. Depth markings and arrows indicate depth and time at which samples were collected.

14

400

10(

30-

(a)

1982

J F M A M J J A S U N U

Fig. 5. Chlorophyll-a concentration in Mondsee 1982: (a) integrated values (mg chl-a m- 2 ) for the total water column (0-40 m; opensymbols) and for the euphotic zone (filled symbols) and (b) vertical distribution of chlorophyll-a concentration. Isopleths in lig 1- '.Depth markings and arrows indicate depth and time at which samples were collected.

15

J F M A M J J A S O N DFig. 6. Depth-time distribution of (a) dissolved reactive silicon concentration (g 1- ') and (b) biovolume of diatoms per unit volumeof water (mm 3 1-') in Mondsee, 1982. Depths markings and arrows indicate depth and time at which samples were collected.

to concentrations below 500 ig 1- I on 19 May(Fig. 6a). A heavy fungal infection during this peri-od resulted in an abrupt decline of the Asterionellapopulation. 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 23December; Fig. 6) when Tabellaria was again a ma-jor component of the population (Fig. 9B). Thediscrepancy is probably due to low pigment contentof the Tabellaria cells.

Cryptophytes and dinoflagellates were other im-portant components of the phytoplankton assem-blage during spring (Fig. 8), but had their maingrowth period between 26 June and 7 August afterthe decline of the diatom population (Fig. 7a, b).

17

m

5-10-12-15-

20-

30-

40-

1982(c)

·..., .. .......

\kA \ I o~~l \ ts,,.. ..... ... ..\x.' \ x \::::.w.~~~~~~~~:::::::::::::

. 01 01.·- . .. ;...w.1 _ \t :.::::.:.:::.:~~~~~~~~~~::::::

\ .. ..

.. . . .. . .5

tJ F 414 4 414 4 9P 4 41 p I 4 f 4 4 4414 4 41 J J A S

J F M A M J J A S O N D

Fig. 7. Depth-time distribution of the biovolume of algal groups in Mondsee, 1982. Isopleths in mm3 1-1: (a) Cryptophyceae; (b)dinoflagellates; (c) Chrysophyceae; (d) blue-green algae. Depths markings and arrows indicate depth and time at which samples werecollected.

- -- - - -- - - ___ - - - - - - - __ - - - - - -

18

Rhodomonas lacustris Pascher and Ruttner, Cryp-tomonas erosa Ehr. and C. marssonii Skuja werepresent through the year (Fig. 9B) and representedthe main components during winter and spring(Figs 8, 10).

Peridinium cf. willei Huitf.-Kaas and Gym-nodinium helveticum Pen. represented the dino-flagellates between March and May, whereas Ce-ratium hirundinella O.F. Mill. formed themaximum between 9 July and 3 September (Figs7b, 8 and 9B).

The Chrysophyceae, represented primarily byDinobryon divergens Imh. and D. sociale Ehr., firstappeared on 1 April. Two small growth phasesparalleled the diatom peak in spring (Fig. 7c). Themain growth period was between 3 September and4 November (Figs 7c, 9A), when Chrysophyceae oc-cupied 13%o to 69% of total phytoplankton bio-volume (Fig. 8).

Blue-green algae were most abundant between 11

June and 4 November (Figs 7d, 8). An Oscillatoriarubescens D.C. population reached a metalimneticmaximum at 10 to 15 m (Figs 4, 9A) followed bymassive development of Microcystis aeruginosaKutz. in the epilimnion (12 mm 3 1-1 at 5 m depthon 3 September, Fig. 9A). Additional species in-cluded Anabaena flos-aquae (Lyngb.) Br6b.,Chroococcus dispersus (Keissl.) Lemm. and Gom-phosphaeria lacustris Chod. (Fig. 9A).Aphanizomenon flos-aquae (L.) Ralfs was not de-tected before 4 November but became increasinglyimportant afterwards.

Numerous green algal species never contributedsignificantly to total phytoplankton biovolume.Among the species present throughout the yearwere Oocystis lacustris Chod., Monoraphidiumcontortum Kom.-Leg. and Eudorina elegans Ehr.Additional species during July and August includ-ed Pandorina morum (Mill.) Bory, Phacotus len-ticularis (Ehr.) Stein, Planktosphaeria sp. G.M.

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

(a)

-2

(P

2

-IJI F n R J J R S o0 D

j 'F 'A 'R" ' J

Fig. 9. Seasonal changes of average biovolume in the euphotic zone, in log.-106 sm3 1- , for various algal species, Mondsee, 1982.A) Blue-green algae and Chrysophyceae: (a) Oscillatoria rubescens, (b) Microcystis aeruginosa, (c) Gomphosphaeria lacustris,(d) Aphanothece clathrata, (e) Dinobryon divergens, (f) Dinobryon sociale. B) Diatoms, dinoflagellates and Cryptophyceae: (a) Asteri-onellaformosa, (b) Fragilaria crotonensis, (c) Tabellariaflocculosa var. asterionelloides, (d) Melosira italica, (e) Ceratium hirundinel-la, (f) Cryptomonas marssonii, (g) Cryptomonas erosa, (h) Rhodomonas lacustris.

19

-

r0

CJE

0

0o

on

(d)

S I rI S O D

·

4

b)

-

:)

(C)

(e)

-I

(d)

(f)

(h)

-,

20

mE

'.0o

o

E

o

o

-0o

-I

-

21

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

significantly (more than 10%) to total biovolume inthe euphotic zone is depicted in Fig. 10. Severalphases can be distinguished:

1. Cryptomonas-Rhodomonas phase: Winterplankton with highest biovolumes of Cryptomonasmarssonii and Rhodomonas lacustris. Additionalspecies are Cryptomonas erosa and C. pusilla withminor contributions from Asterionella formosaand Tabellaria flocculosa var. asterionelloides.

2. Spring diatom phase: Asterionella formosabuilt the first maximum followed by Melosira itali-ca. If this is a persistent sequence the phase mightbe separated into two individual steps. Gymnodini-

um helveticum was co-occurring during the Asteri-onella peak.

3. Tabellaria phase: High biovolumes during theonset of stratification.

4. Oscillatoria-Ceratium phase: Summer phaseduring July and August, followed by Ceratiumhirundinella.

5. Blue-green algal phase: Pronounced develop-ment during August and September. Microcystisaeruginosa occurred at the beginning of this phase,followed by Gomphosphaeria lacustris andAphanothece clathrata (Fig. 9A). Microcystismight be separated into a particular phase if thetype of occurrence is regular.

6. Dinobryon phase: Autumnal development of

, I I I I J J a V PI U

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 10% of biovolume at some time. For details see text. Abbreviationsof species: APH C: Aphanothece clathrata, AST F: Asterionellaformosa, CER H: Ceratium hirundinella, CRY E: Cryptomonas erosa,CRY M: Cryptomonas marssonii, CRY P: Cryptomonas pusilla, DIN D: Dinobryon divergens, 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. asteri-onelloides.

22

Dinobryon divergens and D. sociale (comp. alsoFig. 9A).

7. Diatom-Cryptomonas phase: The late fall,early winter plankton is dominated by Tabellariafenestrata and Fragilaria crotonensis. Developmentof Cryptomonas marssonii by the end of yearreturns to the winter phase.

Comparison of Figs. 9, 10 and 4 reveals thatsome of the successional events coincide with totalbiovolume peaks (e.g. Tabellaria in early June andmid November) whereas others occur during a peri-.od of declining biovolumes (e.g. Oscillatoria andCeratium).

Discussion

Oscillatoria rubescens developed at temperaturesbetween 6 and 8 C, below or at the 1o light level(Figs 2, 7d). In early September maximum develop-ment was around 12 °C, in accordance with obser-vations by Findenegg (1971, 1973). According toKonopka (1981) the vertical stratification of O.rubescens is primarily determined by light intensity,but the position of the layer on the vertical lightgradient is affected by nutrient availability (Schmitt& Olive, 1980). Microcystis grew well at tempera-tures between 15 o and 20 °C, as observed by Ganf& Oliver (1982). This is below the temperature opti-mum of isolates reported by Kriiger & Eloff (1978)or Nicklisch & Kohl (1983). Population increase inearly August seems to have emerged from the sedi-ment (Reynolds & Rogers, 1976) because biovolumevalues below 15 m in Fig. 7d are essentiallyMicrocystis.

Maximum growth of the Dinobryon species (Figs7c, 9A) occurred in most cases in the epilimnionnear the lake surface, indicating a preference forhigh irradiance. Temperature seems to be of minorimportance (cf. Findenegg, 1971).

Of the dinoflagellates, Peridium willei and Gym-nodinium helveticum, are associated with low tem-peratures and low light (Figs 2, 6b), whereas Cerati-um hirundinella has a preference for highertemperatures and light (Heaney & Talling, 1980).The seasonal appearance of C. hirundinella (bimo-dal in July and August, Figs 9, 10) is generally simi-lar to that reported for temperate-zone and subarc-tic populations (Moore, 1981). Cryptophyceae (Fig.7a) occur all the year round (Fig. 9B) but are more

important during winter (Figs 8, 10). Takinglaboratory work (Cloern, 1977; Morgan & Kalff,1979) into consideration, it seems thatCryptomonas-species have high growth rates andare able to adapt to a wide range of light-temperature conditions (Ramberg, 1979). AlreadyFindenegg (1971) considered the genus as 'eury6k'.Various observations in the field on seasonal fluc-tuations of both Cryptomonas- and Rhodomonas-species report high growth rates and persistent oc-currence throughout the year (Ramberg, 1979;Sommer, 1981a, b; Reynolds, 1982, 1984). However,relative winter dominance is probably more areflection of smaller loss rates and reduced grazingpressure (Sommer, 1981b). Feeding by herbivorouszooplankton also seems to be responsible for thelow population densities attained during summer(Fig. 9B).

Spring development and succession of the dia-toms is caused by rapid growth of Asterionella, in-fluence of turbulence on Asterionella andMelosira, and temperature distribution in the watercolumn (Lund, 1955, 1964; Reynolds, 1980). Termi-nation of the growth period of these two species isinfluenced by silicon depletion (Fig. 6a), fungal in-fection and high loss rates through sinking at theonset of temperature stratification. When the rapidgrowth of both species ceased they were replaced byTabellaria which became dominant (Figs 9, 10) de-spite its low division rate. Lower death and sinkingrates, rare fungal infection and higher photosyn-thetic rates are possible factors influencing the sea-sonal appearance of Tabellaria (Knoechel & Kalff,1978). The population increase of Fragilariacrotonensis at the end of the year (Figs 9B, 10) maybe related to temperature distribution, turbulenceand concentration of soluble reactive silicon.

Comparison of the present observations withprevious investigations on the lake (Schwarz, 1979,1981) indicates considerable changes in phyto-plankton composition. Biovolume of Oscillatoriarubescens significantly decreased since 1978, be-cause of reduced nutrient input to the lake and highloss rates through the outflow (Miiller-Jantsch,1979). Unlike 1980 (Schwarz, 1981), Aphanizome-non has not been recorded during summer. Instead,Microcystis has become an important componentof the phytoplankton assemblage in August andSeptember 1982 (Fig. 9A), possibly because offavourable temperature conditions in the epilim-

23

nion. As a result nitrate concentrations decreasedsubstantially in the epilimnion (Fig. 3). Larger dia-toms, such as Asterionella, Tabellaria or Fragilaria,dominate the diatom populations as in the periodpreceeding eutrophication (Findenegg, 1969).These changes in the species composition are alsoconsidered responsible for the observed shift of thebiovolume peak from spring to early September(Schwarz, 1979, 1981).

Associated with the alteration of species compo-sition, total phytoplankton biomass decreased byabout 60%, indicating recovery of the lake aftersewage diversion. Unlike several other Oscillatoria-lakes (Edmondson, 1977; Ahlgren, 1978; Faafeng &Nilssen, 1981; Sampl et al., 1981; Wurzer, 1982) theprocess of oligotrophication in Mondsee is relative-ly quick. Similar observations were made in lakeswith short water renewal time (e.g. Ossiacher See)or lakes whose renewal times have been artificiallyaltered (e.g. Klopeiner See, Sampl et al., 1981;Wurzer, 1982), pointing to the importance of waterrenewal, among other factors, for the recovery ofOscillatoria-lakes.

Use of the classification of phytoplankton perio-dicity by Reynolds (1982) clearly points to themesotrophic line including some eutrophic ele-ments. This result is in general agreement withchemical and biological observations in the waterand sediment of Mondsee and indicates a certaintransitional stage of the lake in 1982, after recoveryfrom eutrophication.

Acknowledgements

We would like to thank Mr R. Niederreiter for hisinvaluable help in collecting the samples, Doz. Dr.H. Winkler for continuous support in data reduc-tion and computer programming, Mr K. Maier fordrawing the figures and Miss I. Gradl for typing themanuscript.

References

Ahlgren, G., 1978. Response of phytoplankton and primaryproduction to reduced nutrient loading in Lake Norrviken.Verh. int. Ver. Limnol. 20: 840-845.

Cloern, J. E., 1977. Effects of light intensity and temperature onCryptomonas ovata (Cryptophyceae) growth and nutrient up-take rates. J. Phycol. 13: 389-395.

Crumpton, W. G. & R. G. Wetzel, 1982. Effects of differentialgrowth and mortality in the seasonal succession ofphytoplankton populations in Lawrence Lake, Michigan.Ecology 63: 1729-1739.

Danecker, E., 1969. Bedenklicher Zustand des Mondsees imHerbst 1968. Osterr. Fisch. 22: 25-31.

Edmondson, W. T., 1977. Trophic equilibrium of Lake Washing-ton. Rep. EPA-600/3-77-087. U.S.E.P.A., Corvallis, Oregon,44 pp.

Faafeng, B. A. & J. P. Nilssen, 1981. A twenty year study of eu-trophication in a deep, soft-water lake. Verh. int. Ver. Limnol.21: 412-424.

Findenegg, I., 1969. Die Eutrophierung des Mondsees im Salz-kammergut. Wass. Abwass. 4: 139-144.

Findenegg, I., 1971. Die Produktionsleistungen einiger plank-tischer Algenarten in ihren natiirlichem Milieu. Arch.Hydrobiol. 69: 273-293.

Findenegg, I., 1973. Vorkommen und biologisches Verhalten derBlaualge Oscillatoria rubescens D.C. in den osterreichischenAlpenseen. Carinthia II 163: 317-330.

Ganf, G. G. & R. L. Oliver, 1982. Vertical separation of lightand available nutrients as a factor causing replacement ofgreen algae by blue green algae in the plankton of a stratifiedlake. J. Ecol. 70: 829-844.

Heaney, S. I. & J. F. 'ITalling, 1980. Ceratium hirundinella -ecology of a complex, mobile, and successful plant. Ann.Rep. Freshwat. biol. Ass. 48: 27-40.

Holm-Hansen, O. & B. Riemann, 1978. Chlorophyll-a determi-nations: improvements in methodology. Oikos 30: 438-447.

Jagsch, A. & G. Bruschek, 1984. Ergebnisse der Wasserchemiedes Mondsee. Arb. Labor Weyregg 7 (in press).

Knoechel, R. & J. Kalff, 1978. An in situ study of the productivi-ty and population dynamics of five freshwater planktonic dia-tom species. Limnol. Oceanogr. 23: 195-218.

Konopka, A., 1981. Influence of temperature, oxygen, and pHon a metalimnetic population of Oscillatoria rubescens.Appl. envir. Microbiol. 42: 102-108.

Kruger, G. H. J. & J. N. Eloff, 1978. The effect of temperatureon specific growth rate and activation energy of Microcystisand Synechococcus isolates relevant to the onset of naturalblooms. J. Limnol. soc. s. Afr. 4: 9-20.

Lund, J. W. G., 1955. Further observations on the seasonal cycleof Melosira italica (Ehr.) Kutz. subsp. subarctica O. M011. J.Ecol. 43: 90-102.

Lund, J. W. G., 1964. Primary production and periodicity ofphytoplanktno. Verh. int. Ver. Limnol. 15: 37-56.

Lund, J. W. G., C. Kipling & E. D. Le Cren, 1958. The invertedmicroscope method of estimating algal numbers and thestatistical basis of estimation by counting. Hydrobiologia 11:143-170.

Mackereth, F. J. H., J. Heron & J. F. Telling, 1978. Water analy-sis: Some revised methods for limnologists. Freshwat. biol.Ass., scient. Publ. 36: 1-120.

Moore, J. W., 1981. Seasonal abundance of Ceratium hirun-dinella (O. E Miiller) Schrank in lakes of different trophy.Arch. Hydrobiol. 92: 535-548.

Morgan, K. C. & J. Kalff, 1979. Effect of light and temperatureinteractions on growth of Cryptomonas erosa (Crytp-tophyceae). J. Phycol. 15: 127-134.

Muller, G., 1979. Grundlagendaten fur Fuschlsee, Mondsee undAttersee, sowie das gesamte Einzugsgebiet. Arb. Labor Wey-regg 3: 10-14.

24

Miiller-Jantsch, A., 1979. Untersuchungen and der Mond-seeache und Sedimentationsmessungen im Attersee. Arb. La-bor Weyregg 3: 107-120.

Nicklisch, A. & J. G. Kohl, 1983. Growht kinetics of Microcystisaeruginosa (Kuitz.) Kfitz. as a basis for modelling its popula-tion dynamics. Int. Revue ges. Hydrobiol. 68: 317-326.

Ramberg, L., 1979. Relations between phytoplankton and lightclimate in two Swedish forest lakes. Int. Revue ges. Hydrobi-ol. 64: 749-782.

Reynolds, C. S., 1980. Phytoplankton assemblages and their pe-riodicity in stratifying lake systems. Holarct. Ecol. 3:141-159.

Reynolds, C. S., 1982. Phytoplankton periodicity: its motiva-tion, mechanism and manipulation. Ann. Rep. Freshwat.biol. Ass. 50: 60-75.

Reynolds, C. S., 1984. Phytoplankton periodicity: the interac-tions of form, function and environmental variability. Fresh-wat. Biol. 14: 111-142.

Reynolds, C. S. & D. A. Rogers, 1976. Seasonal variations in thevertical distribution and buoyancy of Microcystis aeruginosaKuiitz. emend. Elenkin in Rostherne Mere, England. Hydrobi-ologia 48: 17-23.

Reynolds, C. S. & S. W. Wiseman, 1982. Sinking losses ofphytoplankton in closed limnetic systems. J. Plankton Res. 4:489- 522.

Reynolds, C. S., J. M. Thompson, A. J. D. Ferguson & S. W.Wiseman, 1982. Loss processes in the population dynamics ofphytoplankton maintained in closed systems. J. Plankton Res.4: 561-600.

Rott, E., 1981. Some results from phytoplankton counting inter-calibrations. Schweiz. Z. Hydrol. 43: 34-62.

Sampl, H., L. Schulz & N. Schulz, 1981. Bericht uiber die lim-nologischen Untersuchungen der Krntner Seen. Ver6ff.Korntner Inst. Seenforschip. 6: 1-175.

Schmitt, J. & J. H. Olive, 1980. Interacting effects of light, tem-perature, and nutrients on C-14 uptake of Oscillatoriarubescens De Candolle. Hydrobiologia 70: 51-56.

Schwarz, K., 1979. Das Phytoplankton des Mondsees 1978. Arb.Lab. Weyregg 3: 84-92.

Schwarz, K., 1981. Das Phytoplankton im Mondsee 1980.Arb. Lab. Weyregg 5: 110-118.

Sommer, U., 1981a. The role of r- and K-selection in the succes-sion of phytoplankton in Lake Constance. Acta oecol., Oecol.gen. 2: 327-342.

Sommer, U., 1981b. Phytoplanktonbioz6nosen und -sukzessio-nen im Bodensee/Oberlingersee. Verh. Ges. Okol. 9: 33-42.

Wurzer, E. (ed.), 1982. Seenreinhaltung in Osterreich. - Lim-nologie - Hygiene. Maflnahmen - Erfolge. In Wasserwirt-schaft. BM Land- und Forstwirtschaft. 6, 256 pp.