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The Periodicity of Phytoplankton in Lake Constance (Bodensee) in Comparison to Other Deep Lakes of Central Europe

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The Periodicity of Phytoplankton in Lake Constance (Bodensee) in Comparison to Other Deep Lakes of Central Europe Hydrobiologia 138: 1-7, (1986). 1 © Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands. The periodicity of phytoplankton in Lake Constance (Bodensee) in comparison to other deep lakes of central Europe Ulrich Sommer University of Constance, Institute of Limnology, PO. Box 5560, D-7750 Constance, FRG New address: Max Planck Institute of Limnology, PO. Box 165, D-2320 Plon, FRG Keywords: phytoplankton succession, inter-lake comparison, oligotrophic-eutrophic gradient, central Eu- ropean lakes Abstract Phytoplankton periodicity has been fairly regular during the years 1979 to 1982 in Lake Constance. Algal mass growth starts with the vernal onset of stratification; Cryptophyceae and small centric diatoms are the dominant algae of the spring bloom. In June grazing by zooplankton leads to a 'clear-water phase' dominated by Cryptophyceae. Algal summer growth starts under nutrient-saturated conditions with a dominance of Cryptomonas spp. and Pandorinamorum. Depletion of soluble reactive phosphorus is followed by a domi- nance of pennate and filamentous centric diatoms, which are replaced by Ceratium hirundinella when dis- solved silicate becomes depleted. Under calm conditions there is a diverse late-summer plankton dominated by Cyanophyceae and Dinobryon spp.; more turbulent conditions and silicon resupply enable a second sum- mer diatom growth phase in August. The autumnal development leads from a Mougeotia - desmid assem- blage to a diatom plankton in late autumn and winter. Inter-lake comparison of algal seasonality includes in ascending order of P-richness K6nigsee, Attersee, Walensee, Lake Lucerne, Lago Maggiore, Ammersee, Lake Ziirich, Lake Geneva, Lake Constance. The oligo- trophic lakes have one or two annual maxima of biomass; after the vernal maximum there is a slowly develop- ing summer depression and sometimes a second maximum in autumn. The more eutrophic lakes have an additional maximum in summer. The number of floristically determined successional stages increases with increasing eutrophy, from three in K6nigsee and Attersee to eight in Lake Geneva and Lake Constance. Introduction Lake Constance' (Leader: M. Tilzer). The most im- portant physical and chemical environmental varia- It is the aim of this contribution to present a bles (Stabel & Tilzer, 1981), the primary productivi- comparison of the phytoplankton seasonality in ty (Tilzer, 1984), the seasonality of the deep central European lakes of considerable size. phytoplankton (Sommer 1981a, b), of the All are characterized by basins with steep sides, zooplankton (W. Geller, in prep.) and sedimenta- small littoral zones, and a development of mainly tion (H. H. Stabel, in prep.) have been studied at pelagic communities. Unfortunately heterogeneity weekly intervals. In the second section a compari- and incompleteness of available data make an son with other central European lakes is attempted. equal treatment of all lakes impossible. Therefore, The data are taken from literature and from unpub- in the first section the periodicity of phytoplankton lished lists (Ziirichsee, Walensee, by courtesy of U. in Lake Constance is given in greater detail. This Zimmermann). Less frequent sampling and ab- lake has been thoroughly studied from 1979 to 1982 sence of background data on physical and chemical by the working group of the Deutsche Forschungs- conditions and zooplankton make it inevitable in gemeinschaft - sponsored project 'Carbon cycle in some cases to explain developments in those lakes 2 by analogy with Lake Constance. For example, the (2) Mass growth of algae starts with the onset of June minimum of algal biomass in Lake of Geneva thermal stratification in April/May. High nutrient has been interpreted as a grazing-induced clear- availability and low grazing pressure gives rise to an water phase in spite of absence of zooplankton assemblage of rapidly growing, nanoplankton spe- data, because phytoplankton data are nearly identi- cies dominated by Rhodomonas lens, R. minuta, cal to those from Lake Constance for the respective Cryptomonas ovata and small centric diatoms, phase. mainly Stephanodiscus hantzschii. All these algae Inter-lake comparison needs some degree of ab- are heavily grazed by Cladoceran filter-feeders. straction from actual data. Here, the main abstrac- (3) Increased zooplankton grazing leads to a tion is the deduction of a 'main sequence' in the dramatic decrease of phytoplankton densities in succession of phytoplankton (Margalef, 1978). This early June. The scarce phytoplankton of this 'clear- concept is based on the experience that in a strati- water phase' (Lampert & Schober, 1978) is domi- fied body of water nutrients are successively deplet- nated by Cryptomonas ovata and Rhodomonas ed, interactions with higher trophic levels increase minuta. and non-motile algae replaced by motile or buoy- (4) Towards the end of the clear-water phase ant ones. Partial destratification or erosion of the shortage of food leads to a decline of zooplankton thermocline by weather-induced mixing may lead abundance. Algal biomass rises again. As long as to re-capitulation of earlier successional stages nutrients are not yet depleted, Cryptomonas ovata, ('reversions' sensu Reynolds, 1980). In contrast to C. rostratiformis and Pandorina morum are the temporary reversions the autumnal erosion of dominant algae. stratification is a regular event and belongs to the (5) When soluble reactive phosphorus becomes 'main sequence'. An experimental demonstration depleted in July algal dominance shifts to the large for the distinction between the 'main sequence' and and poorly grazed diatoms Asterionella formosa, 'reversions' has been provided by destratification Fragilariacrotonensis, Stephanodiscus binderanus, experiments (Reynolds, Wiseman, Godfrey & But- Melosira granulata. Current competition research terwick, 1983). has shown by continuous culture experiments that several species of net-planktonic diatoms tend to outcompete all other taxa if P is in short supply Lake Constance (Bodensee) and rich silicate supply prevents Si-limitation (Til- man, 1977; Tilman, Kilham & Kilham, 1982; Som- Lake Constance (500 km 2 surface area, maxi- mer, 1983). mum depth 250 m, mean depth 100 m) is a warm- (6) When silicate is depleted (Sommer & Stabel, monomictic lake. The process of rapid eutrophica- 1983) the diatoms are replaced by a dominance of tion was stopped in the late 70's (Elster, 1982). At Ceratium hirundinella (mid-August). present the overturn concentrations of total phos- (7) In some years the Ceratium-stageis followed phorus are about 100/g .1-1. Seasonality and by a diverse phytoplankton dominated by the heter- stratification of nutrients and temperatures are ocystous blue-green algae Anabaenaflos-aquae, A. available from Stabel & Tilzer (1981), and details on planctonica, A. spiroides, Aphanizomenon flos- the counting method and further information on aquae and the Chrysophyceae Dinobryon sociale phytoplankton seasonality from Sommer (1981a, and D. divergens. There were some deviations from b). Phytoplankton species composition throughout this scheme of summer succession. Upwelling of the whole text refers to biomass, measured as cell phosphate gave rise to short growth pulses of Cryp- volume. tophyceae and upwelling of silicate led to a second The seasonal periodicity of phytoplankton spe- sommer diatom stage, which then replaced the cies composition followed with slight deviations the Cyanophyceae-Dinobryon assemblage. The de- same general pattern (Fig. 1) during the whole peri- velopment of the Cyanophyceae-Dinobryonassem- od of study (1979-1982). blage is considered the 'main sequence' of algal (1) During winter a long period of deep circula- succession, whereas the late summer pulses of dia- tion and low light intensities lead to extremely low toms and Cryptophyceae are classified as rever- algal densities. sions. 3 winter spring clear summer september autumn bloom water oh. minimum Z=Z~C surface light day length L MIXING DEPTH 0 - 3 m ZMIX INCREASING stratification NO- - - - - I I - _. XX I SRP 10 , I I t - --~- -_L_ W SRSi i i .......... co IfI grazing t- STH dominant algae RHO -RHO i R M ST STACRY ICY CRY CRY I AST FRA DIA NI CRl * FRA STBMEL 1 I iAST F I FRA DIA AeSTMF/M OUn STA ISTR MCEWR: ULO- STB APH.STAU MEL ANA CLO I IN Fig. 1. Scheme of phytoplankton seasonality and of temporal evolution of environmental parameters in Lake Constance. Units for en- vironmental variables except P and Si are arbitrary. The x-axis is out of scale temporally and rather reflects the number of successional stages than time. Species code: ANA: Anabaena spp. APH: Aphanizomenonflos-aquae, AST: Asterionellaformosa, CER: Ceratium hirundinella, CLO: Closteri- um aciculare, CRR: Cryptomonas rostratiformis, CRY: Cryptomonas ovata, DIA: Diatoma elongatum, DIN: Dinobryon spp., FRA: Fragilariacrotonensis, MEL: Melosira granulata, MOU: Mougeotia thylespora, PAN: Pandorina morum, RHO: Rhodomonas spp., RHM: Rh. minuta, STA: Stephanodiscus astraea, STB: Stephanodiscus binderanus, STH: Stephanodiscus hantzschii, STAU: Stauras- trum cingulum, ULO: Ulothrix subtilissima. (8) After an early autumn (September or early August represents the process of autogenic succes- October) minimum of biomass, which is probably sion, where the main selective environmental varia- caused by deterioration of the light climate bles are a consequence of the previous activities of (decreasing day length, increasing mixing depth), planktonic organisms. For example, ambient nutri- phytoplankton is mainly
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