Interannual variability in the phytoplankton composition of Lake Stechlin (1994-2000)

Padisák, Judit and Scheffler, W. and Kasprzak, P. and Koschel, R. and Krienitz, L. (2003) Interannual variability in the phytoplankton composition of Lake Stechlin (1994-2000). ARCHIV FÜR HYDROBIOLOGIE BEIHEFT ADVANCES IN LIMNOLOGY, 58. pp. 135-155. ISSN 0071-1128

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Abstract

Phytoplankton of the oligotrophic Lake Stechlin (Brandenburg, Germany) were investigated based on weekly and biweekly samples between 1994 and 2000. Because autotrophic picoplankton regularly formed a deep layer maximum in the upper hypolimnion, this fraction of phytoplankton was also included in the regular monitoring program. The observed changes were related to duration of icecover, water column stability, concentrations of main nutrients and zooplankton density. Altogether, 193 algal taxa were found of which 165 can be considered as typical planktonic forms. Floristic similarity between different years was very high indicating that initial floristic composition is rather stable. Most species were very rare, and annually only 10-20 species contributed significantly to total biomass. Number of species that exceeded 1% contribution to total biomass was only four and each of these four species had its main growing season during the isothermal period (stress-tolerant species). Successional sequences during the stratified period largely depended upon selection of phytoplankton species during the isothermal period. There were two irregular phytoplankton patterns in the 7-year study period: mass appearance of Planktothrix rubescens in 1998 and Aulacoseira islandica in 2000. Both species started to grow in the hypolimnion one year before their peak. Dominance of Planktothrix had cascading effects ont he lake’s ecosystem; namely most of the P-pool of the lake during the winter was consumed, therefore spring diatom bloom and clear water phase did not develop; being a large organism, zooplankton standing crop became very low, especially herbivorous species; inorganic N decreased to limiting levels probably triggering more intensive growth of Anabaena lemmermannii; the presence and respiration of a relatively dense Planktothrix population int he hypolimnion led to greater oxygen deficiency than in other years and Secchi transparency increased due to low the light extinction of large cells. During the Aulacoseira bloom soluble reactive silica levels declined to limiting levels. A comparison with other interannual or long-term studies has shown that a large variety of phytoplankton related variables were used for assessing changes along long time scales, and this diversity corresponded to individual character of the studied lakes. In order to understand the effect of global changes on lake ecosystem regular studies (including the often neglected isothermal periods) with appropriate sampling frequency and sufficiently detailed taxonomic accuracy are important. These observation are consequential upon planning long-term monitoring project sin lakes of low nutrient status and winter periods and vertical structures of the phytoplankton assemblage have to be considered when making sampling plans otherwise events int he „vegetation period” are difficult to interpret.

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