Phytoplankton succession in the oligotrophic Lake Stechlin (Germany)in 1994 and 1995

Phytoplankton samples were taken weekly from January to December of 1994 (epilimnion) and 1995 (0-25 m, euphotic zone) from the deep, stratified, alkaline, oligotrophic Lake Stechlin, Baltic Lake District, Germany. The purpose of the study was to gain detailed information about phytoplankton changes including those of picophytoplankton, to relate these changes to stratification patterns and nutrient chemistry of the lake and to compare them to results from other lakes of similar character. During 1994-1995, a total of 142 phytoplankton taxa was encountered in quantitative samples, most being common in deep, oligotrophic lakes. Seasonal development of phytoplankton is characterizedby a definite spring peak followed by a moderate summer peak. Autotrophic picophytoplankton made the largest contribution to the annual total biomass. This is probably true for other, temperate, non-acidic,oligotrophic lakes. Development of the spring assemblage (autotrophic picophytoplankton and centric diatoms) starts in February-March andis terminated by the onset of stratification when diatoms sink to the hypolimnion. Picophytoplankton, especially Synechococcus sp., assembles in a narrow deep-layer maximum in the upper hypolimnion. Our data show that neither deep circulations nor decreased incident radiation under winter ice and snow cover prevent the development of some specially adapted low-light--high-nutrient species. Our views about the length of vegetation period for phytoplankton need to be revised withrespect to winter and isothermal conditions. [ABSTRACT FROM AUTHOR]