PHYTOPLANKTON SUCCESSION IN THE OLIGOTROPHIC LAKE STECHLIN (GERMANY) IN 1994 AND 1995

Phytoplankton samples were taken weekly from January to December of 19 94 (epilimnion) and 1995 (0-25 m, euphotic zone) from the deep, strati fied, alkaline, oligotrophic Lake Stechlin, Baltic Lake District, Germ any. The purpose of the study was to gain detailed information about p hytoplankton changes including those of picophytoplankton, to relate t hese changes to stratification patterns and nutrient chemistry of the lake and to compare them to results from other lakes of similar charac ter. During 1994-1995, a total of 142 phytoplankton taxa was encounter ed in quantitative samples, most being common in deep, oligotrophic la kes. Seasonal development of phytoplankton is characterized by a defin ite spring peak followed by a moderate summer peak. Autotrophic picoph ytoplankton 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 picophytoplan kton and centric diatoms) starts in February-March and is terminated b y the onset of stratification when diatoms sink to the hypolimnion. Pi cophytoplankton, especially Synechococcus sp., assembles in a narrow d eep-layer maximum in the upper hypolimnion. Our data show that neither deep circulations nor decreased incident radiation under winter ice a nd snow cover prevent the development of some specially adapted low-li ght - high-nutrient species. Our views about the length of vegetation period for phytoplankton need to be revised with respect to winter and isothermal conditions.