Success of chrysophytes, cryptophytes and dinoflagellates over blue-greens (cyanobacteria) during an extreme winter (1995/96) in eutrophic shallow lakes

Abstract

Phytoplankton succession in three shallow eutrophic and hypertrophic lakes located in the Scharmützelseeregion (East Brandenburg, Germany) is described for the extreme winter of 1995/96 before and during ice-cover. The lakes have been investigated since 1993 and pass along the tophic gradient from eutrophic, Petersdorfer See and Melangsee to hypertophic, Larger See. All lakes are dominated by cyanobacteria from early spring to late autumn. Due to low water temperature and low light intensity before ice-cover a decline of cyanobaterial biovolume initiated which led to the total disappearance of cyanobacteria in Melangsee for the whole ice-covered period, whereas their portion in Langer See with at least 6% (3 mm³ l^-1) and in Petersdorfer See with 25% (5 mm³ l^-1) of total biovolume was still considerable. The cyanobacterial breakdown was linked to a switch in dominance to flagellates which built up together with diatoms a subsequent peak of biovolume during ice-cover.

Maximum phytoplankton biovolume under the ice peak reached 20 mm³ l^-1 with chl-a concentrations up to 100 µg l^-1. Oxygen saturation increased during ice-cover with a final oversaturation of more than 200% in the upper water column at the end of ice cover. These findings indicate that the winter period is also characterized by intensive photoautotrophic activities. Therefore, this period should be more worthy of note in investigations of phytoplankton succession and ecology.