CHANGES IN THE PLANKTON OF LAKE FELDBERGER HAUSSEE (GERMANY, MECKLENBURG-VORPOMMERN) IN RESPONSE TO BIOMANIPULATION

During the course of a restoration programme, which lowered the extern al nutrient load, the harvest of zooplanktivorous fish and introduced pike-perch, the structure of the plankton community of the highly eutr ophic Lake Haussee changed distinctly. Two of the main differences wer e the changes in suspension-feeding crustacean plankton (Daphnia spp., Eudiaptomus gracilis) and the replacement of the former phytoplankton community of green algae by filamentous cyanobacteria. Field observat ions show that the annual overlap between zooplankton and the blue-gre ens has increased during the last years. In 1985 it amounted to about 30 days, whereas in 1991 and 1992 zooplankton developed nearly all the time in the presence of high concentrations of filamentous cyanobacte ria. The concentration of the blue-greens varied between 2.2 mg WW/L a nd 21.5 mg WW/L (mean july- September, 1987/1992). Regression analysis showed that the filamentous cyanobacteria had a statistically signifi cant influence on the biomass of Eudiaptomus gracilis. The biomass of Daphnia cucullata, Daphnia hyalina, and the egg stocks of D. cucullata and E. gracilis did not vary significantly with the concentration of blue-greens.