Phytoplankton responses to grazing by Daphnia galeata in the biomanipulated Bautzen reservoir

We studied the response of phytoplankton to grazing by Daphnia galeata in the hypertrophic Bautzen reservoir (Dresden, Germany) from January 1995 to May 1996 during a long-term whole-lake biomanipulation experiment. The correlation between clearance rate of D. galeata and total phytoplankton biomass was negative only if biomass of Microcystis aeruginosa was excluded. This suggests that M. aeruginosa is the main grazing resistant phytoplankton species in the Bautzen reservoir. Except for M. aeruginosa and grazing-resistant Staurastrum quadridentatum spec. nov. (Scharf, 1995) no other phytoplankton species was able to reach a biovolume above 1 mm3 L-1 when the clearance rate of Daphnia exceeded 0.1 L L-1 d-1. There was significant positive correlation between mean cell or colony size of phytoplankters and clearance rate of D. galeata, showing an advantage of bigger cells or colonies at high grazing pressure. Cross-correlation indicated a time lag of about one month between changes in grazing pressure and a change in phytoplankton mean size. The phytoplankton species were divided into edible and inedible fractions, depending on their width and length. No edible species were able to reach high biovolumes during high biomasses of D. galeata but a positive correlation was found between the edible fraction of phytoplankton biovolume and the clearance rate of D. galeata. However, this relationship disappeared when the ‘ingestible edible’ fraction of M. aeruginosa was excluded, suggesting a rejection of ‘edible’ Microcystis colonies by D. galeata. A negative correlation was found between the inedible fraction of phytoplankton biovolume without M. aeruginosa and the clearance rate of D. galeata which might be due to superior competition of M. aeruginosa. We could clearly show that biomanipulation might not work well with respect to the reduction of total phytoplankton biomass under hypertrophic conditions and finally discuss a theoretical threshold of phosphorus (probably around 80 μg L-1), below which biomanipulation should become effective.