ZOOPLANKTON GRAZING AND PHYTOPLANKTON ABUNDANCE - AN ASSESSMENT BEFORE AND AFTER INVASION OF DREISSENA-POLYMORPHA

The introduction of the zebra mussel Dreissena polymorpha into Lake Erie raises the question of whether it competes with the existing zooplankton community for phytoplankton. In this study, we approached this question by examining the role of zooplankton grazing in influencing Lake Erie abiotic conditions and biotic interactions during 2 years of varying zebra mussel abundances. In situ grazing experiments (P-32-labeled yeast cells) revealed that cladoceran species contributed > 85% of zooplankton community grazing rate, whereas copepods accounted for < 15%. The western basin of the lake had the highest grazing pressure on phytoplankton populations because its zooplankton community was dominated by cladocerans. Therefore, we developed filtering rate:body length models for the two most important cladoceran species, Daphnia galeata mendotae and D. retrocurva, in western Lake Erie to estimate grazing rates from body size distribution and abundance. The peak Daphnia grazing rates (> 200 mL L-1 d-1) calculated from the models between June and early July coincided with the high Secchi transparencies found during these periods (clear-water phase) in both the year with few zebra mussels (1988) and the year with abundant mussels (1989). The impact of Daphnia grazing on the phytoplankton community is negative, directly reducing edible algal volume to < 5 mL m-3. The release of Daphnia grazing pressure in August was followed by an increase in edible phytoplankton in both nearshore and offshore areas of both years. High edible algal volumes and low Secchi transparencies in August were in contrast to what we would expect if adult zebra mussels that were present throughout 1989 or veligers that were abundant in late July and August were able to overgraze phytoplankton. We concluded that the pulsed Daphnia grazing still controlled edible algal density and water transparency in 1989, even though Dreissena polymorpha was abundant in the lake in 1989.