EFFECTS OF THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA PALLAS) ON PROTOZOAAND PHYTOPLANKTON FROM SAGINAW BAY, LAKE HURON

Direct effects of the grazing activities of the zebra mussel, Dreissen a polymorpha, on the natural assemblage of planktonic protozoa and alg ae from Saginaw Bay, Lake Huron, were studied in September and October 1994. Water and mussels collected from two eutrophic sites were incub ated in an outdoor ''natural light'' incubator at ambient temperature for 24 hours. Experiments were conducted in 4-L bottles with screened (40 or 53-mu m net) or unscreened water and with and without mussels. Despite relatively high growth rates of protozoa on both dates, mussel s lowered protozoan numbers by 70-80% and reduced the species richness of the protozoan community by 30-50%. Large heterotrophic flagellates were reduced up to 100% while peritrichous ciliates attached to the c olonies of blue-greens were reduced only by 50%. Dreissena selectively removed nanoplanktonic Cryptomonas and Cyclotella, but had no signifi cant effect on the predominant phytoplankton species, Microcystis. Ove rall, Dreissena clearance rates were low in the presence of this cyano phyte species. We conclude that zebra mussels, in regions where they a re abundant, can cause significant changes in composition of both the protozoan and phytoplankton communities.