Effects of the zebra mussel (Dreissena polymorpha Pallas) on protozoa and phytoplankton from Saginaw Bay, Lake Huron

Direct effects of the grazing activities of the zebra mussel, Dreissena polymorpha, on the natural assemblage of planktonic protozoa and algae from Saginaw Bay, Lake Huron, were studied in September and October 1994. Water and mussels collected from two eutrophic sites were incubated 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, mussels 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 colonies of blue-greens were reduced only by 50%. Dreissena selectively removed nanoplanktonic Cryptomonas and Cyclotella, but had no significant effect on the predominant phytoplankton species, Microcystis. Overall, Dreissena clearance rates were low in the presence of this cyanophyte species. We conclude that zebra mussels, in regions where they are abundant, can cause significant changes in composition of both the protozoan and phytoplankton communities.