Zooplankton grazing during the zebra mussel (Dreissena polymorpha) colonization of Saginaw Bay, Lake Huron

Weight-specific zooplankton filtering rates were determined at three sites in Saginaw Bay during the period of maximum zooplankton abundance prior to and after the establishment of zebra mussel colonies (June 1991 and June 1992). Biomass-specific filtering rates: were similar in both years (inner bay: 0.24-0.33 mL mu g dry wt.(-1) d(-1); outer bay: 1.27-1.83 mL mu g dry wt.(-1) d(-1)), but large decreases in zooplankton biomass resulted in a decrease, on average, of 58% in community filtering rates between years. As part of a large-scale monitoring program, zooplankton abundance and biomass estimates were also recorded at 13 sites during May-August of both years. Mean biomass in the inner bay was 40% lower in 1992 than in 1991, and in the outer bay, mean biomass was 70% lower in 1992 than in 1991. Zooplankton community composition was the same in both years. We estimated the time required to clear the water volume of the inner bay during the May-June period to be 17 days in 1991 and 37 days in 1992. For these two periods, decreased zooplankton numbers and community filtering rates indicate that grazing by zooplankton was likely not responsible for noted declines in phytoplankton abundance and productivity.