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Changes in zooplankton biomass in the Bay of Quinte with the arrival of the mussels, Dreissena polymorpha and D. rostiformis bugensis, and the predatory cladoceran, Cercopagis pengoi: 1975 to 2008

Long term monitoring of Lake Ontario's Bay of Quinte provides the opportunity to examine the impact of dreissenid invasion on the zooplankton community. Weekly or biweekly zooplankton samples have been collected from 1975 to 2008 at 3 stations: Belleville (B), Hay Bay (HB), and Conway (C) along a trophic and depth gradient down the bay. Rotifers have been collected since 2000. Biomass estimates based on measured zooplankton lengths started in 1995. Archived seasonal composite samples prior to 1995 were reanalysed and biomass recalculated from length-weight equations to allow for comparable data in trend analysis. Mean May 1– October 6 zooplankton biomass was low from 1975 into the early 1980s and peaked between 1982–1983 and 1991. Biomass fell during the cold summer of 1992 associated with the Pinatubo eruption and was low after the invasion of dreissenid mussels. From 1979–1991 (after phosphorus control and prior to dreissenid invasion), biomass averaged 265, 253 and 84 mg m −3 at B, HB and C, respectively. Seasonal biomass of most zooplankton groups, as well as total biomass, was significantly lower at all stations after the dreissenid invasion. Cladocerans still dominated zooplankton biomass after the invasion, averaging 56% to 80% of the total. Cyclopoid numbers and biomass fell dramatically. After the invasion of Cercopagis pengoi in 1999, calanoid and cyclopoid biomass at HB and C decreased by approximately 50%. Bosmina biomass did not change. Seasonal mean rotifer biomass over the 2000 to 2008 period was 1.9% to 4.4% of total zooplankton biomass. Reductions in zooplankton following dreissenid and C. pengoi invasions are thought to be caused by both direct predation of microzooplankton (e.g. nauplii) by mussels and copepods by C. pengoi (at C and HB), and competition between zooplankton and dreissenids for food resources.