Zooplankton bacterivory at coastal and offshore sites of Lake Erie

Two size classes of zooplankton (microzooplankton: 40-200 m; rotifers and nauplii, but protists were excluded; and macrozooplankton: >200 m; cladocerans and copepods) bacterivory at coastal and offshore sites in Lake Erie, USA, were determined in situ using both fluorescent and radiolabeled bacteria during the summers of 1993 and 1994. Bacterial abundance cellular carbon content and productivity were significantly higher at the more eutrophic coastal site (P <0.01). Bacterivorous rotifers usually dominated total rotiferan abundance at both sites. All cladocerans except Leptodora kindtii grazed bacteria, but most copepods did not. Microzooplankton (especially rotifers) were generally more important bacterial grazers than macroplankton (primarily cladocerans) at both sites, and accounted for 56 and 71% of total zooplankton bacterivory at the coastal and offshore sites, respectively. However, on four occasions (out of 16 cases) when cladoceran biomass was>60% of total zooplankton biomass, macrozooplankton bacterivory accounted for 54-95% of total bacterivory. Total zooplankton generally consumed <50% of bacterial productivity at the coastal site, while zooplankton bacterivory often exceeded bacterial productivity at the offshore site. Zooplankton consumed an average of 27 and 114% daily bacterial standing stock at the coastal and offshore site respectively. Considering the total community bacterivory zooplankton were often more important bacterial predators than protists. Our results demonstrate the importance of bacteria as a carbon source for zooplankton in the Lake Erie food web, where rotifers can contribute significantly to total zooplankton bacterivory.