Comparisons of zooplankton community size structure in the Great Lakes

Zooplankton mean size and size spectra distribution potentially reflect the condition of trophic interactions and ecosystem health because they are affected by both resource availability and planktivore pressure. We assessed zooplankton mean size and size spectra using an optical plankton counter (OPC) on 35 site visits among lakes Superior, Michigan, Huron, Erie, and Ontario (2002-2003). The surveys were conducted in both nearshore regions (5-20 m depth) and on associated transects to offshore regions either greater than 8 km from shore or greater than 100 m depth. The survey sites were distributed across a gradient of land use disturbance in watersheds adjacent to the nearshore regions. The mean size, biomass density, statistical size distribution, and normalized biomass size spectra of zooplankton were determined from OPC measurements for all locations. Significant differences among takes were observed in mean size, biomass, and the parameters of size spectra distributions for both nearshore and offshore regions. Significant differences within lakes were observed in some parameters that also allowed for significant discrimination between nearshore and offshore zooplankton communities in lakes Michigan (mean size, biomass, one spectral parameter), Ontario (mean size, three spectral parameters), and Superior (one spectral parameter). Similarly, some parameters allowed for discrimination between offshore epilimnion and hypolimnion waters in lakes Michigan (mean size, biomass, and four spectral parameters), Huron (biomass), and Ontario (two spectral parameters). The use of OPC technology and parameters that characterize spectral shape may have potential as an efficient and economic way for developing a size-based zooplankton metric to discriminate among zooplankton communities in the Great Lakes.