GRAZING BY ZOOPLANKTON AND ITS RELATIONSHIP TO COMMUNITY STRUCTURE

Zooplankton can consume a substantial portion of the phytoplankton, but grazing rates are notoriously variable seasonally and among sites. We developed models relating grazing rates to characteristics of zooplankton communities, food concentration, and water temperature. Grazing rates were measured in 30 zooplankton communities that differed in biomass, size distribution, and taxonomic composition. Crustaceans grazed per day 2-21 % of the chlorophyll in algae <35-mu-m, which is within the range of most grazing rates measured in other studies. Grazing rates were positively related to zooplankton biomass and negatively related to food concentration, although much variation among communities remains unexplained (R2 = 0. 1 9-0.35). Surprisingly, we could not detect a general relationship between zooplankton size distribution and grazing rate. Communities dominated by large zooplankton (mostly Daphnia and Diaphanosoma) did not tend to have higher grazing rates than communities dominated by small zooplankton. Zooplankton taxonomic composition was significantly related to grazing rates but explained little variation among communities. Grazing rates calculated from published allometric equations were biased, usually overestimating phytoplankton removal by zooplankton.