0 Cyr, H. 1998 CANADIAN JOURNAL OF FISHERIES AND AQUATIC SCIENCES OTTAWA NATL RESEARCH COUNCIL CANADA 55 2 414-422 FEB 0706-652X FRESH-WATER ZOOPLANKTON EUDIAPTOMUS-GRACILIS HOLOPEDIUM-GIBBERUM NITROGEN-EXCRETION FILTERING RATES EUTROPHIC LAKE CASTLE LAKE ALGAL SIZE FOOD SIZE PHYTOPLANKTON Many studies suggest that the taxonomic composition of a zooplankton community should determine its grazing rate and selectivity for different types of particles. It is generally believed that copepod-dominated communities should (i) have lower grazing rates and (ii) consume larger particles than communities dominated by large cladocerans. I tested these hypotheses in situ by comparing zooplankton grazing in 19 communities from low-productivity lakes where the zooplankton ranged from >99% copepod biomass to >90% large cladoceran biomass (Holopedium gibberum, Daphnia spp.). The zooplankton grazed 1-14% of total chlorophyll per day and 0-17% of the chlorophyll in algae <35 mu m per day. Grazing rates increased with increasing zooplankton biomass (r(2) = 0.34, P < 0.01), but once the effect of zooplankton biomass was accounted for, similar grazing rates were found in copepod-and in cladoceran-dominated communities. The difference in grazing rates on small algae and on the whole phytoplankton assemblage, on the other hand, varied systematically with zooplankton taxonomic composition. Holopedium-dominate communities were most efficient at grazing algae <35 mu m, Bosmina-dominated communities had similar grazing rates on algae <35 mu m and on the whole phytoplankton assemblage, and copepod-dominated communities had similar or slightly higher grazing rates on the whole phytoplankton assemblage. Qualitative differences in grazing selectivity of different zooplankton taxa are observed in complex natural communities.