Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities

The body size of an individual zooplankton is well related to its grazing r ate and to the range of particle sizes it can ingest? and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms. we tested whether the si ze structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of a lgal sizes they graze. We compared community grazing rates on individual al gal taxa in two communities dominated by small cladocerans, three communiti es dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were po orly related to algal size alone. The range in size of grazed algae increas ed with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial le ngth dimension (GALD)=16-36 mu m] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28-78 mu m). Copepod-dominated communities followed the same general relationship as cladocerans, Daphnia-dominated c ommunities grazed the broadest range of algal sizes, and their total grazin g rates were up to 2.4 times their grazing rates on small (<35 mu m) "highl y edible" algae, a difference of similar magnitude to those found in succes sful trophic cascade biomanipulations.