Calanoid copepods versus cladocerans: Consumer effects on protozoa in lakes of different trophic status

Through their consumption of protozoa, cladocerans and copepods link classical food chains and microbial food webs in aquatic ecosystems. Published results of studies of the effects of these metazooplankton on protozoa in lakes allow few generalizations to be made. To determine if general patterns exist along atrophic gradient, we measured the effects of cladocerans and calanoid copepods on heterotrophic nanoflagellates (HNF) and ciliates in four lakes that ranged from ultraoligotrophic to eutrophic using the same methodology. Copepods (Boeckella spp.), and to a lesser extent cladocerans (Daphnia, Ceriodaphnia), had significant negative effects on the growth of protozoa, and the rates at which both groups cleared protozoa from the water were higher in nutrient-poor conditions than in nutrient-rich conditions. In two oligotrophic lakes, calanoid copepods ingested HNF at biomass-specific rates that were 2.2 times higher than those of cladocerans. Rates of ciliate ingestion by copepods, relative to cladocerans (top-down effects on the ciliate community), increased with lake productivity from 2.5 times higher in an ultraoligotrophic lake to 9.5 times higher in a mesotrophic lake. Our study shows that copepods are more effective consumers of protozoa than cladocerans, particularly in eutrophic conditions.