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

Through their consumption of protozoa, cladocerans and copepods link classi cal food chains and microbial food webs in aquatic ecosystems. Published re sults of studies of the effects of these metazooplankton on protozoa in lak es allow few generalizations to be made. To determine if general patterns e xist along atrophic gradient, we measured the effects of cladocerans and ca lanoid copepods on heterotrophic nanoflagellates (HNF) and ciliates in four lakes that ranged from ultraoligotrophic to eutrophic using the same metho dology. Copepods (Boeckella spp.), and to a lesser extent cladocerans (Daph nia, Ceriodaphnia), had significant negative effects on the growth of proto zoa, and the rates at which both groups cleared protozoa from the water wer e higher in nutrient-poor conditions than in nutrient-rich conditions. In t wo oligotrophic lakes, calanoid copepods ingested HNF at biomass-specific r ates that were 2.2 times higher than those of cladocerans. Rates of ciliate ingestion by copepods, relative to cladocerans (top-down effects on the ci liate community), increased with lake productivity from 2.5 times higher in an ultraoligotrophic lake to 9.5 times higher in a mesotrophic lake. Our s tudy shows that copepods are more effective consumers of protozoa than clad ocerans, particularly in eutrophic conditions.