Factors Regulating in situ Filtering Rates of Cladocera

Individual filtering rates of four species of Daphnia, as well as Holopedium, Diaphanosoma, Ceriodaphnia, and Bosmina, were measured in 10 lakes under various field conditions to identify important factors regulating in situ filter-feeding for these cladocerans. Linear regression analysis indicated that carapace length accounted for almost 60% of the total variance in individual filtering rates when data from all 10 lakes were pooled. We suggest that zooplankton in these lakes filter-feed in proportion to their volume, since the functional slope of this regression approximated 3. Taxonomic affiliation had generally little effect on filtering rates. Temperature, ranging from 12 to 22 °C, had no statistically significant effect on filtering rate; there were also no significant differences between rates measured at dawn and dusk compared with midday rates for animals ranging in size between 0.30 and 1.30 mm. Addition of phytoplankton concentration as a second independent variable to the regression relating filtering rate to length further increased explained variance to 64%. Since the two main determinants of in situ filtering rates were zooplankton carapace length and concentration of edible phytoplankton, both of which are independent of taxonomic affiliation, investigators interested in assessing grazing losses should seriously consider measuring zooplankton in terms of size distribution rather than taxonomic affiliation.