In situ filtering rates of Cladocera: Effect of body length, temperature, and food concentration

The individual filtering rates of the cladocerans in Creteil Lake were measured in the daytime with 14C-labeled ChZorella during a seasonal survey. This mesotrophic, shallow, polymictic lake is characterized by small algae (~25 wrn) and cladocerans (< 1.3 mm). Multiple regression models were established for each genus and for all the cladocerans. Body length alone explained from 44 to 57% of the total variance in the filtering rates of Daphnia spp., Ceriodaphnia spp., and Diaphanosoma brachyurum. An additional 23-34% of the variance was attributable to temperature. The inclusion of the Chl a concentration finally yielded r2 values ranging between 0.79 and 0.84. On the other hand, body length and temperature explained only 16% of the total variance in filtering rate of Bosmina longirostris. By taking into account the effect of factors other than length of the animal, the fit of the model established for all cladocerans improved considerably (from r2 = 0.47 to r2 = 0.83). Species-specific responses and thermal effects in the lake show the difficulty of applying models based solely on body length to obtain sufficiently accurate estimates of cladoceran filtering rates. Cladocerans are a common group in freshwater zooplankton and typically are the most efficient filter feeders among the lake pelagic communities. They play an important role in energy transfers along the food chain and in the algal seasonal pattern (Bergquist et al. 1985; Lampert et al. 1986; Sommer et al. 1986). Thus, measurements of their grazing activity and knowledge of the factors that control their filtering rates are essential in achieving better understanding of the functioning of planktonic communities. There is an increasing mass of data on phytoplankton and zooplankton dynamics. Many laboratory experiments have been conducted to analyze the trophic interactions between zooplankton and algae. Laboratory results, however, can hardly be extended to the field, and direct predictive models of in situ grazing activity are needed. Field studies on zooplankton grazing rates