Filtering rate inhibition of Daphnia (Crustacea; Cladocera)

The ability of lacustrine phytoplankton to limit zooplankton grazing rates has been well documented in the last two decades by Ryther, Rigler, McMahon, Burns, and many others. The experimental part of this study was designed to distinguish between the effects of seston concentration and dissolved substances on the rates of filtering and feeding by Daphnia pulex in the water of a small eutrophic lake (Wintergreen Lake, Kalamazoo County, Michigan) during the late summer (August 16-18, 1971). Estimates of filtering and feeding rates of field-collected animals in modified lake water were obtained by adding low concentrations of 32P-labeled yeast, Rhodotorula sp.; five different dilutions of each of three treatments were used as experimental media: 1) Whole lake water diluted with its own filtrate; 2) Lake water filtrate diluted with a physiologically neutral water; and 3) Lake water seston resuspended in neutral water diluted with the neutral water. Lake water seston was found to limit grazing rates at ambient concentration during the study period, but the lake water filtrate also exhibited inhibitory properties. In order to explain the results obtained in these experiments, a physiological model of daphnid feeding is proposed. It is suggested that feeding rates may be under the direct control of gut receptors at high food concentrations, whereas filtering rates are proximately regulated in the labral region. Finally, mathematical models of the grazing and assimilation processes are derived. These are useful in clarifying and describing the mechanisms by which these processes function.