Filtering Rate Inhibition of Daphnia Pulex in Wintergreen Lake Water1,2

The difference 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 was studied during late summer (16–18 August 1971) . Low concentrations of 3 2 P‐labeled yeast, Rhodotorula sp., were added to whole lake water, lake water filtrate, and seston resuspended in filtered tapwater to measure grazing by field‐collected animals. Lake water seston limited grazing rates at ambient concentration; “dissolved” inhibitors capable of limiting filtering rates at lower seston concentrations were also present, but their effects were masked by high seston concentration. Possibly these dissolved substances were excreted into solution by the dense phytoplankton population, notably including Anabaena and Anacystis. Mathematical expressions for filtering and feeding rates as functions of seston concentration are derived from Holling’s disc equation and fit to the data.