Comparison of Filtering Rates of Daphnia Rosea in Lake Water and in Suspensions of Yeast1

Filtering rates of Daphnia rosea in natural lake water and in pure cultures of a yeast, Rhodotorula glutinis, were measured by adding 32 P‐labeled yeast cells to the medium. The effects of the concentration of food, body length, and water temperature on filtering rates and feeding rates were examined. Results showed that filtering rate increased with increasing body length and increasing temperatures up to 20C. Above a concentration of 0.25 × 10 5 yeast cells/ml, filtering rate decreased with increasing cell concentration; below this concentration, filtering rate was maximal. From the results of these experiments, theoretical maximum filtering rates for the Daphnia population in Heart Lake over summer were predicted. A comparison of these theoretical rates and those measured in lake water in the laboratory revealed that from June to November the Daphnia were filtering at less than maximal rates.