Phosphorus Release by Daphnia1

The flux of 32 P from fully labeled Daphnia rosea, feeding on the yeast Rhodotorula, has been used to produce a multiple regression model relating the rate of phosphorus release (per unit of animal dry weight) to temperature, food cell concentration, food phosphorus content, and animal dry weight. This model adequately predicted the release rate of phosphorus by other daphnids and by D. rosea feeding on other foods. The validity of the model’s predictions was further confirmed by comparing the phosphorus excretion rate with assimilation minus growth and reproduction. These corresponded closely at higher food levels, and even at the lowest level the correlation was considerably better than the extreme variation in the literature. The model appears to support other tracer studies in suggesting that experiments of long duration substantially underestimate the excretion rates of aquatic animals.