Dependence of the rate of release of phosphorus by zooplankton on the P: C ratio in the food supply, as calculated by a recycling model1

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Abstract
In two enclosure experiments, Daphnia pulex ingested cryptophytes, bacteria, and probably detritus particles. The specific clearance rate of the zooplankton increased when the concentration of food decreased. The P : C ratio of the food also increased. More than 92% of the particulate phosphorus was located in the living cells (algae and bacteria); the detritus was practically phosphorus‐free. The specific release rate of phosphorus estimated for the daphnids by use of the recycling model increased as the P : C ratio of the food increased and became zero at a critical low P : C ratio, Q, of 6–8 µg P mg−1 C. At this concentration, all the ingested phosphorus is needed for growth and reproduction, and no release of the element can be expected. This indicates that Daphnia may experience P limitation in nature, since the P : C ratio of P‐starved algae and detritus may be considerably µg P mg−1 C.The use of mass balance also appears to be suitable for modeling phosphorus and carbon fluxes through zooplankton in pelagic ecosystems.