ESTIMATION OF PHOSPHORUS RELEASE RATES FROM NATURAL ZOOPLANKTON COMMUNITIES FEEDING ON PLANKTONIC ALGAE AND BACTERIA

Phosphorus release rates from Daphnia galeata, the dominant metazoan, were estimated in eutrophic Lake Nesjovatn during summer by applying our previously published mass balance model. For both juveniles (2.0 mu g C ind.(-1)) and adults (6.6 mu g C ind.(-1)), three different estimates of P release were made by two different parametrizations and by considering the importance of ingestion of detrital particles. As a basis for estimates of P release, clearance rates of algae and bacteria were determined as well as a thorough characterization of the food compartments. The specific release rate of P varied between 0.16 and 1.18 mu g P mg C-1 h(-1) for juveniles and between 0.06 and 0.96 for adults, independent of estimate. These release rates are considerably lower than those obtained by previously published models. Most of the observed variation was due to variation in the P content of the food, thus emphasizing the importance of including this variable in studies of P release from zooplankton. Calculations of rates of population P release suggest that D. galeata contributed <1/3 of the P requirements of algae and bacteria.