ESTIMATION OF ORTHOPHOSPHATE EXCRETION RATE OF ZOOPLANKTON IN FRESHWATER MICROCOSMS

In the later period of the experiments to investigate the top-down effects of Oreochromis niloticus or Hypophthalmichthys molitrix and Aristichthys nobilis in freshwater microcosms, the specific orthophosphate(PO4-P) excretion rate of Daphnia magna, the sole species of zooplankton in the microcosms, was measured for the different size classes of the animal and in the different incubation time. Moreover, the rate of PO4-P cxcretion (Epz) by D. magna population in the microcosms was estimated with the equation Epz= Σi=1D Espi*Bi, where Esp=the spceific PO4-P excretion rate, B=biomass (dry weight), and n=size classes of the animal. The results indicated that the specific PO4-P excretion rate of D. magna decreased exponentially with the increase of individual weight and reduced markedly as incubation time increased. And the rate of PO4-P excretion by D. magna population and its ratio to the rate of PO4-P uptake by phytoplankton in the microcosms stocked with fish were significantly lower than those in the fish-free microcosms, whereas the estimated values of the above indices in the microcosms with Oreochromis niloticus or Aristichthys nobilis were obviously smaller than those in the microcosms containing Hypophthalmichthys molitrix alone. It appears that phosphorus excretion by D. magna population have made some contributions to PO4-P regeneration in the microcosms stocked with Hypophthalmichthys molitrix alone but contributed inconsiderably to the recycling of phosphorus in the microcosms with Oreochromis niloticus or Aristichthys nobilis under the given experimental conditions.