ESTIMATION OF THE TURNOVER TIME OF ORTHOPHOSPHATE IN FRESHWATER MICROCOSMS

Experiments have been conducted to investigate the effects of Oreochromisniloticus or Hypophthalmichthys molitrix and Aristichthys nobilis on the communitystructure, metabolism and nutrient levels of freshwater microcosms. consultsdemonstrated that in the microcosms stocked with fish, orthophosphate (PO4-P)concentrations were lower but primary productivity was higher than those in thefish-free microcosms, probably indicating an intense cycling of phosphorus in theexperimental microcosms. In the later Period of the experiments, the tUrnover bine ofPo4-P in the microcosms was estimated through the index of the bine required foruptake of the total amount of PO4-P in the water column by phytoplankton, in whichthe PO4-P uptake rate of phytoplankton was eshmated by the decrease of PO4-Pconcentrations in the sample bottles suspended in the microcosms in the given bimeintervals. The results from both experiments showed that the rates of PO4-P uptake byphytoplankton were significantly higher while the indices for PO4-P tUrnover bine weremarkedly lower in most microcosms with fish than those in the fish-free microcosms.In each experiment the rates of PO4-P uptake by phytoplankton were showed to bein significantly posihve correlation with the density and primary preduction ofphytoplankton but in insignificantly negative correlation with PO4-Pconcentrahons inthe water column. It appears that Oreochromis niloticus had much greater influence ascompared witrh Hypophthalmichlhys molilrix and Aristichthys nobilis on phosphoruscycling in the microcosms, whereas the effect of Aristichthys nobilis was obviouslygreater than that of Hypophthalmichthys molitrix. The reduchon of PO4-P turnover timein the microcosms containing fish was attributed mainly to the accelerahon of PO4-Pregeneration by fish.