Abstract: This paper studies the dynamics of Nand Pin freshwater microcosms with varying densities of Nile tilapia(Oreochromis niloicus). The concentrations of ammonia (NH₃-N), particulate phosphorus (PP) and total phosphorus(TP) in microcosms with Nile tilapia are higher than those in control, but the contents of or thophosphate(PO₄-P) and sediment phosphorus are lower. There are significant differences in measured values of some indices among different density treatments, but none of these indices are found to be regularly changed with stocking density of Nile tilapia. In the microcosms, PO₄-Pconcentration is significantly correlated with zooplankton and phytoplankton densities and primary productivity, and NH₃-Nconcentration is related to phytoplankton density. The top -down effect predicted by the trophic cascade hypothesis is not existed between phytoplankton density and TPconcentration, but on the contrary, there is a tendency of existing bottom-up effect. The variation of Pdistribution in the microcosms can reveal the main mechanism of accelerating nutrient cycling in the systems, and thus, enhancing their eutrophication processes by Nile tilapia.

Key words: Microcosm, Nile tilapia, Nutrient dynamics, Orthophosphate, Ammonia, Phosphorus distribution