Abstract: This paper described analyses of plankton species composition and phytoplank-ton diversity and the relationship between hydrological parameters and fish vield.Significant negative relationship was observed between summer water level and fishyield,Y=25.98-1.33X(r=-0.7113).At summer high water level,microplankton was observed with decreasing biomass al1d increasing phytoplankton diversity.However,at summer low water level microplankton was perceired with increasingbiomass and decreasing phytoplankton diversity.According to the theory of cascad-ing trophic interaction,this variation was related possibly to fish predacity.