Abstract: The Effects of aquatic macrophytes of various life form on phytoplankt on were studied in the large-scale enclosures established in sublake Shuiguohu of Lake Donghu, Wuhan during June-October, 1994. The results is showed that both floating and submerged macrophytes could decrease the standing crops of phytoplankton and resulted in the changes in species composition. The coefficients of productivity vs. biomass of waters in the enclosures with macrophytes were lower than those in the control enclosure and lake water. It is also demonstrated that water hyacinth(Eichhornia crassipes)could effectively suppress the Microcystis bloom and then facilitate the proliferation of submerged macrophytes. The water blooms and physico-chemical parameters were monitored for four months. It is indicated that the competition from small-sized algae such as Cytromonas, Merispodia and Cyclotella might have principally resulted in the dissappearance of blooms of large-sized cyanophytes based on the analyses of the succession of phytoplankton communities and nutrient level among the various enclosures and lake water.