Abstract:Hedi Reservoir, located in the north of Leizhou Peninsula, is a large and mesotrophic reservoir. To understand the structure and variation of phytoplankton community in the reservoir, phytoplankton was sampled at five sites in February, July, September and December of 2003. Total phytoplankton biomass ranged from 0.156~2.548 mg L-1, mainly contributed by cyanobacteria and diatom. The structures of phytoplankton community at five sites varied markedly and seasonally from spring to winter, and the dynamic pattern of phytoplankton community was quite similar among the five sampling sites. The phytoplankton biomass in flood season was higher than that in dry season. The phytoplankton benefited from high temperature in flood season and nutrient input with inflow. Phytoplankton biomass showed a spatial gradient from riverine zone to lacustrine zone, corresponding to a decrease in phosphorus concentration. Phytoplankton species was dominated by thermophilous species in tropics, and the dominant species in dry season were markedly different from those in flood season. The dominant phytoplankton species in dry season were Rhizosolenia sp., Cyclotella sp., Melosira granulate and Melosira ambigua, while Cylindrospermopsis raciborskii, and Pseudanabaena limnetica in flood season. Phosphorus was the main factor regulating the variation of the dominant phytoplankton species in flood season. The biomass of first eight dominant species contributed 85%~92% of total biomass, significantly less than that in the temperate lakes with steady structure of phytoplankton community.