Abstract:Fuxian Lake is a typical deep plateau lake located in Yunnan Province, China. In order to describe phytoplankton community of the Fuxian lake and its response to ecological changes, investigation on phytoplankton community and its related environmental factors were carried out from June to July in 2005. there existed 56 genera, 32 families and 7 phyla in totally eighty phytoplankton species from the survey. The dominant taxa were Cyanophyta, Chlorophyta and Bacillariophyta, with the species number in the order of Cyanophyta (46.0%) > Chlorophyta (25.0%) > Bacillariophyta (15.0%) and quantity in the order of Cyanophyta (61.6%) > Bacillariophyta (16.1%) > Chlorophyta (9.1%). The abundance of phytoplankton community was 87.58×104 ind./L, and the dominant species was Mougeotia sp. with a relative abundance of 35.9％. The average density of phytoplankton community was higher in Northern lake than that in Southern lake (P＜0.05), and the average density of phytoplankton community in littoral area is higher than that in lake centre area (P＜0.01). The vertical distribution of phytoplankton demonstrated a significant stratification in the open waters. Phytoplankton mainly accumulated in 0-30 m depth, and Cyanophyta dominated the community in density from top to the bottom in lake. Environmental analysis indicated that the horizontal distribution of phytoplankton density is controlled by the nutrients while its vertical distribution is determined by the physical variables. The survey results in different periods showed the increased trends both on the species number and on the density of phytoplankton in Fuxian Lake. It is the increased nutrient levels of the lake that cause the change of phytoplankton community. Large filamentous Mougeotia sp. has growed as the dominant species of the community since the mid-1990s,due to its competitive advantages and the top-down control of zooplankton by Neosalanx taihuensis.