Abstract:Hydrological conditions, water quality and the distribution of phytoplankton assemblage were surveyed in Tangxi Reservoir, Yaoping County （Guangdong Province）, in 2003, to analyze the effecting factors on seasonal changes in phytoplankton growth. Results demonstrated that phytoplankton abundance in flood season, with the highest in July （〉104 cells ml-1）, was significantly higher than that in dry season. In January, Bacillariophyta was the dominant alga （63.1%）, while Cyallophyta was less than 20%. In March, May and July, Cyanophyta was the dominant one, with high proportions of 45.6%, 55.9% and 87.7%, respectively, whereas the proportions for Bacillariophyta decreased to 30.1%, 25.9%, and 1.1%, respectively. In November, proportions for Bacillariophyta and Chlorophyta were equivalent, 40% each, and the Cyanophyta composed below 20%. In December, Bacillariophyta made up 25.6% and Cyanophyta 38.2%. In March, May and July, dominant species of Cyanophyta were Microcystis aeruginosa, Pseudoanabaena, and Gloeothece linearis. In dry January, November and December, species of Bacillariophyta were dominated by A chnanthes sp., Melosira ambigua, Melosira granulata and Cyclotella meneghiniana. No close correlation was found between nutrient concentrations in reservoir and seasonal change of phytoplankton community throughout the year. In flood season （from May to July）, obvious variation of dominant species seemed influenced mainly by temporal change of hydrodynamics. Water temperature was positively related to the abundance of Cyanophyta, but negatively to that of Bacillariophyta. It is indicated that water temperature seemed robe the main factor affecting seasonal change ofphytoplankton community in Tangxi reservoir.