Abstract:Phytoplankton, water quality and hydrology in Feilaixia reservoir, Guangdong, were investigated to understand temporal pattern ofphytoplankton community. The reservoir, located in the southern tropic region, is a typical riverine reservoir with a short residence time, averaging 15 days a year. Total cell abundance of phytoplankton was measured to be less than 106 cells L-1, and the concentration of chlorophyll a varied between 1 and 5 μg L-1. In total, 140 taxa were observed in all samplings in various seasons, in April 83 taxa, in July 92, in September 96 and in December 51. The variation in species composition of phytoplankton was mainly contributed by green algae and diatoms. In the dry season, diatoms were the important groups, which were mainly composed of Melosira variant, Cyclotella menighiniana and Syaedra spp.. Cryptophyta were the most important groups,dominated by Cryptomonas erosa and Chroomonas acuta, which appeared in relatively long residence time of water in reservoir （67 days） in December. Green algae, cyanophyta and diatoms were rich in flood season with high water temperature and high nutrient concentration. However, there was no significant differences in seasonal composition of green algae, in which no obvious dominant species were observed. Such green algae were Dictyosphaerium pulchellum. Actinastrum hantzschii and Scenedesmus bijuga. The cyanophyta were dominated by Gloeocapsa sp.. Merismopedia elegans and Pseudoanabaena sp..