Abstract: CHANGES IN ALGAL COMMUNITY STRUCTURE AND ALGAL DENSITY IN RELATION TO TN AND TP IN THE THREE GORGES RESERVOIR  Algal community structure and algal density in the Three Gorges Reservoir were investigated from January 2003 to December 2004. Eleven sampling sites were selected, five of which were in the mainstream and six in the anabranch. Samples were taken once a month. More than 1 000 samples of algae were identified. The results were discussed in three periods, i.e. before water storage (2003.01-06), one year duration after water storage (2003.06-2004.05), and one and a half year duration after water storage (2004.06-12). It was shown that the algal species composition and cell densities in the reservoir varied markedly among the three periods. At present, the algal community in mainstream was dominated by Bacillariophyta, and then Chlorophyceae. In the tributary, species of Chlorophyta were most frequently observed, and then species of Bacillariophyta. The mainstream and the tributary after water storage could be referred to the mesotrophic and to meso and eutrophic types, with water quality deteriorating than before water storage. The cell densities of algae in the three periods showed an increasing trend especially in the tributary. It proved that the water quality of the reservoir has been caused by the significant reduction in current speed and the increase of nutrients concentration. In order to protect the water quality of the Three Gorges Reservoir, it is important to prohibit the discharge of wastewater with high phosphorus and nitrogen concentration.

Key words: Three Gorges Reservoir, algae, mainstream, tributary