Abstract: An investigation on the community structure of phytoplankton and environmental impact assessment was carried out in Harbin section of Songhua River in August,2008. The results showed that a total of 72 species 43 genera 7 phylums of phytoplankton were recorded, the community structure of which mainly belongs to Chlorophyta, Bacillariophyta and Cyanophyta. The dominant species are completely from Chlorophyta Cyanophyta and Cyclotella of Bacillariophyta.Based on the dominant species, abundance and biomass, pollution indicator community and physical-chemical indicators, the comprehensive assessment indicates that Harbin section of Songhua River was eutrophication in summer.The construction of Dadingzishan Navigation and Hydropower Project has been leading to raising the water level and reducing the flow rate in Harbin section of Songhua River, resulting the slow succession of phytoplankton community structure from blue and green algae to the diatom in this water in summer.And the water bloom will break out easily.

Key words: Songhua River, Harbin section, phytoplankton, community, impact assessment