Abstract: The community structure and species diversity of planktonic algae in Gehu Lake of Jiangsu Province were investigated from April 2004 to January 2006, and their relationships with the physicochemical parameters of the lake were analyzed. A total of 267 planktonic algae species belonging to 8 phyla and 147 genera were identified, among which, Chlorophyta (130) was the dominant, followed by Bacillariphyta (59) and Cyanophyta (36). The dominant species were Microcystis aeruginosa, M. incerta, Merismopedia punctata, M.tenuissima, Rhabdoerma lineare, and Goelastrum microporum. The mean annual total algal density and biomass were 4 871.09×10⁴ and 7.302 mg·L^-1, respectively. The density was the highest in autumn, and the biomass was higher in spring and autumn than in other two seasons. The biodiversity reached the maximum in spring, and was related positively to algal density but negatively to algal biomass. The standing crop of planktonic algae decreased gradually from the north to the south area, with significant difference between these two areas but no significant differences between the north and middle area and the middle and south area.

Key words: Allelochemical, Phenolic acids, Weed-suppressive effect, Orthogonally gyral regression design