Abstract: In August September 2008, an investigation was made on the phytoplankton in southern Taihu Lake, and the water quality at nine sampling sites was analyzed. A total of 47 phytoplankton species belonging to 28 genera and 7 phylums were collected, with 9 species of Cyanophyta, 13 species of Chlorophyta, 18 species of Diatoms, 3 species of Euglenophyta, 2 species of Cryptophyta, 1 specie of Chrysophyta, and 1 specie of Dinophyta. The phytoplankton density was 3.56×10⁶-7.99×10⁶ cells·L^-1, Shannon Weaver index was 0.29-2.77, evenness was lower than 0.5, and trophic state index (TSIM) was higher than 64. Based on the evaluation of the dominant species composition, density, and ShannonWeaver index of phytoplankton, and of the TSIM, the southern Taihu Lake was considered at the stage of eutrophication.

Key words: Water deficit, Supplemental irrigation, Winter wheat, N uptake, N residue