Lake Tai (Tai Hu) is located in the S part of the Yangtze River delta, has a surface area of 2,425 km2, a mean depth of 2.12 m, and a volume of 5.15 km3. The climate of the region is characterised by an average annual air temperature of 15.7°C, precipitation of 1,178 mm and evaporation from the water surface of about 1,024 mm. The average annual water temperature is 17.1°C. A positive and a negative thermocline may occur in one day, a characteristic of polymictic lakes. The average water level is 3.03 m above Wusong datum. Total annual inflow was 8.7 km3 with a total outflow of 9.0 km3 and a residence time about 212 days during an average water year. Influent-effleunt currents and wind currents both occured in the lake with a velocity of between 0,1 and 0,3 m s−1; the height of wind generated waves up to 1 m when the wind velocity was about 13 m s−1. The average transparency of lake was 0.39 m and the water colour about XV-XVI. The salinity of water was 157.66 mg 1−1, alkalinity 1.22 meq 1−1, total hardness 1.523 meq 1−1 and the pH 8.0. The lake water belonged to calcium bicar-bonate and calcium-sodium bicarbonate types during the study period. The biota had 114 genera of phytoplankton, 122 species of zooplankton, 68 species of benthos, 61 species of aquatic macrophytes and 106 species of fish. Ten species were considered predominant:Microcystis aeruginosa, Anabaena spiroides, Chroomonas acuta, Strobilidum velox, Tintinnopsis conicus, Corbicula fluminea, Zizania latifolia, Phragmites communis, Coilia ectenes taihuensis, and Protosalanx hyalocranius.