According to the data collected from the five monitoring sites in front of the dam in Thousand-island Lake in September 2015, the vertical distribution characteristics of dissolved oxygen (DO), water temperature, pH, turbidity, conductivity and phytoplankton were analyzed. The influencing factors of special stratification of DO and vertical distribution of phytoplankton were also discussed. The results showed that:① The vertical distribution of DO presented "the surface is higher than the underlying" mode. DO ranged from 1.95 mg·L-1 to 8.25 mg·L-1 and the average concentration was 5.10 mg·L-1. Low-oxygen zones appeared between 12 m and 20 m and the minimum concentration was 1.95 mg·L-1 at 17 m. The concentration of DO maintained at a high level between 0 m and 12 m and the vertical variance was small. Sudden drop of DO occurred between 12 m and 20 m and there was an anoxic zone (<4.0 mg·L-1). The concentration of DO returned to normal level between 20 m and 38 m. Then it reduced under 38m as the water depth increased. The vertical distribution of pH showed the same trend with that of DO and their sudden change regions appeared in the same zone. ② Vertical phytoplankton biomass displayed significant difference. The phytoplankton biomass of S1, S2 and S3 presented:mid-layer > surface > bottom. But S4, S5 presented:bottom > surface > mid-layer. In addition, the depth between 20 m and 30 m was the best for the growth of phytoplankton. ③ The correlation analysis showed that DO and water temperature in the thermocline were significantly correlative. The range and degree of low-oxygen zones in Thousand-island Lake was determined by physical processes like thermal stratification of water and direct or indirect effects of the plankton. Phytoplankton and DO, pH showed significant negative correlation. The phytoplankton was mainly influenced by turbulent mixing and DO stratification in the surface layer while it was mainly influenced by light intensity at the bottom.
Keywords: Thousand-island Lake; dissolved oxygen; physical and chemical factors; phytoplankton; vertical distribution.