Changes of Water Quality and Eutrophic State in Recent 20 Years of Dongting Lake

Temporal and spatial trends of water quality and eutrophic level in Dongting Lake were analyzed using monitoring data for recent 20 years.The results showed that,by the combined effects of the socioeconomic development,water quality deteriorated with eutrophication,and the eutrophic level in the east Dongting Lake was more serious than that in the west and the south Dongting Lake.Total nitrogen and total phosphorus were the major pollutants in the lake.The annual average contents of these two pollutants varied from 1.08 to 1.93 mg/L and from 0.026 to 0.203 mg/L,respectively.A significantly positive relationship between the concentration of chlorophyll a and the total nitrogen was found in the water phase.Phytoplankton quantity was positively related to the total nitrogen and the total phosphorus,and was negatively related to the maximum water flow.Although chemical oxygen demand in the water phase decreased after remediation of the paper-making enterprises in 2007,concentrations of the total nitrogen and the total phosphorus still showed an increasing trend,and phytoplankton abundance increased significantly.Our results suggest that eutrophication control in Dongting Lake should pay more attention to the non-point pollution.