Abstract: Lake/reservoir eutrophication has become a more and more serious problem. Watershed pollution prevention is the key of eutrophication control. Calculation of water environmental capacity is the important basis of both the pollution prevention and the capacity distribution in the basin. This study is aimed to assess the eutrophic status and estimate the environmental capacity of the Lake Xiannv in Jiangxi Province, a lake with good water quality, in order to provide the reference for the aquatic environmental protection and capacity gross control of main pollutants in this important ecological function zone. From the year of 2011 to 2013, seasonal monitoring of physical and chemical parameters in Lake Xiannv was carried out and parameters included total nitrogen (TN), ammonia nitrogen (NH₃-N), total phosphorous (TP), transparency (SD), chemical oxygen demand (COD_Mn) and chlorophyll a (chl_a). Samples were collected at 4 sites, located in the upper lake-Qianyang Lake (S1-S2) and the lower lake-Wulong Lake (S3-S4), respectively. A comprehensive eutrophic state index was applied to assess the trophic status, and the Vollenweider-Dillon model was employed to analyze the nutrient loading and assimilation capacity of Lake Xiannv. COD, ammonia, TN and TP were used as the control index in calculation of the water environment capacity. Results showed that the water quality of Lake Xiannv could meet Level Ⅱ-Ⅲ standards of water quality for surface water. The mean values (range in parentheses) for TN, TP, NH₃-N, COD_Mn and chl_a were 0.59 mg/L (0.32-0.91 mg/L), 0.028 mg/L (0.017-0.080 mg/L), 0.31 mg /L (0.012-0.59 mg/L), 2.85 mg/L (1.61-5.59 mg/L) and 0.56 μg/L (0.37-0.95 μg/L), respectively. The water quality tended to improve from the upper lake zone to the lower one of the lake. TP was in the status of Level Ⅲ standards in some months, whereas the other indicators were steadily in Level Ⅱ standards. In general, Lake Xiannv was mesotrophic, with higher nutrient levels in the Qianyang Lake area, and the Secchi depth was in the highest trophic state (slightly eutrophic) by the analysis of the single factor index. Loading of COD, ammonia, TN and TP in Lake Xiannv were 21 208.0 t/a, 3 528.8 t/a, 4 991.2 t/a and 248.1 t/a, respectively. The ratio of the remaining capacity of these four pollutants were 56.88%, 68.25%, 62.89% and 13.67%, respectively. TP was selected out as the greatest impact factor on water environmental capacity of the lake. Nutrient loading should be reduced by 72% for phosphorus to achieve water quality meeting Level II standards throughout Lake Xiannv. In this study, the main factors influenced on water environment carrying capacity were analyzed, as well as the variation tendency of water quality and how to improve the water quality of the lake. Based on the analysis of the main influence factors on water environmental capacity, the constructive scheme was proposed for improving water environment carrying capacity in the end.

Key words: Lake Xiannv, eutrophication, comprehensive assessment, water environmental capacity, water quality goal