Eutrophication Evolution, Driving Mechanism And Bloom Risk Assessment For Xiannv Lake

Lake Xiannv (Formerly known as Jiangkou Reservoir) is one of the four largest reservoirs in Jiangxi Province, which located in Xinyu city.It was founded in 1958 and known as the famous national 4A level scenic attraction, and also the main source of drinking water of Xinyu city and the surrounding counties and cities. As a typical large reservoir, providing both domestic and industrial water, it also plays an important role in regulating climate and local ecology. While, with the rapid development of the economy in the watershed, the increasing industrial and domestic sewage emissions put the water quality of Lake Xiannv at risk in recent years. For ensuring the safety of its water resources and reservoir ecology, it is very important to protect its water quality and also does research on its eutrophication.In this paper, some main conclusions on researching the eutrophication evolution, driving mechanism and water bloom risk assessment of Lake Xiannv were as follows:(1) Through the study on the spatio-temporal dynamics of water quality in Lake Xiannv Basin, its general water quality was in class III, and class II was found in the downstream lake outlet. The water quality was various from different zones of the lake. The overall water quality of upstream lakes was inferior to that of the lower lakes. And in the high-water and normal-water period, the water quality was better than that in dry season throughout the watershed. In the dry season, the pollution of total nitrogen, total phosphorus was more serious than other seasons.(2) Due to the morphology characteristics of Lake Xiannv and its huff and puff flow characteristics, its water quality and eutrophication situation varied from different lake regions and different seasons. The transparency in the inflowing estuary was low and the velocity of flow was fast with the shortest HRT and lowest nutrient contents. The transition zone with small water fluctuation was the main area of suspended matter deposition and its bottom sediment nutrient concentrations was high. And its transparency became higher than the inflowing estuary. The growth rate and biomass of phytoplankton increased greatly in the transition zone detected by Chl_a. The lacustrine zone of the lake was located in the dam which water quality was the best and the eutrophication was lowest in the whole lake.Among the different water environmental factors, chl_α、TP、TN、SD、CODMn were detected as the main factors of eutrophication in Lake Xiannv. While the impact of some factors such as DO, conductivity, BOD5, NH4-N, was weak for the contribution to water eutrophication. The phosphorus was the key element for the control of the nutrition abundance. The continuous import of phosphorus would make its eutrophication aggravate.(3) Through the Grey System Prediction Model (GM) and BP Neural Network Model, the main indexes of water quality were predicted in Lake Xiannv and its risk of water bloom was also assessed. We found that the annual mean of pH almost had no changes in all measured values (2008-2014), with weak upwarding tendency of predictive values in 2015 and 2016 over the values of the previous years, and all in alkaline. There was small deviation in the prediction results of DO from the detected values of the former years. The concentration of DO was always better than class Ⅲ of water quality standard. The annual mean of TN detected was close to class Ⅳ. The annual mean of NH4-N had large difference among all measured values (2008-2014), with weak upwarding tendency about the predictive values of NH4-N over the measured ones of previous years. TP was in the worse increase, and there was in a little increasing tendency judging from its measured values and predicted results. The comprehensive nutrition state index was growing, and there was in the risk of water bloom through the predicted values.