Evolution Of Eutrophication In Lake Taihu And Driving Force Analysis

Lake eutrophication and water ecological security are major water environmental issues in the world including in China at present.The dominant factors,nutrient concentration thresholds and occurrence mechanisms of lake eutrophication are still controversial,and this controversy also greatly affected the effects of water environmental management measures of government.Water quality mechanism model can realize the flexible management of the nonlinear water system.The water quality mechanism model can not only analyze the current situation of the lake,but also establish appropriate scenarios to predict the future development trend of the lake.In this paper,the long-term historical water quality data were used to construct the Taihu Lake nutrient characteristics database.The Random Forest and Partial Least Squares Regression method were used to identify the environmental and human-driven factors of Taihu Lake’s nutritional status change,and various factors were analyzed for Taihu Lake water environment.The Bayesian Hierarchical Model was conducted to explore the eutrophication process in Taihu Lake and the internal coupling mechanism of different nutrients.The results show that during the period of year from 2007 to 2016,the concentrations of TN and TP in Taihu Lake showed a downward trend,and the N/P value did not change significantly.The concentration of N in the summer and autumn of Taihu Lake is lower,and the concentration of P in spring and winter is higher.Water temperature,DO,TP and N/P have a great influence on Chl a concentration in Taihu Lake.In general,the TN concentration should be controlled below 1.23 mg/L,the TP concentration should be controlled below 0.07 mg/L,and the N/P ratio should be controlled between 20-30.The results of human-driven factors in the change of nutritional status in Taihu Lake indicate that factors affecting the concentration of N in water bodies include rural wastewater,livestock farming,aquaculture and agricultural planting in Taihu basin.For P,the more important factors in the change of water concentration include rural wastewater,livestock farming and agricultural planting in Taihu basin.For the N/P ratio of Taihu Lake,main factors affecting the concentration change in the water body include urban wastewater,rural wastewater and aquaculture in the basin.According to the Bayesian Hierarchical Model,the average real growth rate of phytoplankton in Taihu Lake was 28 mg N·m^-3·day^-1 between 2007 and 2016,and the exchange rate between P and sediment was 0.5 mg P·m^-3·day^-1.N exchange rate with sediment is 112.9 mg N·m^-3·day^-1.Nitrification rate is 99 mg N·m^-3·day^-1.Denitrification rate is 30.4 mg N·m^-3·day^-1.The large fluctuations of nutrient circulation in Taihu Lake can largely affect the nutrient concentration in the lake.The non-synchronous seasonal dynamics of the internal N and P cycles eventually lead to the seasonal conversion of N and P limits in Taihu Lake,i.e.P is the limited factor in from spring and winter,while N and P are both limited factors in summer and autumn.Scenario analysis shows that seasonal reduction of N and P is better than continuous reduction of N and P.The growth of phytoplankton can only be responded after a continuous reduction of 4-5 years.After years of implementation of emission reduction measures,Taihu Lake has been transformed from a lake driven by external loads to a lake now controlled by internal processes in the past decade,which makes the N and P concentrations in Taihu Lake more responsive to seasonal changes caused by internal processes.Therefore,the control strategies for N and P also need to be changed to give priority control in different seasons.It is becoming more and more difficult to achieve water quality improvement through external control.Therefore,in the future,it is necessary to regulate the internal source circulation process in particular,rather than just controlling external input and it is necessary to reconsider the setting of nutrient control targets for Taihu Lake.Seasonal changes in nutrient concentration and environmental drivers should be further considered.