Research On Driving Factors And Regulation Of Eutrophication In Arid Area And Semi-closed Lake

The lakes in arid areas of northwest China are an important part of lake ecosystem in China.With the rapid development of social economy,the ecological environment security of lakes in arid areas is facing a serious threat.Bosten Lake is located in Xinjiang Uygur Autonomous Region and belongs to temperate continental desert climate.The drainage system of the basin is underdeveloped and the entrances and exits are concentrated in the west of the lake area,which leads to slow water flow and poor hydrodynamic conditions,showing typical characteristics of semi enclosed lakes in arid areas.Influenced by human activities,hydrodynamic conditions and regional climate,Bosten Lake has suffered from many water environment problems in recent years,such as water quality deterioration and eutrophication level rise.This paper takes Bosten Lake as the research object,analyzes the temporal and spatial variation characteristics of the water quality of the lake,and clarifies the eutrophication level and change trend of the lake;To explore the main driving factors affecting the change of eutrophication level of Bosten Lake;Establish a numerical model for lake eutrophication,simulate and evaluate the improvement effects of different regulatory measures on lake eutrophication levels.The research results can provide scientific guidance for the further development of Bosten Lake eutrophication control,and also provide reference for the eutrophication control of semi enclosed lakes in similar arid areas.The main conclusions of this paper are as follows:（1）The temporal and spatial variation characteristics of water quality in Bosten Lake were analyzed,and the eutrophication level and change trend of the lake were clarified.From 2018 to2021,the water quality of Bosten Lake met the ClassⅣstandard of surface water,and the average TLI index was 33.05,which was at the moderate trophic level and showed an increasing trend year by year.At the age scale,TP,COD_Mn,COD and other indexes showed an increasing trend.The indexes of TN and SD showed a downward trend.The concentration of pollutants showed obvious seasonal increase and decrease,and the maximum appeared in summer every year.The average concentrations of Chl,TP,DO and NH₃-N in the water quality indicators of Bosten Lake are the highest in the west of the lake area;The average concentrations of TN and BOD were the highest in the center of the lake area;SD,COD,COD_Mn and p H average value are the highest in the east of the lake area.TLI index was high in summer and autumn and low in winter and spring.Among the water quality functional areas,the TLI index of Zone I was the highest,and the TLI index of Zone IV was the lowest.（2）The main driving factors influencing the change of eutrophication level of Bosten Lake were explored.The driving factors of eutrophication in Bosten Lake and their degree of action are as follows:pollutant concentration factors>lake hydrodynamic factors>climatic factors.The average annual concentration of total nitrogen in the inflow of Bosten Lake is about 0.67mg/L,and the ratio of nitrogen and phosphorus in the lake is as high as about 69,and the accumulation of excess nitrogen and phosphorus substances provides sufficient nutrients for algae and other plankton in the water body,leading to the formation of eutrophication,and the uneven distribution of pollutants such as nitrogen and phosphorus causes spatial differences in eutrophication levels.Bosten Lake is a semi-closed lake,the entrance and exit are concentrated in the western part of the lake area,the average flow rate of the whole lake is only 0.0101m/s,the hydrodynamic conditions are poor,resulting in weak migration and diffusion ability of pollutants in the water body,and pollutants are easy to be enriched in the lake area,providing a suitable environment for eutrophication algae growth.The arid area where Bosten Lake is located has high temperature and little rain all year round,and meteorological conditions such as temperature,wind,evaporation and precipitation are conducive to photosynthesis of algae in the lake,which increases the chlorophyll concentration produced,which together drives the eutrophication of Bosten Lake to a higher level.（3）The numerical model of eutrophication in Bosten Lake was used to simulate the improvement effect of the reduction of pollution load into the lake,water replenishment on the north shore and the implementation of comprehensive regulation and control conditions on the level of eutrophication.After the implementation of reducing the pollution load into the lake,the concentration of various pollutants in the lake area has been reduced to a certain extent.The average annual TLI index of the three working conditions were 32.95,32.35 and 31.73respectively.After the implementation of the north shore replenishment condition,the average velocity of the whole lake was increased,and the local hydrodynamic conditions were improved.The average annual TLI index of the three working conditions are 33.62,33.41 and 33.25respectively.After the implementation of pollution load reduction and comprehensive water refill,the pollutant concentration in the lake area decreased significantly,and the average annual TLI index value was 29.70,which decreased by 12.11%compared with the current situation,indicating that the eutrophication level of Bostan Lake reached the lowest level,and the TLI index change amplitude of each water quality functional zone was ranked as follows:Ⅰ>Ⅲ>Ⅱ>Ⅴ>Ⅳ.