| Dongting Lake is the first Yangtze-connected lake in the middle reaches of the Yangtze River after the Three Gorges reservoir.It receives the water from four upstream branches(Xiang,Zi,Yuan and Li)and three water inlets(Songzi,Taiping and Ouchi)of the Yangtze River,then the water reintroduce into the Yangtze River from Chenglingji.Connected by the coupling network,Dongting Lake,Yangtze River and the upstream rivers form a complex,dynamic relationship between ‘Dongting lake and Yangtze River’,as well as ‘Dongting lake and its upstream rivers’.After 2013,the cascade reservoirs in the upper reaches of the Three Gorges were gradually built and put into operation,and the operation of the Three Gorges sediment dispatching experiment changed the amount of water and sediment into Dongting Lake by the three water inlets.In addition,the implementation of Xiangjiang River pollution prevention policy further aggravated the complexity and uncertainty of water environmental protection in Dongting Lake.Therefore,it is necessary to predict the impact of changes on the risk of water blooms in Dongting Lake.on the basis of accurately analyzing the current situation of changes in Dongting Lake,exploring the relationship between hydrology,water quality factors and water blooms.(1)Distribution characteristics of water quality factors in Dongting Lake and its relationship with chlorophyll aNitrogen and phosphorus nutrients were important factors for algae growth.So the temporal and spatial distribution characteristics of N and P flux in Dongting Lake from 2013 to 2017 were analyzed,including monthly change,annual change and spatial comparison.The analysis found that the Xiangjiang River and Yuanjiang River were the most important sources of TN and TP fluxes entering the lake respectively,and the fluxes mostly entered Dongting lake from April to September along with the discharge in flood season.After the construction and operation of the Three Gorges Reservoir,the average annual water inflow from the three water inlets decreased by one-third,but its TP flux increased by 7.8% after2013,and the total phosphorus composition of Dongting Lake inflow was mainly dissolved phosphorus,which was easily utilized by algae.In addition,the concentration of chlorophyll a(Chl-a)in Dongting Lake was significantly lower when nitrogen and phosphorus concentrations were not lower than other lakes.The reason may be that Dongting Lake had relatively good connectivity,low nitrogen and phosphorus retention rates,and hydrological conditions were not conducive to algal growth.It was found that the peak of Chl-a concentration occurred in September,and the overall Chl-a concentration was East Dongting Lake > West Dongting Lake > South Dongting Lake.Using correlation analysis,multiple regression and other statistical methods,the correlation between hydrological factors and Chl-a and multiple regression equation were established.It was found that the influencing factors of Chl-a concentration were water temperature,ammonia nitrogen,total nitrogen,total phosphorus and suspended solids.In addition,through comparative analysis,the concentration of suspended solids and current speed in the area of water blooms were significantly lower than the average of the whole lake.(2)Construction of the two-dimensional hydrodynamic-eutrophication model of Dongting LakeAiming at the complex characteristics of the water system in the Dongting Lake Basin,unstructured grids and Navier-Stokes governing equations were used to solve the problem,and a two-dimensional hydrodynamic-eutrophication model was established by MIKE 21.Through sensitivity analysis method and gradual calibration of model parameters,the model output value was close to the measured value,which was reflected in the Pearson correlation coefficient of most points was above 0.75,and the MRE was relatively low.(3)Result analysis and optimization of the two-dimensional model of Dongting LakeAfter the model calibration was completed,the results of the model were analyzed,and the hydrodynamic conditions such as velocity and water level,as well as the spatial and temporal distribution of DO,Chl-a and other water quality conditions in Dongting Lake were described.In addition,based on the research on the relationship between suspended solids and chlorophyll a,the model was optimized and improved by Ecolab,and the function of suspended solids affecting the light conditions in the water was added,which improved the accuracy of the model to predict the concentration of chlorophyll a.(4)Influence of Water Bloom Risk in Dongting Lake Under the Changing River-Lake RelationshipDue to the gradual construction of cascade reservoirs and other water conservancy facilities in various tributaries of Dongting Lake,it was expected that the the amount of water entering Dongting Lake would continue to decrease during the impoundment period,and the amount of sediment and suspended solids would also decrease.It was found that the decrease of water volume in the impoundment period slowed down the flow rate,lowered the water level of the lake and made the lakeshore exposed in advance.In October,the concentration of chlorophyll a in East Dongting Lake,South Dongting Lake and West Dongting Lake increased by 1.64%,9.39% and 13.60% respectively.The decrease of suspended matter concentration increased the overall risk of water bloom in Dongting Lake to a certain extent.From July to September,the concentration of chlorophyll a in East Dongting Lake increased by 5.11%,6.57% and 1.08% respectively.The increase of TP flux from three water inlets only slightly increased the risk of water bloom in Dongting Lake.Based on the above analysis,it is found that Dongting Lake is already under suitable water temperature and nutrient background conditions for water blooms,and high suspended matter concentration and water flow velocity are important factors limiting the occurrence of water blooms.The flow during the impoundment period and the sharp drop in the amount of water and sediment entering the lake have significantly increased the risk of water blooms in certain areas of Dongting Lake.Apart from these,the Three Gorges Reservoir regularly discharges water and sediment during the impoundment period may play important role in improving water blooms in Dongting Lake. |
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