Spatial-temporal Change Of Lake Surface Water Temperature In The Lower Yangtze River Basin And The Implications From Climate Change

Lakes play an important role in maintaining water biodiversity and providing key ecosystem services to humans.Changes in lake water temperature have important effects on the growth of aquatic organisms,physical and biogeochemical processes.However,the existing research on the long-term characteristics of the surface water temperature of shallow lakes and its key climate driving factors are still not clear enough.In this study,39 lakes in the lower Yangtze River Basin were used as the research object.Through long-series surface water temperature observation data（2006–2019）,the characteristics of temporal and spatial changes in the temperature of lakes in the lower Yangtze River Basin were analyzed,and statistical models were used to analyze the driving factors of spatial differences in water temperature.At the same time,taking Taihu Lake as an example,using a one-dimensional lake heat balance model（Freshwater Lake model,FLake）to identify the main climatic driving factors of the long-term change trend of lake water temperature,it is predicted that the water temperature and heat balance of Taihu Lake will be under climate change scenarios from 2020 to 2099（RCP2.6,RCP6.0,RCP8.5）potential change trends.Finally,a three-dimensional hydrodynamic-water quality model（The Environmental Fluid Dynamics Code,EFDC）is used to predict the potential changes in the circulation of nutrients and algae growth in Taihu Lake under the background of lake warming（RCP8.5）.The results of the study show that during the period 2006–2019,the annual variation of the average water temperature of the lakes in the lower Yangtze River Basin showed an upward trend,with a rising rate of 0.06 ^oC/year.There are big differences in the annual variation trend of water temperature in different seasons.The rising trend of water temperature in spring is more significant,followed by winter,where the heating rate reaches 0.11 and 0.07 ^oC/year,respectively.Near-surface air temperature,cloud cover,rainfall and water temperature are all positively correlated;while net long-wave radiation is negatively correlated with water temperature.Different factors explain the spatial difference of lake water temperature in the lower reaches of the Yangtze River,in descending order of surface air temperature>net long wave radiation>cloud cover>rainfall.Using the FLake model to simulate the changes in the water temperature of Lake Taihu,the study found that during 2006–2019,the main climatic driving factor for the increase in the interannual water temperature of Lake Taihu was downward solar radiation,which caused the water temperature of Lake Taihu to rise by about 0.0054^oC/year,and the relative contribution ratio reached 24.1%;Secondly,the climatic driving factors that cause the interannual water temperature increase in Lake Taihu also include water vapor pressure and cloud cover,contributing 2.3%and 21.1%.The FLake model is used to predict the water temperature changes of typical lakes under three climate change scenarios from 2020 to 2099.The results show that the water temperature of Lake Taihu will rise at a rate of 0.2–0.5 ^oC/decade.Using EFDC to simulate the nutrient cycle and algae biomass in Lake Taihu,the results show that as the water temperature rises,the seasonal fluctuations of the nutrient cycle flux within Lake Taihu may increase.Compared with 2019,the amplitude of fluctuations in the future ammonia nitrogen（NH₄⁺-N）may increase.Increased by 6.47%–59.14%,the fluctuation range of total phosphorus（TP）during the year may increase by 1.12%–1.58%;the annual algal bloom outbreak in Taihu Lake may be earlier and last longer,which will also have an impact on the community structure.In the future,the number of days when the concentration of chlorophyll a（Chl a）exceeds 10μg/L may increase from 272 days to 274–323 days.Cyanobacteria will dominate,and their biomass may increase by 10.92%–14.36%.In the context of lake warming,the results of this study can provide a reliable basis for the long-term management of eutrophic lakes in the lower Yangtze River Basin.