Study On Formation Process Of Large-small West Lake Of East Dongting Lake Under Water Level Fluctuation In Chenglingji

In recent years,with the rapid development of social economy in Dongting Lake Basin,the water environment problems in the lake area have become increasingly prominent,which are manifested in the shortage of water resources,the deterioration of water quality,the aggravation of eutrophication degree and the frequent occurrence of seasonal bloom,etc.,which have attracted extensive attention.Among them,the seasonal bloom problem is the most prominent,especially in the Large-small West Lake area of east Dongting Lake.The bloom area increases year by year and tends to be normalized.Chenglingji Hydrological Station,the nearest to the Large-small West Lakes,has a great impact on the water environment of the Large West Lake and Small West Lake due to the fluctuation of water level,especially in the aspects of water area,land area,aquatic plant coverage,nutrient concentration and water environment capacity.However,in the current research on Dongting Lake,the research on water bloom only stays at the simple field description level,lacking the linkage analysis of the apparent phenomena and many influencing factors.This paper starts with the fluctuation of water level,starting from the environmental effects of water level fluctuations in Chenglingji,analysis of different water level fluctuation on the Large-small West Lake of Dongting Lake phytoplankton biomass,community structure and the influence of sports migration patterns.In order to provide the basic data and theoretical basis for constructing the water bloom formation mechanism of a typical river connected lake of Dongting Lake.The main conclusions of the study are as follows:（1）The water level of Chenglingji was significantly correlated with the water area of Large-small West Lake of east Dongting Lake（r=0.871 P<0.01）,and negatively correlated with land area and large aquatic vegetation cover.The environmental capacity of total phosphorus（TP）and total nitrogen（TN）in each hydrological period is higher in the wet season than that in the normal season,the normal season is higher than that in the dry season.The fluctuation of chenglingji water level affects the water level of Large-small West Lake and the amount of lake water entering,thus affecting the water area,land area,aquatic vegetation coverage area and water environmental capacity of the region.The water movement pattern in each hydrological period is mainly wind-generated flow,which is not significantly affected by the fluctuation of chenglingji water level.（2）The monitoring results of each hydrological period show that the succession process of the phytoplankton population structure in the Large-small West Lake is:wet season（cyanophyta,bacillariophyta and chlorophyta）—normal season（bacillariophyta,chlorophyta,cyanophyta and euglenophyta）—Dry season（chlorophyta,bacillariophyta,cyanophyta and Cryptophyta）.Chenglingji water level fluctuation affects TN and TP concentration,euphotic zone,water temperature and dissolved oxygen by changing the amount and water level of the Large-small West Lake,thus affecting the succession of phytoplankton in the region.The results of continuous monitoring in water-prone areas showed that:The water level fluctuation of Chenglingji affects the biomass of phytoplankton in this area by affecting the water level,SD,phosphorus nutrients,D-Si,etc.The response relationship of change can be expressed as a linear equation:Y _biomass=2.13X _{water level}-44.389（R²=0.715 P<0.01）.（3）The direction of phytoplankton movement in the Large-small West Lake of east Dongting Lake is consistent with the direction of wind-generated flow,drifting towards the downwind and gathering,and changes continuously with the change of the wind direction and flow direction.The horizontal drift rate of phytoplankton is affected by the combined effect of wind speed and flow velocity.The fitting relationship between the horizontal drift rate of phytoplankton and wind speed and flow rate in this area can be expressed by a linear equation:V _algae=0.0027V _wind+0.6659V _{water flow}+0.0115（R²=0.9768 P<0.01）.