Study On The Relationship Of Algal Bloom And Phosphorus Cycling Pengxi River Of Three Gorges Reservoir Area

Phosphorus（P）is the main nutrient that causes eutrophication in fresh waters,and most P in lake ecosystems usually is stored in the bottom sediment.Therefore,when exogenous phosphorus has been controlled,the concentration of phosphorus in water still remains high,providing a continuous supply of nutrients for the massive growth of algae.This research located in Pengxi River which is a first order tributary of Yangtze in the Three Gorges Reservoir region,and located in Gaoyang Lake which is the middle reach of the Pengxi River.We sampled for eight months（December 2019 to September2020）on seven sites of Pengxi River;we sampled for 33 days（April 2019 to May 2019）during the algal bloom season;and we sampled for one year（September 2019 to September 2020）in the water-level-fluctuation zone（WLFZ）of Gaoyang Lake of Pengxi River.Chlorophyll-a（Chl-a）,different forms of P in three water layers（surface,middle and bottom）,water quality parameters,P in sediment and spatiotemporal variation of phosphorus in WLFZ under flooding and dryness period were investigated.From the point of view of P,the relationship between algae,water and sediment in Pengxi River was explored,which provided some scientific basis for the further study on the mechanism of the outbreak of algal bloom and the prevention of eutrophication in the Three Gorges reservoir area.From December 2019 to September 2020,the water quality（including water temperature（WT）,dissolved oxygen（DO）,p H,Chl-a and P）of the Pengxi River and the dynamic change of P at the water-sediment interface were continuously monitored at the end of each month,and the following conclusions were drawn:（1）The P content in the sediment of Pengxi River increased from December,2019to September,2019.During this period,the sediment of Pengxi River was the"source"of P in winter,spring and summer;and the"sink"of P in late summer and autumn.The environmental factors（WT\DO\PH）at the bottom of the Pengxi River were closely related to the dynamic change of P at the water-sediment interface,which mainly showed that high WT and p H,low DO were conducive to the release of P in the sediment.（2）P in the sediment has become the main source of total dissolved phosphorus（DP）in water of Pengxi River,and it was mainly released in the form of available phosphorus.When there is a rainstorm,especially in summer,the surface sediment of Pengxi River is easily scoured,which makes a large amount of nutrients deposited in the upper reaches of Pengxi River（PX7-6）to the middle and lower reaches of Pengxi River（PX5-1）.（3）Water disturbances may inhibit algal growth and aggregation.However,water disturbance maked algae mainly concentrate in the bottom layer instead of the surface and middle layer in water,which may further affect the water quality of the bottom layer in water.However,water disturbance maked algae mainly concentrate in the surface and middle layers moving down,and more algae growed in the bottom layer.Moreover,since the essential problem of excessive nutrition in the water has not been solved,once the disturbance was stopped,the algal bloom would still occur on a large scale.From September,2019 to September,2020,the P change in the WLFZ of Pengxi River was monitored in situ for one year,and the P content and soil physical and chemical properties in the WLFZ in different periods were analyzed,and the following conclusions were drawn:（1）The total phosphorus（TP）content of soil in the WLFZ increased after a year of spatiotemporal variation,showing a"P sink",and the content of TP in PX160 was the most obvious,which was 1.6 times of the content of PX160 in 2019.Soil organic matter（OM）and available phosphorus（AP）in the WLFZ changed little in whole one year,and their average contents increased slightly.Combined with the changes of both contents in the two periods of flooding and drying,it was found that flooding was beneficial to the accumulation of OM and AP,while drying would reduce the contents of both.（2）During the flooding period,the soil P in the WLFZ was mainly accumulated,and a large amount of particle phosphorus（mainly sand）was deposited in the WLFZ of Pengxi River.The highest content of TP,AP and OM in the sediment in the low-altitude area（PX160）was about 1.4 g kg^-1,0.12 g kg^-1 and 53.47 g kg^-1.（3）During the drying period,there was P loss in the WLFZ,mainly in PX165 area,and the TP content in the surface soil（0-3cm）of PX165 decreased by 0.47 g kg^-1.However,the TP content in the soil of the WLFZ increased overall due to the input of exogenous phosphorus during this period.During the severe algal bloom period（April to June）,exogenous phosphorus was imported into the WLFZ.Therefore,in order to prevent the P from being transported into the water,efforts should be made to control its stability during the drying period.A 33-day continuous sampling was conducted on Gaoyang Lake,which is the most serious area of algal bloom in the middle section of Pengxi River.The interrelation of various factors in the whole water environment was analyzed.The following conclusions were drawn:（1）The intensity of water stratification is crucial to the phosphorus cycle model between water and sediment.In the period of strong stratification(e.g.0.72°C m^-1),the available phosphorus in the bottom sediment mainly maintains a dynamic balance relationship with the DP in the bottom water,but has little relationship with the DP in the surface water（r=-0.208）.Therefore,continuous high intensity stratification will lead to the depletion of the DP in the surface water due to the continuous growth of algae.When the stratification intensity of the water weakened(e.g.0.42°C m^-1),the available phosphorus in the bottom mud increased with the DP in the bottom water（r=0.423）,and together with the DP in the surface water,it cyclically communicated with the DP in the surface water,and never supported the continuous growth of algae.Therefore,in the season of water bloom,P is in a continuous circulation state in the sediment-water media.Blooms occur every year unless excess nutrients from the water-sediment are gradually removed from the body of water as a whole.（2）In algal bloom period,the sediment of Gaoyang Lake became the main source of P.And the bottom water environment parameters（WT,DO,p H）could affect the content of sediment available phosphorus（SAP）.Among them,WT could indirectly impact SAP by affecting DO and p H.In addition,as the WL decreased,it enhances the p H and T increasing and DO decreasing.Therefore,the reduction in WL enhances the P cycle throughout the bottom water-sediment surface.