Study On The Mechanism Of The Difference Of Algal Bloom Degree Between Pengxi And Modao River In The Three Gorges Reservoir Area

Since the impoundment of the Three Gorges Reservoir（TGR）,water eutrophication has occurred in many tributaries of the reservoir,and even large-scale algal bloom has occurred in some tributaries of the backwater area,and the algal bloom has shown the trend of time ahead,frequency increasing and scope expanding,which has seriously affected the water environment quality of the reservoir area.Pengxi River,a first-class tributary located on the North Bank of the TGR area,is one of the tributaries with the most frequent algal bloom in the reservoir area;while Modao River,a first-class tributary located on the South Bank of the TGR area,has similar geographical conditions with Pengxi River,but few algal bloom reports.At present,the research on Pengxi River mainly focuses on eutrophication,spatiotemporal distribution of phytoplankton and water body stratification.The research on Modao River mainly focuses on heavy metal pollution,biological community structure and so on.Few scholars have studied the differences between the two rivers algal bloom.Therefore,in this study,the hydrological and water quality characteristics of Pengxi and Modao River are monitored continuously and dynamically to master the hydrological and nutrient characteristics of the two rivers.By comparing the nutritional differences between the two rivers,the main environmental control factors of different degrees of algal bloom in Pengxi and Modao River are explored,which provides a theoretical basis for further research on the prevention and control measures of algal bloom in the tributaries of the TGR.This study started in November 2018 and ended in October 2019.Samples of water body,sediment and water-level-fluctuation zone were collected in Pengxi and Modao River on a monthly basis for detection and analysis.The results are as follows:（1）In the period of high incidence of algal bloom,the concentration of Chl-a in Pengxi River was significantly higher than that in Modao River（p<0.05）,and the highest concentration of Chl-a was 148.96μg_·L^-1 in Pengxi River from March to June;there was no algal bloom in Modao River,and the highest concentration of Chl-a was only 10.92μg_·L^-1.In horizontal spatial dimension,the concentration of Chl-a in the upper reaches of Pengxi River was significantly higher than that in the lower reaches（p<0.05）,while there was no significant difference in the concentration of Chl-a in the upper and lower reaches of Modao river.（2）The water temperature of Pengxi River is about 25℃.There is no significant difference between Modao River and Pengxi River in water temperature stratification,which is not the limiting factor of algal bloom.The increase of pH and DO content is the response result of the algal bloom outbreak in Pengxi River,not the factor influencing the algal bloom outbreak.The Cond of Pengxi River was significantly higher than that of Modao River（p<0.05）,which was supposed to be caused by other kinds of ions and different concentrations except nitrogen and phosphorus.（3）There is no significant difference in TN and TP concentrations between Pengxi and Modao river.TN and TP are not the important limiting factors of the algal bloom,nor are they the key factors of the bloom difference between the two rivers.The difference in ammonia nitrogen content in spring and summer is the most important difference in nutrient content between the two rivers,and also one of the factors affecting the algal bloom degree difference.（4）The proportion of DTN and DTP in Pengxi River decreased continuously from spring to summer.On the one hand,the input of exogenous nutrition led to the increase of TN,TP and particulate nitrogen and phosphorus content;on the other hand,phytoplankton absorbed and transformed dissolved nitrogen and phosphorus,resulting in the decrease of DTN and DTP content in water.In spring and summer,TN in Pengxi and Modao River played a more important role in regulating water nutrient structure than TP;in autumn and winter,TP in Pengxi and Modao River replaced TN as the main factor in regulating water nutrient structure.（5）The mean of Zn ion concentration in Pengxi River is 0.042 mg_·L^-1,and that in Modao river is 0.056 mg_·L^-1.There is a significant negative correlation between Zn and Chl-a in Pengxi River（p<0.05）.There is no significant correlation between Zn ion concentration and Chl-a in Modao river.It is speculated that the difference of Zn ion concentration in Pengxi and Modao River during the algal bloom season is one of the reasons for the difference of algal bloom.（6）The average contents of TN,TP and organic matter in the sediment of Pengxi and Modao River are 1.11 g_·kg^-1 and 1.03 g_·kg^-1,0.82 g_·kg^-1 and 0.73 g_·kg^-1,respectively.The organic matter content and nitrogen and phosphorus nutrition in Pengxi River bottom mud are higher than those in Modao river.The increase and accumulation of organic matter content in Pengxi River bottom mud is one of the reasons for the increase and accumulation of organic matter content caused by the gradual decomposition of the bottom mud after the disappearance of algal bloom.With the continuous accumulation of organic matter and nitrogen and phosphorus nutrition in the bottom mud,the algal bloom in Pengxi River will become more and more intense.（7）The average content of organic matter in the sediment of Pengxi and Modao is68.41 g_·kg^-1 and 66.71 g_·kg^-1,respectively.The content of organic matter in the sediment of Pengxi and Modao River increases with the increase of flooding time.Compared with the content of organic matter in the sediment,the content of organic matter in the sediment of Pengxi River is higher.The thickness of the sediment in the sediment of Pengxi River is significantly greater than that of Modao River,indicating that the soil loss degree of Pengxi River is greater than that of Modao River,which is one of the reasons for the significant difference of ammonia nitrogen nutrients between the two rivers.