Endogenous Mechanism And Risk Analysis Of Water Eutrophication In Duliujian River Basin

In recent years,China has done a lot of work in reducing land-based pollutant emissions,and achieved remarkable results,but eutrophication in many coastal waters still occurs frequently.Nitrogen（N）and phosphorus（P）are the main pollution factors of water eutrophication.However,under the background of enormously reduction of land-based emissions,the increases of N and P in water body may be determined by the concentrations of N and P in sediments（bottom-top）,or by the demands for N and P by bacterial metabolic activities（top-bottom）.Therefore,it is important to have a correct understanding of the endogenous mechanism for N and P releases from sediment,which is helpful for solving the problem of water eutrophication at the river basin scale.In this study,the overlying water and sediments in the catchment area（CA）,estuarine area（EA）and offshore area（OA）of Duliujian River Basin in Tianjin were selected as the research objects.The distribution characteristics of N and P in sediments were measured and analyzed,and then combined with the eutrophication status of overlying water,physicochemical properties and bacterial flora characteristics of overlying water and sediments,to explore the endogenous mechanism and risk state of waters in Duliujian River Basin.The results are helpful to understand the endogenous mechanism of eutrophication from the top-down and bottom-up perspectives,and provide theoretical support for the control of water eutrophication in coastal river basins.The main results are as follows:（1）The distribution of N and P in sediments of Duliujian River Basin has spatial heterogeneity.TP in EA was significantly higher than that in CA and OA;The concentrations of different P forms（Ex-P,Fe/Al-P,Ca-P）in EA and OA was significantly higher than that in CA.The distribution of TN and NO₂-N in sediments is opposite to that of TP,while the distribution trend of NH₄-N and NO₃-N is the same as that of P.The contents of N and P forms（TP,TN,NO₂-N,NO₃-N,NH₃-N）in overlying water showed the trend of CA>EA>OA.The variation trends of N and P concentrations and forms in sediments and overlying water are different,indicating that the concentration of nutrients in sediments may not be the decisive factor of water eutrophication.Redundancy analysis（RDA）results show that the change of N and P in overlying water in Duliujian River Basin is significantly affected by salinity and MC,while the distribution of N and P in sediments is significantly affected by SOM and p H.（2）The diversity index of bacterial community and the abundance of bacterial functional metabolism in the sediments of Duliujian River Basin showing the trend of CA>EA>OA in different spatial units.It suggests that the main cause of eutrophication of overlying water may be caused by the releases of N and P from sediments mediated by bacterial communities.The results of RDA analysis shows that the diversity index,dominant flora and functional metabolism level of bacterial community in Duliujian River Basin are mainly affected by environmental factors such as salinity,MC and SOM.The correlation heat map shows that SOM is the key factor affecting the morphological changes of P in sediments,and the change of salinity affects the morphological transformation process of N and P to a certain extent.（3）From CA to OA in Duliujian River Basin,the eutrophication index（EI）of the overlying water gradually decreases,but the EI is more sensitive to the change of P than that of N in the overlying water,which means that the increase of P is the key factor leading to the eutrophication at the river basin.There is a nonlinear correlation between P concentrations in sediments and overlying water.It suggests that the concentration of P in sediments may not the key factor affecting the eutrophication of overlying water.The dynamics（diversity and functional metabolic abundance）of bacterial community in sediments are linearly correlated with the concentration of P and N in overlying water.In addition,the slope of the fitting curve between bacteria and P is significantly higher than bacteria and N,indicating that the release of P in sediments mediated by bacteria is the key factor of eutrophication of overlying water.Standardized principal axis（SMA）regression analysis shows that sediment bacterial diversity and functional metabolic abundance can mirror the risk assessment of water eutrophication,but the risk index was more sensitive to bacterial diversity.