Spatial And Temporal Variability In Sources,Distributions,and Fluxes Of Nutrients And Trace Elements In The Yangtze River And Their Responses To The Human Activities

Exports of trace elements and nutrients in the major river have a significant impact on the ecological environment of its estuary and coastal areas.On the basis of riverine comprehensive survies in May 2017 and August 2018,and monthly observations at the Datong station in the lower reaches of the Yangtze River(Changjiang)in 2013-2019,dissolved and particulate nutrients were determined to study the variations in concentrations,composition,distribution and controlling factors with an end of exploring the impact of human activities,like the Three Gorges Dam project,on nutrients transport in the major river and the regarding ecological environment of coastal areas next to the river mouth.Again,concentrations of dissolved trace and major elements were also analyzed for determining the spatial variations,source identification,and riverine fluxes using various multivariate statistical techniques,including correlation analysis,principal component analysis(PCA),and cluster analysis(CA)with an aim to exploring the influence of natural factors and human activities on the distribution and loading of major and trace elements in the Yangtze River water environment.Major results in the thesis are summarized as follows:(1)The concentrations of nitrogen(N)and silicon(Si)were much higher than that of phosphorus(P)in the Yangtze River.Nitrogen was mainly presented in dissolved speices in the Yangtze River Basin.The N/Si/P ratios in the water body were much higher than the Redfield-Brzezinski value(16:16:1),and there were thus potential phosphorus nutrient limitation in water.The concentrations and fluxes of nutrients in the downstream water had obvious seasonal patterns.Nitrogen and phosphorus were significantly negatively correlated with runoff,showing low values in wet season and high values in dry season,while the silicon had opposite changes.Except for ammonia(NH4+)and nitrite(NO2-),the fluxes of nutrients were dominated by runoff,which showed the characteristics of high values in wet season and low values in dry season.(2)Concentrations of nitrogen and phosphorus in the upper reaches of the Yangtze River were significantly lower than the middle and lower reaches in the past 20 years.It was considered that high variations in nutrient concentrations in the middle and lower reaches were affected by regional differences in human activities and land policies.In the past 60 years,the long-term annual changes of the nitrogen and phosphorus in the Yangtze River could be roughly divided into three stages based on their concentrations and fluxes.The first stage was a slow growth in contents from 1960 to 1980,and followed an exponential rapid increase from 1980 to 2000,and the last one had a slight decline trend in the past 20 years.The declineing concentrations and fluxes of silicon in the Yangtze River were related to human activities,and mainly induced by river damming.(3)The spatial distribution of trace elements show that Cu,Zn,Pb,Cd,and As were the major elements affected by human activities in the Yangtze River,and their concentrations in the downstream were significantly higher than those in the middle and upper reaches(P<0.05).All elements had a fairly high concentration values in both the channels of the Yangtze River mainstream in Chongqing city and Hanjiang River in Wuhan city,which were mainly related to the enhanced human activities.However,the low concentrations of multi-elements in the reach of the Yangtze River in Yichang were largely due to the retention effect of Three Gorges Project on the element transport,which declined the riverine loadings of multi-elements.Principal component analysis(PCA)indicates that Na,Mg,K,Ca,Fe,Mn,Co,Ni,Mo,Cr,and V were mainly associated with the weathering and erosion of various rocks and minerals in the river basin.And Cu,Zn and Pb were mainly affected by the enhanced human activities such as industry wastewater,metal smelting and mineral mining,while Cd and As were mainly related to agricultural activities.The spatial distributions of trace and major elements show that concentrations of some elements in the Yangtze River channels were enhanced by human activities.Generally,the heavy metal pollution in the Yangtze River Basin was lower than those in other rivers of the world.However,the annual fluxes of Cu,Zn,Pb,Cd,and As would have far-reaching ecological effects on the Yangtze River estuary and offshore ecological environment.