Comparative Study Of Sedimentary Biogenic Elements In Reservoirs And Estuary Of The Yangtze River

Rivers are important link between mainland and ocean.Human activities in river basin,such as industrial and agricultural development,dam construction,have played an important role in altering ecological environment of reservoirs and estuaries.The impact of dams on reservoirs and estuaries has become a high focus of ecological and environmental sciences.The present mostly on two aspects:one is how materials and ecological environment of the reservoir changes after impoundment of the dam,and the other is how the estuarine ecological environment responds to upstream environment change.The difficults of above research are to find quantitative indicators to reflect the changes of ecological environment in reservoir and estuary.The biogenic elements of sediments are important proxies to invert the changes of water ecological structure and have become an effective means to explore the changes of water environment.In this study,we obtained 8 sediment cores'samples from 5 typical reservoirs in the upper and middle reaches and the Yangtze River Estuary,measeared the cores'eco-environment proxies of particle size,biogenic elements including TOC?total organic carbon?,TN?total nitrogen?,BSi?biosilicon?and chronology proxies of ²¹⁰Pb and ¹³⁷Cs,and collected the annual nutrient flux from Datong Station.Based on above data,we want to address the filtering effect of the reservoir in nutrient flux and eco-environment change of the river and estuary.The important conclusions of this study can be made as follow:Firstly,dam construction induces obvious change of riverine natural nutrient transport:1.After the construction of dam,the content of TOC,TN,BSi,and BSi/TOC in sediments generally increased,indicating that the reservoir significantly retained nutrients and increased growth amount and proportion of diatoms;It is estimated that after the construction of reservoir,the average annual retention of C,N,P and Si in Yangtze River Basin have increased by 10.49,2.53,0.42 and 24.35 g/m².a respectively,among which the retention of Si is most significant.The increase of nutrients from human emissions enhances the retention effect of reservoir,instead the reduction of upstream sediment flux entering reservoir weakens the retention effect of reservoir.2.From 1998 to 2015,there had been annually averaged 9,100,1,500,and 87,900tons of nitrogen,phosphorus,and silicon retained in whole reservoirs of the Yangtze River basin,respectively,and the average annual inflow flux of DIN,DIP and DSi in datang station was 1.63,0.039 and 2.55 million tons.The average annual retention of N,P and Si in the basin accounted for 0.56%,3.85%and 3.44%of the annual inflow flux,respectively.Above results induced that annual DSi flux into sea decreased to 41.9%from 1998 to 2015,however,annual DIN and DIP flux increased to 61.4%and 74.2%respectively due to the dramatic increase in human fertilizer application and wastewater discharge.3.Different nutrient fluxes have led to the increase of sedimentary TOC and TN,but decrease of sedimentary BSi and BSi/TOC in the Yangtze River Estuary during recent decades.This results implicate the effect of reservoir retaining nutrient that directly induced the algae blooming and the proportion of diatoms decreasing in the Yangtze River estuary.Secondly,the nutrient retention rate is different in reservoir and estuary4.When sediments particle size is similar,the retention rate of biogenic elements in reservoir is higher than that in estuary.This probably relates to the high salt and unstable environment in the estuary,that not suitable for algae growth and causes the sediments easier to re-suspend and to decompose biogenic elements.