| Suspended sediment concentration(SSC)is an important natural attribute of estuaries.Variations in estuarine SSC are related to factors such as river discharge,suspended sediment discharge(SSD),tides,waves,and stratification.Estuarine SSCs are a key component of the biological,physical,geomorphological,and geochemical systems that develop in coastal environments.With respect to ecology,spatial and temporal variations in estuarine SSC affect primary productivity by changing the intensity of light that penetrates the water column.For sedimentology,the SSC is positively correlated with the rate of deposition,and the difference between the rates of deposition and erosion determines the net erosion–accretion balance over each tidal cycle.In terms of geochemistry,suspended sediments are carriers of pollutants because of their high capacity to absorb chemical species.Therefore,it is necessary to carry out research on the variation law and mechanism of suspended sediment concentration in estuarine and coastal areas.Over the past few decades,human impacts in the Yangtze River Basin have been very intense.There are now more than 50,000 reservoirs in the basin,with a total storage capacity of more than 300 km~3,representing one-third of and 1.14 times the annual river discharge and average river discharge during the dry season,respectively.The construction of large numbers of reservoirs in the basin has led to a sharp decline in the SSD from the Yangtze River into the sea:during a 10–year period(2011–2020),SSD decreased by 77%relative to the level in 1956–1968;this decline is significantly greater than the average decline of global rivers.In addition,the Yangtze Estuary is frequently hit by typhoons,which causes the Yangtze Estuary to be in a state of strong swell(the significant height of total swell is greater than 0.93 m)for an average of 56days a year.With global warming,the intensity of typhoons tends to increase,which will lead to an increase in the duration of strong swell and high SSC in the Yangtze Estuary.The effects of reservoir construction and typhoon events(and the combined effects of both)on the concentration of suspended sediment in the Yangtze Estuary and its adjacent waters are major scientific issues that require in-depth study.This paper documents original research on these scientific issues.One research strand uses surface SSC data from 10 stations in the Yangtze Estuary and its adjacent waters since the 1990s,combined with other hydrological data(river discharge and SSD data at Datong Station and wave and tidal data in the Yangtze estuary),to study the response of interannual estuarine SSC variation to human activities such as dam construction.The second research strand involves the application of typhoon track information and hydrological data(turbidity,velocity,and waves)measured by a submarine tripod observation system equipped with a variety of high-resolution instruments in the waters near the mouth of the Yangtze River before,during,and after typhoons to study the response of estuarine SSC to typhoon events.The third research strand focuses on evaluation of the combined effects of human activities(e.g.,dam construction)and interdecadal changes in typhoon intensity on SSC in the Yangtze Estuary and predicting the SSC changes in the Yangtze Estuary and adjacent waters in the next few decades.The main results and conclusions are as follows.(1)Responses of SSC variation in the Yangtze Estuary to human activities such as dam construction in the Yangtze basin.Between the periods 1990–1999and 2003–2020(the Three Gorges Reservoir began to store water in 2003),the annual average SSC at Datong station(the tidal limit in the Yangtze River)decreased by 57%,mainly because of sediment retention resulting from dam construction and the implementation of soil conservation measures in the drainage basin.At the same time,the annual average SSC in the Yangtze Estuary(10 stations)decreased by only 27%,markedly less than the relative SSC decrease at Datong.This difference arose primarily because wave-and tide-influenced sediment resuspension can mitigate the influence of the fluvial SSD decline on the SSC in the Yangtze Estuary.As a result of tidal and wave influences,there is a time lag of 1–14 years between the change in SSD from the Yangtze River and the subsequent response of SSC levels in the Yangtze estuary.The time lag increases from 1 year at the tidal current limit(Xuliujing station)to 8–14 years in the mouth bar area of the estuary,as the marine force becomes stronger and the riverbed sediments become finer seaward.The continuous construction of new reservoirs in the Yangtze River and the seasonal regulation of reservoirs have resulted in the average river discharge in the dry season becoming significantly greater(a 49%increase),and enhanced stratification caused by the increase in the average river discharge in the dry season aggravates the decline of the suspended sediment concentration in the dry season in the Yangtze Estuary.(2)Response of SSC variation in the Yangtze Estuary to typhoon events.During the five studied typhoons,the average significant wave height at the 7-m-depth station(outside the Yangtze Estuary)was 4.2 times higher than that in calm weather,and the mean surface SSC at the station was 3.6 times higher than that in calm weather.During two typhoons(Meranti and Malakas),the average significant wave height at the 20-m-depth station(outside the Yangtze Estuary)was 2.78 times greater than that in calm weather,and the near-bottom SSC was 8.3 times greater than that in calm weather.During the impact of remote typhoon Malakas,swells dominated sediment resuspension in the Yangtze Estuary.The tide-averaged bed shear stress derived from swells(0.011-0.372 N/m~2)was 3–16 times the bed shear stress derived from wind waves(0.003-0.056 N/m~2).The correlation between shear stress derived from swell and SSC(R~2=0.91)was much better than that between the shear stress derived from wind wave and tidal dynamics and SSC(shear stress derived from wind wave:R~2=0.05;tidal range:R~2=0.38).During typhoon Malakas,the contribution of swell to sediment resuspension in the Yangtze Estuary was 81%.For three days after the typhoon disappeared 1,500 km from the Yangtze Estuary,its swells still affected the estuary.(3)Combined effects of human activities,such as dam construction in the Yangtze River,and interdecadal variation of typhoon intensity on SSC in the Yangtze Estuary.Contrary to the decreasing trend of the SSD from the Yangtze River into the sea in recent decades,the duration of super typhoons(central wind speed>51m/s)in the Northwest Pacific has been increasing in recent decades.Between 1990–2002 and 2003–2020,the suspended sediment flux into the sea from the Yangtze River decreased by 69%and the duration of strong typhoons in the Northwest Pacific increased by 87.2%.Additional resuspension of sediment resulting from the recent increase in typhoon intensity in the Northwest Pacific may have mitigated but not completely offset the decline in the suspended sediment flux into the sea caused by the construction of reservoirs in the Yangtze River Basin;thus,the SSC in the Yangtze Estuary and the surrounding marine area still exhibits a downward trend.In the next few decades,with increasing reservoir construction in the basin,the flux of suspended sediment from the Yangtze River into the sea is likely to further decline,and the coarsening of surface sediments caused by the erosion of the underwater delta of the Yangtze River may also increase.In contrast,in the context of global warming,the intensity of typhoons affecting the Yangtze Estuary may continue to increase.The combined influence of these positive and negative factors may result in a reduction of the suspended sediment concentration in the Yangtze Estuary and its adjacent waters,but the extent of the reduction is not yet properly understood.(4)Environmental effects of the SSC decrease in the Yangtze Estuary.The SSC decrease in the Yangtze Estuary has led to subaqueous delta erosion,coarsening of surface sediments,and an increase in the frequency of algal blooms.The Yangtze subaqueous delta has been suffering from erosion since 2003.From 2002to 2017,the average erosion rate in the 3961 km~2 region around the Yangtze subaqueous delta front was～0.58 cm/yr,and the maximum erosion rate was>15cm/yr.Subaqueous delta erosion resulted in extensive coarsening of surface sediments and a landward retreat of the boundary between the outer delta and the residual sands of the Late Pleistocene East China Sea.As the shading effect of suspended sediments in seawater can limit photosynthetically available radiation,the SSC decline might affect phytoplankton growth in the Yangtze.From 2000 to 2018,the concentration of chlorophyll a(Chl-a)off the Yangtze has increased by 61.4%(from 3.45 to 5.57mg/m~3,p<0.05)and the frequency of algal blooms is more than five times greater(from 6.27 to 31.6,p<0.05).The Chl-a concentration and the frequency of algal blooms are significantly negatively correlated with SSC in the Yangtze Estuary(p<0.001).To summarize,the reduction in SSD into the sea caused by dam construction in the Yangtze River is the main reason for the long-term SSC decline in the Yangtze Estuary.A typhoon can cause the SSC to rise rapidly over a short period(i.e.,one to several days).The potential SSC increase caused by the interdecadal intensification trend of typhoon intensity has not completely offset the SSC decrease caused by dam construction in the Yangtze River,so the SSC in the Yangtze Estuary still shows a significant decrease.The SSC decline in the Yangtze Estuary has an important impact on geomorphic change and ecosystems,and this should be studied further.The results described in this study can provide scientific guidelines for evaluating the impact of reservoir construction in the river basin and extreme weather events such as typhoons on the estuarine environment.The results are also of great significance for maintaining the security of the estuary ecosystem and for the coordinated development strategy of the river basin and the estuary. |
