Multi-scale Spatiotemporal Dynamics And Driving Mechanisms Of Suspended Sediment In The Yellow River Estuary And Its Adjacent Seas

Estuarine deltas are key zones for land-sea interaction and have important ecological service functions and socio-economic values.Suspended particulate matter is not only the core of the coupled system of estuarine dynamics-sediment transport-geomorphic evolution,but also the main carrier of organic carbon,nutrients,pollutants,etc.,which has an important impact on the nearshore water ecosystem.Because of its special geographic location,the suspended sediments near the estuary show highly complex spatial and temporal heterogeneity due to runoff-tidal interactions and the intervention of human activities.Ocean color remote sensing technology has become an important approach for large-scale,quantitative,refined,and long time-series continuous monitoring of nearshore suspended sediments due to its many advantages.Therefore,it is of great significance to explore the multi-scale spatiotemporal dynamic characteristics and driving mechanisms of estuarine suspended sediments using remote sensing technology for integrated management and decision-making of river-delta systems.The Yellow River Estuary(YRE)and its adjacent seas are one of the most turbid waters in the world due to the large amount of sediment discharged,and are now facing new circumstances such as the new situation of riverine water and sediment load,the shift of estuarine dynamics environment,and major geomorphic transformation of the delta.With this background,in this study,we developed a remote sensing inversion model of Suspended Sediment Concentration(SSC),as well as remote sensing extraction algorithms of High Turbidity Zone(HTZ)and Yellow-Blue Demarcation Line(YBDL),and used multi-source satellite data to reveal multi-scale spatiotemporal dynamics of SSC/HTZ/YBDL in the YRE.The drivers affecting the SSC/HTZ/YBDL changes in the YRE were characterized.We also explored the relationship of SSC with geomorphic evolution and Particulate Organic Carbon(POC).The main findings and conclusions of this study are summarized below:(1)SSC retrieval model and HTZ/YBDL remote sensing extraction methods were developed for the YRE.The existing YRE regional SSC models and global general SSC models were evaluated and summarized using our measured data,and based on which a multi-sensor consistent SSC inversion conditioning algorithm was developed.The algorithm outperformed existing models(R~2>0.95,RPD<24%)and there was high consistency between the inversion results from each sensor(R~2>0.9,RPD<30%).The effect of the satellite observation scale was assessed using buoy data,and it was found that coarse-resolution imagery resulted in a significant loss of detail in the nearshore SSC distribution,while low-frequency satellites had difficulty in capturing abrupt changes in SSCs.In order to solve the problem of limited satellite temporal and spatial scales,a multi-source image temporal and spatial fusion scheme was proposed to achieve the 30-m hourly SSC monitoring in the YRE.Aiming at the characteristics of the water body in the YRE,the HTZ remote sensing extraction method based on the SSC threshold and the YBDL identification method based on the Canny edge detection operator were proposed.The HTZ extraction accuracy was verified to be 93.4%and the YBDL extraction error was only 10.53%.(2)Multi-scale spatiotemporal dynamics of SSC/HTZ/YBDL in the YRE were revealed.Spatiotemporal dynamics of SSC/HTZ/YBDL in the YRE and its adjacent seas were analyzed using 775 Landsat TM/ETM+/OLI,1116 GOCI and 292 Sentinel-2 MSI images.It was found that two high SSC centers existed in the YRE,and the SSC increased slowly from 1984 to 1998 and then decreased exponentially from 1998 to2022 in the last 40 years,and there was a significant difference between the north and south.The SSC in the Bohai Sea also showed a significant decrease(-1.38 mg/L/yr)in the last 10 years,especially in its southern part.The contribution of YRE to the high-turbidity water distribution in the Bohai Sea(74.46%)was much higher than that of other estuaries.Suspended sediment in YRE was mainly transported to the Yellow Sea through the south of Bohai Strait,while the north of Strait was dominated by inflow.The changes in the distribution area of HTZ over the last 40 years also showed a trend of increasing and then decreasing,peaking in 1996-1998.It experienced four main morphological transitions:circum-coastal striped(1984-1993),southeastward ovoid(1994-2007),round-like(2007-2017),and northeastward ovoid(2018-2022).Estuarine YBDLs occurred on a large scale in the last 5 years and were mainly distributed within10 km.They could be categorized into two types:short straight shapes along the branch mouths and long curved shapes around the branch mouths,with the latter being the dominant type.(3)Driving mechanism of SSC/HTZ/YBDL changes in the YRE was elucidated.The driving mechanisms of SSC,HTZ and YBDL changes in the YRE were analyzed in terms of riverine water and sediment discharge,ocean dynamics,extreme events and human activities.The results revealed that the riverine sediment delivery and the frequency of strong northerly winds were the main controlling factors of the SSC variations in the last 40 years with comparable contributions,which could explain 74%of the interannual SSC changes.The decrease of SSC in the Bohai Sea over the last 10years was also caused by the weakening of the wind-wave force,and the significant wave heights could explain 59%of the monthly SSC variations.The estuarine HTZ was dominated by a combination of geomorphology and strong northerly winds,with the frequency of strong northerly winds explaining 71%of its variability after 1996.Riverine water and sediment discharge could explain 45.9%of the daily variations in YBDL length,and the contribution of sediment load was higher.The large amount of sediment once discharged by the Yellow River and the continuous sediment source brought by coastal erosion,as well as the high-flow zone produced by the special topography together led to the long-term existence and continuous evolution of the two high SSC centers.On the intra-annual scale,sediment delivery could only influence SSC variability within 10 km of the estuary,and riverine sediment at high discharge could explain 75%and 45%of the monthly variations in SSC within 3 km and 5 km of the estuary.The ebb and flood tides were the primary driving force controlling intraday variability of the estuarine plume and YBDL.Sustained ebb and flood tides could contribute to the development of long-arc YBDL on the front side of the plume,while suppressing YBDL on the back side of the plume.During the lunar cycle,high SSC occurred on the new moon or full moon and the 2 days following it,and low SSC appeared on the 2-3 days after 1/4 moon or 3/4 moon.Water-rich and sediment-rich floods during water and sediment regulation scheme could significantly increase the SSC and HTZ/YBDL distributions in the estuary,while the effect of water-rich and sediment-poor floods was markedly weaker.The effects of strong storms near the YRE could last for 2-3 days,while the impacts of short-duration storms would disappear after a few hours.The East Asian monsoon regulated the seasonal changes of SSC and HTZ in the YRE by driving the dispersion of high SSC centers.(4)Relationships of estuarine SSC with geomorphic evolution and POC were explored.Coastal erosion was an important source of offshore suspended sediment,and SSC and HTZ distributions were closely related to bathymetry.The SSC decline in the north of the delta over the last 40 years was associated with the reduced rates of land erosion and seabed scour in this region.The construction of the engineered protection zone not only stabilized the shoreline,but also its coastal groins demonstrated the effect of intercepting sediment and promoting accretion,which further reduced sediment supply and resulted in a continuous decrease in SSC.The geomorphic evolution of the present and abandoned estuaries at Qingshuigou was the most dramatic,altering the regional hydrodynamic environment,which in turn affected the SSC distributions.The strong accretion zones in this region corresponded to the distribution of high SSCs.The hydrodynamic environment at the leading edge of the sand spit was enhanced by the continually protruding topographic structure during early period,which induced an increase in the estuarine SSC and a widening of the HTZ distribution from 1984 to 1996.The interception of riverine sediment by the Kendong 12 Road Island resulted in a geomorphologic pattern of northern accretion and southern erosion.The concentration of POC in YRE was high but the content in suspended matter was low,and there was a very strong nonlinear relationship between POC concentration and SSC(r=0.97).Based on this,a remote sensing inversion model of POC concentration using SSC as a proxy was developed in this study.Similar to the driving mechanism of SSC,estuarine POC changes were also influenced by a variety of factors such as runoff-tidal action,wind and waves,and geomorphology.In summary,this study revealed the multi-scale spatiotemporal dynamics of SSC and HTZ in the YRE over the last 40 years and its driving mechanism in a more comprehensive way based on the innovative methods,and was the first time to monitor and reveal the spatiotemporal dynamics of the YBDL and its driving mechanism.The research results could provide scientific and systematic data support and decision-making support for the comprehensive and integrated management of the ecological protection,Shengli oilfield development,and ecotourism planning in the Yellow River Delta.