The Lateral Transport Behavior And Flux Of Dissolved Organic Carbon In Chongming Dongtan Salt Marsh Wetland And Its Effect On Estuarine Water Color

Estuarine wetlands are an important component of the coastal ecosystems,contributing tremendously to the global blue carbon sink.The concentration of dissolved organic carbon（DOC）in salt marsh water and its spectral characteristics（light absorption characteristics of colored dissolved organic matter（CDOM）in DOC）are highly dynamic under the influence of factors such as tidal exchange and seasonal change,and have an impact on estuary water optics,biogeochemistry,ecology and carbon budget.At present,researches on estuarine salt marshes under the influence of regional rivers are mainly carried out.Little is known about the DOC tidal exchange between salt marshes affected by world rivers and their adjacent estuaries,and the biogeochemical gradient of salt marshes DOC in estuaries.Therefore,this study takes the Shanghai Chongming Dongtan Salt Marsh and its adjacent waters under the influence of the world’s largest river,the Yangtze River,as the research area,focusing on the temporal and spatial dynamic changes of DOC concentration and properties in the salt marsh wetland and at the wetland-estuary interface.The lateral transport behavior and flux of DOC in salt marsh water were studied by establishing the DOC lateral transport flux observation system in the salt marsh wetland.The influence of lateral transport of DOC in salt marsh water on the water color of the estuary was analyzed by means of large-scale navigation and satellite remote sensing observation.The main conclusions are as follows:（1）Based on high-frequency observations of DOC in salt marsh water,it was found that the concentration and spectral properties of DOC in salt marsh water showed significant dynamic changes in different time scales.Waters in the ebbing tide during summer and fall seasons had elevated DOC concentration,high absorption and aromaticity,and correspondingly lower spectral slope,relative to the waters collected in the winter and spring.In different seasons,the DOC concentration in the wetland at ebb tide was always twice that at flood tide.The physical and chemical properties of DOC in the water of salt marsh wetland change gradually during the process of migration to the adjacent estuary under the effect of tidal dynamics.The photochemical process significantly affected the colored components in DOC pool,while microbial degradation and transformation significantly affected the DOC concentration.（2）Based on high-frequency observations of salt marsh water discharge and DOC flux,it was found that the lateral transport flux of DOC was significantly correlated with the productivity of salt marsh vegetation and water flux.On the scale of tidal cycle,the DOC flux increases significantly in the early and middle periods of high tide and low tide,and close to zero in the high tide and low tide period and the end of low tide,and the semi-diurnal tide with higher water level will show more intense changes in DOC flux.On a seasonal scale,the lateral transport flux of DOC in salt marsh water in summer and autumn was much higher than that in early spring and winter.（3）Based on the large-scale navigation observation of bio-optical and spectral parameters of the wetland-estuarine interface,the CDOM and DOC remote sensing retrieval algorithms were developed respectively for Landsat-8 OLI/-9 OLI-2 and Sentinel-2A/B MSI satellites applicable to surface water of the wetland-estuarine interface.Firstly,the Landsat-8/9 and Sentinel-2A/B satellite data are atmospherically corrected using the La SRC and Sen2Cor atmospheric correction models（which perform best among the nine atmospheric correction methods）.Comparing linear,nonlinear,multiple linear regression and neural network models,using the most accurate genetic algorithm-optimized backpropagation neural network to establish CDOM retrieval models for Landsat-8/9 and Sentinel-2A/B respectively（mean absolute percentage deviation is 9.5%and 8.4%,N=66）.A satellite DOC retrieval algorithm was developed based on a strong linear correlation between DOC concentration and its optical property a _CDOM（300）（r=0.92,p<0.001）,and showed high performance in OLI/OLI-2 and MSI products（r=0.82,p<0.001）.（4）Based on the long-term DOC concentration data retrieved from satellites during 2013-2022,it was found that salt marsh is an important source of DOC,and the lateral transport of DOC in salt marsh water will affect the water color of estuaries.However,the Yangtze River discharge plays a dominant role in the distribution of DOC at the wetland-estuarine interface.On different tidal scales,the distribution of DOC at the wetland-estuary interface during flood tide is mainly affected by the estuary and coastal background(<1.6 mg L^-1);there is an obvious high-concentration DOC plume near the mouth of the wetland tidal creek at the beginning of the ebb tide(1.6-2 mg L^-1),but it was affected by the dilution of the Yangtze river discharge;in the middle and late ebb tide,the DOC concentration gradient(1.4-2.2 mg L^-1)at the wetland-estuary interface was significantly affected by river discharge.On different seasonal scales,the DOC concentration(1.4-2.2 mg L^-1)at the wetland-estuary interface was basically consistent with the seasonal variation of the Yangtze River discharge(1.4-2.5 mg L^-1),but was significantly different from the observations(1.36-6.78 mg L^-1)in Dongtan wetland water at different seasons.（5）In the annual cycle,the concentration distribution of DOC at the wetland-estuary interface was mainly controlled by the Yangtze River discharge.According to the continuous wavelet analysis results,the annual correlation between DOC concentration and river discharge and normalized difference vegetation index was the most significant（R²>0.6）,followed by sea surface temperature and wind speed（R²>0.5）,and the weakest correlation with rainfall（R²<0.3）.The dominant effect of river discharge on the distribution of DOC at the wetland-estuary interface is more significant under the special climate background.This study highlights the important influence of large rivers on the lateral transport gradient of DOC in salt marsh waters in their estuary regions.