Remote Sensing Estimation And Spatial And Temporal Distribution Of Surface Organic Carbon In Eastern China Sea

Particulate Organic Carbon(POC)and Dissolved Organic(DOC)play important roles in the global carbon cycle,which are closely related to biological activities and primary productivity in the aquatic environment.Compared to in situ ship measurement,satellite remote sensing provides greater advantages,whch obtains synoptic observations of biogeochemical parameters at different time and spatial scales.Therefore,the development of ocean color remote sensing algorithms,such as DOC and POC for example,has great significance to further explore the distribution and fate of DOC and POC in the ocean.We have collected and studied POC and DOC in the Eastern China Sea(i.e.Eastern China Seaontinental Shelf Seas of China,ECSC)by six cruises about POC samples and nine cruises about DOC samples,as well as other parameters of each match.The nearshore waters of ECSC were observed high turbidity and complex optical properties,which was not well applied by single proxies of remote sensing algorithm.In this study we adopted the optical classification approaches of remote sensing reflectance spectra(R_rs)and the spectral slope coefficient of chromophoric dissolved organic matter(CDOM),respectively,to develop the remote sensing algorithms of POC and DOC for Geostationary Ocean Color Imager(GOCI)in the ECSC.By applying the POC and DOC algorithms to GOCI,we further analyzed the variation and the potential factors driven the POC and DOC at daily,monthly,seasonal and interannual scales,and explored the response mechanism of POC and DOC during extreme events(e.g.,red tide and typhoon).The main achievements were as follows:(1)Combining water mass analysis and factor analysis,the distribution characteristics and influencing factors of in situ DOC and POC determined were exploredThe distribution of DOC and POC in the ECSC demonstrated significant spatial variation from in situ observations.Concentrations of DOC and POC in the coastal areas were much higher than those in offshore waters.The concentrations of POC decreased as at the Yangtze River estuary and adjacent sea areas>the Bohai Sea>the Yellow Sea>the East China Sea;the DOC concentrations showed a decreasing trend from the Yellow River Estuary to the whole Bohai Sea,and gradually decreased from the Bohai Sea to the North Yellow Sea and the South Yellow Sea.Factor analysis and water masses analysis showed that POC and DOC were influenced by terrigenous input and phytoplankton;diferent water masses exerted different influences on the distribution of organic carbon in the ECSC.(2)We have developed satellite ocean color algorithms of POC in the ECSC,and the spatial-temporal distribution of POC at daily,monthly,seasonal and interannual variations in the ECSC were explored.The entire water body in the ECSC is highly turbid and R_rs is greatly affected by the total suspended Matter(TSM)concentration.The entire water body in the ECSC were divided into three optical types based on the spectral curve of R_rs:high total suspended matter(TSM)concentration(HT)waters,moderate TSM concentration(MT)waters and low TSM concentration(LT)waters.Remote sensing retrieval of POC concentrations has been carried out in the ECSC,which were selected appropriate optical remote sensing proxies for POC in different water types.TSM was chosen as remote sensing proxy for POC in HT waters,Chlorophyll-a(Chla)was chosen as remote sensing proxy for POC in LT waters,and the POC in MT waters was estimated with R_rs of direct empirical algorithm due to relatively complex particulate matter.A POC remote sensing estimation model for the ECSC(POC?ECSC)based on this optical classification has been developed and can accurately retrieve the POC concentration in the ECSC(the root mean square error(RMSE)is 42.11?g/L;the mean relative error(MRE)is 32.08%).Application of POC algorithm to the Bohai Sea showed good agreement between field and satellite observations in the diurnal variation trend of POC at station P of Yellow River estuary(MAE=16.57?g/L and MRE=15.29%).Furthermore,the daily variations of POC in HT and LT water bodies,were greatly affected by tidal action and phytoplankton,respectively,and the POC in MT water body was influenced by those two factors.The monthly and seasonal variations of POC was that the largest POC value area(>1000?g/L)nearby the Yangtze River estuary was the smallest in summer and the highest in winter,and the high POC value area in the northeastern part of the Yangtze River Estuary(150?g/L-1000?g/L)was observed the largest in summer and smallest in winter,which was mainly affected by the seasonality of the runoff of the Yangtze River and the coastal currents of northern Jiangsu.The annual variations of POC were mainly influenced by river runoff in coastal areas,and by multiple environmental factors in coastal-offshore mixed areas and offshore areas.The POC concentrations in 2016 were the highest in 2012-2016,which may be influenced by El Nino.(3)We have established the satellite remote sensing retrieval algorithm of DOC in the ECSC,and the spatial-temporal distribution and differences of DOC for daily,monthly,seasonal and interannual variations in the ECSC were also explored.We established the remote sensing retrieval model of DOC using the 275-295 nm spectral slope coefficient of CDOM,S_275-295,as the basis of the classification of water body,which can effectively separate the ECSC into two parts:the waters were mainly affected by terrestrial sources(S_275-295?0.025 nm^-1)and the waters were mainly influenced by marine phytoplankton(S_275-295>0.025 nm^-1).The DOC remote sensing algorithm based on S_275-295 classification(DOC?ECSC)was established.The algorithm has been validated and the estimation error was relatively small(RMSE is 27.98?mol/L;MRE is 20.58%),which was well applied by remote sensing retrieval of POC in the ECSC.DOC in the ECSC had significant diurnal variation in the nearshore area,while there is no significant diurnal variation of the DOC in the offshore area.The monthly and seasonal variations of DOC was that the highest DOC concentrations area(more than about 120?mol/L)at Yangtze River estuary and nearshore was observed the largest in summer and the smallest in winter;while the higher DOC concentrations area(about80-120?mol/L)at the north of Yangtze River estuary was observed smaller in summer than other seasons.The daily,monthly and seasonly variations of DOC were mainly related to marine phytoplankton.The interannual variations and their impacts of DOC were similar to those of POC.(4)Mechanisms driven the POC and DOC variability during red tide and typhoonThe concentrations of POC and DOC in red tide waters were higher than non-red tide waters,which were indicated that phytoplankton could generate a lage amount of organic matters and influence the distribution of POC and DOC.In red tide water,the distribution and diurnal variation of POC and DOC were observed higer in 12:30-13:30than that in 8:30,which were mainly affected by the vertical migration and generation of phytoplankton.Before typhoon,obvious red tide phenomenon occurred in Yangtze River estuary and Zhejiang coast,which was observed higher POC and DOC concentrations than adjacent waters.Due to the strong but temporary influence of typhoon,the red tide waters were migrated from Zhejiang coast to Jiangsu coast.The high area of POC and DOC at Yangtze River estuary was observed the similar hortiztonal distribution variation.Our results were indicated that abnormal circumstance(such as red tide waters and typhoon)would obviously influence the distribution of organic carbon.