Optical Properties And Controlling Factors Of Dissolved Organic Matter In China Marginal Seas And The Adjacent Western Pacific Ocean

The marginal sea,as a connecting zone between land and open ocean,is one of the most active regions of ecosystem and biogeochemistry on the earth.The cycle of dissolved organic matter(DOM)in the marginal sea is obviously affected by terrestrial input,in situ biological activities and the intrusion of adjacent open ocean water mass.Under the influence of climate change and human activities,and with the guidance of strategic consensus on CO2 emission reduction and storage increase,the sources,fate and dynamic processes of DOM in the marginal sea have attracted more and more attention.In this study,the China shelf marginal seas,the basin of South China Sea and the adjacent tropical western Pacific Ocean were taken as the study area.Based on the dissolved organic carbon(DOC),chromophoric DOM(CDOM)and fluorescent DOM(FDOM),we aim to elucidate the content,abundance,composition,distribution features and budgets of DOM in this continuous system.This work has yielded the following research achievements.In spring,Both DOC concentration,CDOM and FDOM abundance and the specific absorption coefficient(SUVA254 and SUVA325)in the Bohai Sea,Yellow Sea,East China Sea,Beibu Gulf,Taiwan Strait and the northern shelf area of South China Sea showed a similar decreasing trend,indicating that there was significant difference of DOM content and composition in the marginal sea of China,reflecting the difference of terrestrial input and intrusion of water mass from open ocean.CDOM and FDOM in the Bohai Sea,Yellow Sea,East China Sea,Taiwan Strait and Beibu Gulf showed negative correlations with salinity,indicating that they are controlled by the conservative mixing.CDOM and FDOM were mainly controlled by in situ biological activities in the northern shelf area of South China Sea.There were obvious differences of DOM content and properties in the major rivers of eastern China,which was subjected to human activities and river watershed background.Minjiang River endmember had the lowest DOC concentration,SUVA325 and the highest ratio of aCDOM(325)to aCDOM(254)(aCDOM(325)/aCDOM(254)),indicating that riverine DOM of Minjiang River was controlled by terrestrial source.The Yangtze River had the lowest CDOM abundance and SUVA values,but it carries a lot of anthropogenic DOM(high DOC concentration,high CDOM abundance,low aCDOM(325)/aCDOM(254))from Huangpu River to the East China Sea.The plot of DOC to acDOM(325)/aCDOM(254)could well reveal the difference of DOM sources,properties and cycles in the China marginal seas and rivers.Compared with other marginal seas in the world,the SUVA values of China marginal seas are significantly lower,which is closely related to the destruction of river watersheds and coastal wetlands,dam construction and human activities.Despite potential importance in the global carbon cycle,dissolved organic matter(DOM)dynamics in the epipelagic North Pacific low-latitude western boundary current system(<200 m)remain unclear.The input North Equatorial Current(NEC)had much higher DOC concentrations but lower specific UV absorbance than the input New Guinea Coastal Current/Undercurrent(NGC(U)C).This suggests DOM in the NEC is highly photo-degraded while in the NGC(U)C is more biodegraded.The quasipermanent Mindanao cold eddy upwelled humic-rich water into euphotic zone,and thus could be a potential sink region of humified DOM.The Halmahera and another warm eddy had much lower CDOM and FDOM abundances than the surrounding waters,while the DOC concentrations below the mixed layer were much higher.This result suggests that warm eddy could enhance the photochemical transformation of DOM and become an important pathway of humic-poor DOM exporting to ocean interior.Subsurface maxima were observed for DOC,CDOM and FDOM at different depths,which are subjected to different regulations by physical mixing,primary production and microbial metabolism.A mass balance model revealed that DOM properties of the Indonesian Throughflow and North Equatorial Countercurrent was regulated by the mixing of the input NEC and NGC(U)C.These two export currents could transport?220 and 910 Tg C yr-1 of DOC into the Indian and tropical Pacific Ocean,respectively,which emphasize the importance of lateral DOM flux in regional and global marine carbon biogeochemistry.The abundances of CDOM and FDOM in the aphotic SCS(>200 m)were significantly higher than that in the adjacent Western Pacific Ocean(WPO),which was not coupled with DOC patterns,suggested that DOM composition and structure in the SCS was completely different from that in the WPO.A three-endmember mixing model showed that the aphotic SCS were occupied by three water masses:the mixed water in the upper SCS(SCSUIW),the denatured North Pacific Intermediate Water(NPIW*)and the denatured Circumpolar Deep Water(CDW*),which contributed 6%,17%and 77%of the total volume of aphotic SCS,respectively.Model result also showed that the unit oxygen consumption rates in the intermediate SCS(200?1000 m)and deep SCS(>1000 m)were 0.91 ?mol kg-1 yr-1 and 0.58 ?mol k-1 yr-1,respectively,which were much higher than the mean value in the global aphotic ocean(?0.4 ?mol kg-1 yr1).Along with the oxygen consumption,humic-like FDOM and UVA-CDOM had obvious addition,which mainly comes from the remineralization of organic matter.Without considering water mass mixing,the in situ production rate of humic-like FDOM and UVA-CDOM per unit oxygen in the intermediate SCS will be slightly underestimated,but the production rate in the deep SCS will be greatly overestimated.The in situ production rate of humic-like FDOM and UVA-CDOM in the aphotic SCS was significantly higher than that in the open ocean interior.Thus,the aphotic SCS,which only accounts for 0.17%of the global aphotic ocean,supplied 0.20?0.33%of the annual in situ yield of global dark ocean.The bio-refractory carbon produced in situ will not accumulate in the SCS interior,but will be continuously exported to the WPO in the deep-intermediate SCS.Therefore,the SCS can be regarded as an "accelerator"for the storage of refractory carbon in the ocean.The tropical SCS showed different distribution patterns,in situ consumption and production processes of DOC,CDOM and FDOM with the subtropical Mediterranean Sea and temperate Japan Sea,suggesting the complexity and diversity of carbon storage process in the marginal seas,which are closely related to the differences in the hydrological processes,primary productivity and export productivity.