Spatial Distribution And Transport Of Surface Dissolved Organic Matter In The Southeast Coast Of China And Adjacent Northwest Pacific

Marine dissolved organic matter(DOM)is an important carbon reservoir in the marine environment and plays an important role in the biogeochemical cycle of the entire marine environment.The marginal sea area of the shelf is located in the transition zone from terrestrial environment to open ocean environment,which is not only affected by land-based materials brought by rivers but also marine environmental impact caused by seawater intrusion,and has a more complex hydrological environment and active biological activities than the open ocean.The biogeochemistry of marine DOM has attracted widespread attention to the research on dissolved organic matter in the marginal sea area of the continental shelf.As an extremely important marginal sea in the western Pacific region,the East China Sea serves a key role in transporting a considerable part of land-based materials from the East Asian continent to the Pacific Ocean,and the DOM in the seawater also undergoes inevitable changes during longdistance transcontinental shelf transportation.In addition,the Chinese southeast coast spans across the subtropical and warm temperate climatic zones,leading to different natural climatic environment,while and the regional economic development status is also different.Most,of the past studies are for DOM in limited regions,but those for the entire Chines coastal seas are relatively rare.As a result,our understanding of the distribution and change characteristics of DOM in the Chinese coastal seas is still limited.This thesis examined,,DOM in surface seawater in the southeast coast of China and the East China Sea to the northwest Pacific Ocean.Specifically,the surface seawater samples of the navigation section from the East China Sea to the Northwest Pacific Ocean(20 sampling stations)and the surface seawater samples from the southeast coastal offshore area(24 sampling stations)were collected in the autumn of 2022.UVVis spectroscopy,three-dimensional fluorescence spectroscopy,dissolved organic carbon content and seawater salinity were determined to characterize the spatial distribution characteristics of DOM in surface seawater and the change process of DOM from estuary to ocean.The main conclusions are as follows:(1)In autumn,there are obvious horizontal and longitudinal transport processes of DOM from estuaries to the ocean in the East Chinese Sea-Northwest Pacific Ocean and southeast coastal areas,respectively.Specifically,the influence of land-based substances brought by estuarine areas on DOM in surface seawater gradually weakened,while the influence of sea-source substances on DOM gradually increased,and the spectral characteristics of DOM were characterized by reduced concentration,weakened aromaticity,decreased relative molecular weight,decreased degree of humification,and increased proportion of newly generated DOM in situ.(2)During the horizontal transport of DOM in the East Chinese Sea and Northwest Pacific Ocean,there were significant differences in the spectral characteristics of DOM between the inner and outer continental shelves.The correlation between salinity and spectral parameters shows that the mixing of water masses of different properties in the outer shelf region is an important reason for the change of spectral characteristics in this region.Similarly,in the early stage of longitudinal transport,the DOM of surface seawater in the southeast coast also experienced the spectral characteristics of a significant decline in land-based signals,but the transition range was shorter,and a more significant transition from land-based signals to sea-source signals occurred in a small spatial range of the transition from Zhoushan Islands to Zhejiang and Fujian coasts,and the overall performance of DOM in surface seawater in this area.(3)During the transportation of DOM from the estuary to the surface of the ocean,special geographical environment factors,such as the topography of the strait,will have an important impact on the spectral characteristics of DOM.In the process of transverse transport of DOM in surface seawater in the East Chinese Sea-Northwest Pacific Ocean,the strait has an important impact on the spatial distribution of DOM spectral characteristics,but only some spectral parameters are reflected,such as the proportion of DOM contribution of in-situ freshly produced DOM increased significantly,and the relative molecular weight size increased.Similarly,the influence of the topography of the strait on the spectral characteristics of DOM was also observed during the longitudinal transport of DOM in the surface seawater of the southeast coast,and the influence of the Taiwan Strait on the longitudinal transportation process is more obvious in the spectral characteristics,and almost all the spectral characteristics can observe obvious changes in the Taiwan Strait and its north and south regions.(4)Seasonal factors have a significant influence on the horizontal transportation of DOM from surface seawater to the northwest Pacific Ocean and the longitudinal transportation process from north to south along the southeast coast.By comparing with previous studies,it can be found that during the horizontal transportation,the hindrance effect of Kuroshio’s surface seawater DOM to the ocean in the outer shelf area is more obvious in spring,but in this study,the spectral characteristics of DOM in the surface seawater of the outer shelf in autumn are not significantly different from those in the far sea area east of Kuroshio.At the same time,the main power source of vertical transportation is the Zhejiang-Fujian coastal current,the main power source of vertical transportation has seasonal variability in strength and also exerts influence range of river flushing water in the southeast coast,thereby affecting the spatial distribution pattern of DOM.