Composition And Spatial Distribution Of Dissolved Organic Matter In Summer Water From The Yangtze River Estuary

Estuarine is an important link between terrestrial ecosystem and marine ecosystem.The biogeochemical process in the estuary directly determines its final reduction during the transportation of terrestrial organic matter to the ocean through rivers.The study on the composition and distribution of organic matter in the Yangtze River Estuary East China Sea shelf area is helpful to understand the transportation,sedimentation and deposition process of terrigenous organic matter in the Yangtze River Estuary.In this paper,the composition of chromophoric dissolved organic matter(CDOM)was determined by ultraviolet visible spectroscopy and fluorescence spectroscopy.The carbon isotope values of dissolved organic carbon(DOC),particulate organic carbon(POC)and sedimentary organic carbon(SOC)were measured by stable isotope ratio mass spectrometer.The biomarkers in the sediments were extracted and determined.The biogeochemical behavior of terrigenous organic matter in the Yangtze River Estuary and its influencing factors were discussed by combining microcosmic and molecular characterization.The main results are as follows:(1)The DOM properties of the Yangtze River estuary are mainly controlled by the salinity of the water body,the source of organic matter and the physicochemical processes experienced.Physical mixing of salty and fresh water,local transformation of turbidity maximum zone,photodegradation and primary productivity are the main factors affecting the horizontal distribution of DOM in the Yangtze River Estuary.The transformation of microorganisms,the release of interstitial water and the mixing of water masses are the main factors affecting the vertical distribution of DOM in the Yangtze River Estuary;(2)There are significant differences in DOM properties among the four regions,i.e.estuary,turbidity maximum zone,marginal zone of fresh water and sea area.DOM in estuarine area is affected by both terrigenous input and primary productivity,and the variation range of DOC concentration is small;the mixing of salt and fresh water in turbidity maximum zone is intense,and DOC concentration drops sharply;the edge of flushing water is affected by upwelling,and the heterogeneity is strong;DOC concentration in marine area shows a trend of homogenization,but there is a role of microorganism in the process from surface to bottom.It can be seen that hydrodynamic condition is an important factor controlling the DOM composition and spatial distribution in the Yangtze Estuary East China Sea shelf;(3)The low correlation between doc and POC indicates that POC is not the key factor controlling doc distribution.The relative content distribution of terrigenous organic matter in the sediments reveals that the deposition process of terrigenous particulate organic matter occurs in the maximum turbidity zone,which confirms the role of the maximum turbidity zone in the"sink" of terrigenous organic matter.(4)The sedimentary organic matter in the Yangtze Estuary-East China Sea shelf area is rich in indicators indicating the input of terrigenous ancient mature organic matter,including the distribution of geological configuration hopanes and steranes.At the same time,the strong odd carbon advantage of high carbon n-alkanes and the high content of low carbon fatty acids reveal the deposition contribution of terrigenous soil organic matter and marine productivity.