Distributions And Seasonal Variations Of Chromophoric Dissolved Organic Matter(CDOM) In The Changjiang(Yangtze River) Estuary And Its Adjacent Sea

Estuarine and coastal regions are the hot areas of marine biogeochemistry research.The biogeochemical research of biogenic elements in estuarine and coastal areas is very challenging due to a large amount of terrestrial materials inputs,complex biogeochemistry processes and strong human activities.Chromophoric dissolved organic matter(CDOM)is the main constituent of dissolved organic matter(DOM)in estuarine areas,based on its stable optical properties,the distributions of CDOM in the area can be characterized,and then the biogeochemical process of DOM in the area can be explained.This essay selected the Changjiang(Yangtze River)Estuary and its adjacent sea as the study area,then choose the distributions and seasonal variations of CDOM as the research object.The spatial and temporal distribution characteristics of CDOM in the study area were elaborated by measuring the absorption spectra and the excitation-emission matrix spectrum(EEMs)of CDOM,and the main conclusions are as follows:(1)The distribution characteristics of DOC and CDOM in the study areaIn March and July 2019,seawater samples were collected at the surface and bottom layers in the Changjiang Estuary and its adjacent sea,respectively.At the same time,the absorption spectra of CDOM samples were measured,and the absorption coefficient a₂₅₄,molecular aromaticity degree SUVA₂₅₄ and spectral slope S_275–295 were calculated to study the properties of CDOM.The results showed that in March and July2019,the distributions of CDOM in the study area had a trend of decrease in concentrations,aromaticity and average molecular weight with the increase of offshore distance.The concentrations,aromaticity and average molecular weight of CDOM at the surface layer were significantly higher than those at the bottom layer.Meanwhile,the concentrations and aromaticity of CDOM in July were significantly higher than those in March,but the average molecular weight of CDOM did not show significant seasonal change.The correlation relationships between salinity and CDOM parameters showed that the CDOM in the Changjiang Estuary and its adjacent sea is relatively conservative.At the same time,some non–conservative behaviors of CDOM were found and showed some differences in different layers and seasons.(2)The composition of fluorescent dissolved organic matter(FDOM)in the study areaThe EEMs of CDOM samples were also measured and applied parallel factor analysis(PARAFAC)to characterize the FDOM components.There were mainly three components of FDOM,including a humic-like component C1 and two protein-like components C2 and C3 in the study area.The concentrations of the three components showed higher values near shore and lower values offshore.The concentrations of the three components at the surface layer were significantly higher than those at the bottom layer.Meanwhile,the component C1 and C2 did not change significantly between March and July,while the component C3 significantly decreased in July.For the three fluorescence components,the main source of component C1 and C3 was terrigenous substances carried by the Changjiang Diluted Water(CDW).However,the component C2 not only had the terrigenous input,but also the autochthonous source.The HIX,BIX,FI values showed that terrestrial substance is the main source of CDOM in the coastal area,while autochthonous CDOM is the most important source in the ocean.(3)Research on the non–conservative behavior of CDOM in the study areaIn order to describe the non–conservative behavior of CDOM in the Changjiang Estuary and its adjacent sea,variations of CDOM parameters at different stations with similar salinity and the stations near the special salinity contour in July 2019 were analyzed,and we found various non–conservative behaviors in the Changjiang Estuary and its adjacent sea mainly caused consequence which the concentrations of CDOM,aromaticity and average molecular weight decreased.Simultaneously,some stations showed increases on those parameters of CDOM,the main reasons are attributed to the generation and production of protein-like component C2 and the inhibition of photodegradation by the Turbidity Maximum Zone(TMZ)in the Changjiang Estuary and its adjacent area.However,different flows of CDW showed different non–conservative behaviors in the study area.