Combined Spectrum And Chromatographic Technique To Trace The Sources, Compositions And Transformations Of Dissolved Organic Matter In Changjiang-East China Sea Continuum

Dissolved organic matter(DOM)is one of the most important carbon pools on the earth.DOM plays important role in the biogeochemical processes of aquatic ecosystems.The composition of DOM can affect the role that aquatic ecosystems play in the carbon cycle and affect carbon flow in aquatic ecosystems.Rivers play the important roles in linking terrestrial and marine ecosystems,as large amounts of terrestrial organic matter are transported to the coastal ecosystems by rivers.And riverestuary-coastal continuum are intensively affected by human activities.During the transport from rivers to the marine ecosystems,there are many factors(e.g.hydrological condition,light irradiation)that can modify the composition and reactivity of DOM.The study of the DOM composition in the river-estuary-coastal continuum can help us to reveal the transformation mechanisms involved in the alteration of DOM compositions.There are a few studies that focus on the DOM compositions in different aquatic environment.However,there are limited studies that combine the spectral and chromatographic technique to study the transformation of DOM composition in riverestuary-costal continuum.Here,we used the ultraviolet-visible spectroscopy and fluorescence spectroscopy to measure the chromophoric dissolved organic matter(CDOM),the gas chromatography was used to measure the dissolved lignin phenols,and used the ultra-high performance liquid chromatography to measure the dissolved black carbon(DBC)in Changjiang,Changjiang estuary and East China Sea.The study investigated the composition,sources and transformations of DOM in Changjiang-Changjiang estuary-East China Sea continuum and the factors that affect their behaviors.(1)We quantitatively analyzed the modification of DOM in the mid-lower reaches of Changjiang.Under different hydrological conditions,the compositions,sources and transformations were different.During the high water discharge period,due to the extensive soil erosion,the dissolved organic carbon(DOC)and CDOM contents were higher,while primary production contributed to the protein-like matter during the low water discharge periods.The dissolved lignin concentrations were similar under different hydrological conditions;however the differences in the upstream water supply due to impoundments the Three Gorge Dam and the agriculture activities in the mid-lower reaches resulted in the variation of S/V and C/V ratios of dissolved lignin phenols.For the low water discharge period,the degradation process is dominated by flocculation;whereas during the high water discharge period,bio-degradation and photo-degradation played more important roles.Beside the effect of the Three Gorge Dam,groundwater transported large amount of DOC and CDOM to the mid-lower reaches during the low water discharge period and the fluxes are comparable with the fluxes of Dongting Lake.(2)DOM composition and content at Xuliujing are affect by the erosion of the drainage basin.We studied the DOM composition for a hydrologic year at the Xuliujing station,and calculate the DOM flux transport to the East China Sea.The Changjiang annually transports 1.18×106 t C DOC to the ocean,which accounts for 0.45% of the global river transport.The Changjiang annually transports 71.95×103 t C DBC and 6.93×103 t C dissolved lignin to the ocean.Two thirds of the DOM contents are transported during the higher water discharge period.The drainage basin erosion add particulate and dissolved matter to the river,which lead to the good correlations between total suspended matter and DOM contents at Xuliujing station.The drainage basin erosion is the important source of DOM at Xuliujing station.At the Xuliujing station,the erosion and dissolution of soil black carbon are the dominant sources of DBC.The proportion of DBC increased with the increase of water discharged at Datong,which may explain the preferential erosion of black carbon compared to the other components of OM from soil.The recalcitrant property of DBC also contributes to the high proportion during high water discharge condition.The content of vallinic lignin phenols in large rivers globally has showed significant positive correlation with the forest coverage rate of the river basin,indicating the importance of forest coverage of the basin as a dominant factor controlling the dissolved lignin content.(3)We also analyzed the effect of turbid maximum zone(TMZ)on the DOM composition.The DBC and CDOM contents show significant positive correlation in Changjiang estuary during the summer,illustrating the mixing of riverine DOM and marine DOM as the dominant process taking place within the estuary.At the mid salinity ranges(15~20)the DOM composition are altered significantly due to the extensive photo-degradation and active the primary production of phytoplankton.In July 2011,the re-suspension of the sediment and the sorptiondesorption processes taking place within the TMZ,alters the DOM composition significantly.At the TMZ,dissolved lignin concentration increased,and the particulate organic carbon,particulate dissolved lignin increased extensively,suggesting the re-suspension of the sediment contributes to the addition of large amounts of terrestrial organic matter.However,DBC were remove from the water at the TMZ.The black carbon content of the sediment increased at this region,suggesting that DBC absorbed to the particles and was buried in the sediment subsequently.The DBC content was removed by a factor of 11±23% while dissolved lignin was added by a factor of 42±27%,indicating the refractory component of DOM(e.g.,DBC)are preferential buried in the sediments,while the semi-refractory component(e.g.,lignin)were preferentially dissolved in the water.,This strengthens the burial of refractory organic carbon at the TMZ.(4)The Changjiang diluted water,Taiwan warm current and Kuroshio are dominant drivers that alter the variation and budget of DOM in East China Sea.Rivers transport large amounts of terrestrial DOM to East China Sea,and about 80% DOM is delivered by the Changjiang.The DOM pattern in East China Sea are dominated by the mixing of Changjiang diluted waters,which is low in salinity but high in DOC content,with the Taiwan warm current and Kuroshio,which have higher salinity but lower DOC content.Phytoplankton,photo degradation and microbial processes can also affect the DOM content of East China Sea.Based on the different water exchange data,we built the box model for DOC,dissolved lignin and DBC.The results indicate that Taiwan warm current is the most important source of DOM in East China Sea,while the DOM of Kuroshio is also important.River and atmospheric deposition transported more terrestrial DOM.The fluxes were about 3.4% for DOC,14.8% for dissolved lignin and 7.4% for DBC.According to the calculation of the box model,we can get more detail information about the sources and budgets of DOM in East China Sea.In this study,we analyzed the DOM of Changjiang,Xuliujing,Changjiang estuary and the East China Sea.During the transport of DOM from river to the coastal region,DOM composition are modified by multiple factor,for example,the hydrological condition,the erosion within the drainage basin,mixing within the estuary,the TMZ,photo degradation and microbial process.Terrestrial organic matter is preferentially degraded,the aromaticity of DOM is decreased.Using the different components of DOM one can better understand the transformation of terrestrial organic matter during its transportation within river-estuary-coastal continuums.