Characteristics Of Bulk Organic ?¹³C And C/N In Jiulong River And Estuary And It's Indication For The Sources And Transport Of POM

As a confluence of terrestrial and marine ecosystem,estuary is a major passage way of terrestrial organic matter into the ocean.While the complex hydrodynamic conditions,e.g.large salinity gradient and intensive biogeochemical processes make estuaries one of the most complex systems for the global carbon cycle research.Thus,understanding the sources,processes and flux of the estuarine carbon provides a fundamental knowledge for global carbon cycle study.In order to understand the sources,distribution patterns and transportation processes of the estuarine organic matter,a case study was carried out in the Jiulong River Estuary,a typical macro-tidal estuary located at southeast China.Investigations,covering flood and dry seasons,were carried out from 2015 to 2017 along the Jiulong River catchment and estuary to collect different sources/types of organic matters,including terrigenous organic matter(terrestrial plants and terrestrial soil),riverine POM,estuarine organic matter(estuary POM and estuary surface sediment).Samples were analyzed for the TSM and POC concentrations,?13C values and C/N ratios,and particle size composition.Finally,an ETM-POM transportation model were developed to understand the POM processes,including suspended transport,vertical settlement,deposition and resuspension processes.The low C/N ratios show that POM in Jiulong River mainly from in situ phytoplankton with little contribution of terrestrial organic matter.While the concentration of POM in the surface water of the Jiulong River Estuary(JRE)was increased with the increase of TSM concentration in ETM,which was significantly higher than that in the basin.In the downstream of ETM,large amount of POM flocculated and settling,and the POC concentration drops sharply.There exists a linear correlation between the ?13C values of POC and the salinity in JRE.The linear correlation and low C/N ratios indicates that POM is a conservative mixture of riverine POM and marine POM,which is confirmed by the end-member analysis.However,the mixing extent showed large different with the changes of runoff and tide.There were more intense mixing in the winter,which may cause by more intense tide and contribution of other sources outside the estuary.There is also an increasing trend from river to sea for ?13C of sediments(-26.0‰to-22.5‰).The C/N ratios were range from 10 and 20,which is significantly higher than the C/N ratios of POM.Although there also shows river-to-sea trend,the terrestrial detrital organic matter make up a large proportion in sedimentary organic matte in JRE(20%to 60%),This may cause by the large size of the terrestrial organic matter,which is difficult to capture in the surface water,but stored in the estuaruy sediments.Horizontal transport of POM dominated by particles smaller than10?m(>70%),TSM and POC concentration and ?13C and C/N value showed obvious changes with tide,the ?13C increased at high(-24.0‰),and decreased at low tide(-26.0‰),the fluctuation in TSM and POC concentration and POM C/N significantly at maximum flood and maximum ebb.The grain size,?13C and C/N ratio in organic matter of settling particulates were consistent with sedimentary organic matter at the JRE ETM,the high settling flux showed that settling particulates mainly derivered from the resuspension of sediment.Sediment in the riverbed would resuspend at the conditions of high flow velocity,sediment in ETM undergoing the cycle of deposition,resuspension,and deposition again.