Dynamics Of Organic Matter In The Zhangjiang River-estuary-Dongshan Bay Continuum And Its Influencing Factors

As an important transport zone of organic matter from land to ocean,the riverestuary-coastal ocean continuum is of great significance to global climate change and ocean carbon cycle.Dissolved and particle organic matter(DOM and POM),as important components of carbon transport in the continuum,are significantly affected by natural and human factors.At present,DOM and POM studies are mostly limited to rivers,estuaries and bays with relatively single spatial scale,and lack of systematic integration studies across the continuum.In this study,we taken the continuum as the research object.Based on the field investigation of spring,summer and winter cruises,UV-Vis absorption spectra and fluorescence spectroscopy were used as characterization means to conduct qualitative and quantitative characterization of DOM and POM components to explore the characteristics,spatial and temporal distribution,source changes and main influencing factors of DOM and POM in the continuum under the regulation of hydrological process and human disturbance process.The main research results are as follows:(1)DOM content varied greatly in the continuum,DOC concentration ranged from 84 to550 μmol L-1,and the absorption coefficient aCDOM(350)varied from 0.44 to 8.51 m-1.DOM qualitative indexes also have great changes.The spectral slope S275-295 varies from 0.0146 to 0.0297 nm-1,and the specific aromaticity SUVA254 varies from 0.45 to 4.84 L mg-1 Cm-1.DOC,aCDOM(350)and four fluorescent components(C1-C4)increased firstly and then decreased along the course from Fengtou Reservoir to Dongshan Bay,and reached the highest values at the downstream of Zhangjiang river and estuary,reflecting the comprehensive influence of human activities,aquaculture wastewater discharge and mangrove tidal export.The spectral slope S275-295 showed a gradually increasing trend along the continuum from the Fengtou Reservoir,while the specific absorption coefficient SUVA254 and HIXb were opposite,indicating that the molecular weight,aromaticity,and decrease of DOM during the transport from the river to the ocean The spatial trend decreasing degree of humification.(2)The quantitative and qualitative indexes of DOM in the continuum also have obvious seasonal variations.The upstream and downstream reaches of the Zhangjiang River usually have high DOM abundance in spring,which is mainly related to the"initial flushing effect" in spring.The lower DOM abundance in the summer of 2019 was associated with the dilution effect of previous continuous rainfall events caused by El Ni(?)o.In winter,the water residence time is longer,DOM component presents a strong signal of biological origin,and the concentration of DOC in winter is also significantly higher than that in spring and summer.DOM components were added in different degrees in the estuary and Dongshan Bay,but there were significant seasonal differences.The addition degree of DOM components was weak in summer,and the addition behavior was more obvious in spring and winter.(3)In the continuum,the absorption coefficient aCPOM(254)of base-extracted particulate organic matter(BEPOM)varied from 0.083 to 2.566 m-1,and the absorption coefficient aCPOM(254)*of the concentration normalization of BEPOM suspended particulate matter varied from 0.004 to 0.247 mg-1.The absorption spectra of BEPOM samples showed absorption peaks at 280 nm and 390-440 nm.The fluorescence spectra all contained strong protein-like component(peak T and peak B),and the percentage of protein-like components in the fluorescence component was higher than CDOM,especially in winter,which reflected that there was a strong biological activity signal in BEPOM in the continuum.It is suggested that BEPOM analysis can be widely used to trace the source contribution of biological activities in POM of aquatic ecosystem.(4)aCPOM(254)and four fluorescence components(C1p,C2p,C3p and C4p)of BEPOM samples showed a decreasing trend from river to estuary.The relative contribution of authigenetic particulate organic matter is significant,and the distribution and behavior of BEPOM in estuaries are related to resuscitation,wetland output,and estuarine mixing process,and so on.The relative contribution of authigenic particulate organic matter in rivers is relatively significant.The distribution and behavior of BEPOM in the estuary area are related to resuspension,wetland output,and estuary mixing processes.Therefore,the BEPOM abundance in the continuum is lower than that of the US wetland estuary but higher than that in China,such as the Yangtze River estuary and the Jiulong River estuary,which are dominated by river sediment transport.