Tidal changes and influencing factors of dissolved organic matter source and composition in the tidal reach of Dagu River
Tidal river reaches,where estuaries are affected by both human activities and tidal oscillations,play a crutial role in understanding of dissolved organic matter sources and composition,which is key to studying estuarine organic matter and carbon cycling.In this study,we collected water samples over the tidal cycle in tidal reaches of Dagu River(TDGR)during the dry(May)and wet season(October)of 2022.Measurements included salinity,chlorophyl,dissolved organic carbon,UV-Vis absorbance and 3D fluorescence spectra of DOM.The results showed that in the wet season,TDGR experienced large runoff and high primary productivity,with DOM mainly consisting of aromatic compounds and humic substances from runoff.Additionally,aquaculture wastewater discharged during ebb tide contributed high amount of humic-like substances to nearshore stations.In the dry season,with reduced runoff and low primary productivity,DOM was composed of terrestrial humic-like substances and protein-like substances from domestic sewage,which has higher content during ebb tide.Principal component and correlation analysis showed land runoff and primary productivity are important factors driving seasonal differences in dissolved organic matter(DOM)content and composition.In the dry season,tidal oscillations modulated the transport of terrestrial inert substances(aromatic compounds and humic like substances)and active protein-like substances through controlling the mixing ratio of freshwater and seawater masses,which increased the turnround time of DOM biogeochemical cycle changing the seaward export flux of organic carbon.The results of this study provide valuable data and insights for understanding the transport and transformation of estuarine organic matter.
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