Source-to-Sink Pathways of Dissolved Organic Carbon in the River-Estuary-Ocean Continuum: A Modeling Investigation

Abstract. Transport and cycling of dissolved organic carbon (DOC) are most active in estuaries. However, a comprehensive understanding of the sources, sinks, and transformation processes of DOC throughout the river-estuary-ocean continuum is yet to be derived. Taking the Changjiang Estuary and adjacent shelf sea as a case study area, this study applies a physics-biogeochemistry coupled model to investigate DOC cycling the river-estuary-ocean continuum. DOC is classified into two types depending on the origin, namely terrigenous DOC (tDOC) and marine DOC (mDOC). Simulation results were compared with observation and showed a satisfactory model performance. Our study indicates that in summer, the distribution of DOC in the Changjiang Estuary is driven by both hydrodynamics and biogeochemical processes, while in winter, it is primarily driven by hydrodynamics. The spatial transition from terrigenous-dominated DOC to marine-dominated DOC occurs mainly across the contour line of a salinity of 20 PSU. Additionally, the source-sink patterns in summer and winter are significantly different, and the gradient changes in chlorophyll-a indicate the transition between sources and sinks of DOC. A five-year averaged budget analysis of the model results indicates that the Changjiang Estuary has the capability to export DOC, with tDOC contributing 31 % and mDOC accounting for 69 %. The larger proportion of mDOC is primarily attributed to local biogeochemical processes. The model offers a novel perspective on the distribution of DOC in the Changjiang Estuary and holds potential for its application in future organic carbon cycling of other estuaries.