Distribution, Composition and Transformation, and Multi-Process Coupling Mechanisms of Dissolved Organic Matter in the Changjiang Estuary

To clarify the migration and transformation mechanisms of dissolved organic matter (DOM) in estuaries under the influence of multiple factors, a comprehensive survey was conducted in the Changjiang Estuary and its adjacent sea area in summer 2021. By integrating multiple parameters including the absorption and excitation-emission matrix (EEM) fluorescence spectroscopy of Colored DOM (CDOM), dissolved organic carbon (DOC) concentration, and the composition and enantiomeric ratios of total hydrolyzable amino acids (THAA), the distribution characteristics, sources, transformation, and key controlling processes of DOM were systematically investigated. The results showed that both DOC concentration (1.21~2.11 mg/L) and the CDOM absorption coefficient a(355) (0.48~2.14 m^-1) exhibited a significant seaward decreasing gradient and were significantly negatively correlated with salinity (r=-0.89, p < 0.01), confirming that physical mixing is the primary factor controlling the macroscopic distribution of DOM. Parallel factor analysis identified two humic-like and two protein-like components. Their relative composition shifted from a dominance of terrestrial humic-like components (accounting for 65%~75%) in the nearshore to a dominance of marine-sourced protein-like components (accounting for 45%~60%) in the offshore waters. The spectral slope ratio showed a positive correlation with salinity (r=0.83, p < 0.01), indicating a decrease in molecular weight and aromaticity of DOM during its seaward transport, attributable to photochemical degradation. Molecular-level amino acid information further elucidated the spatial differences in DOM sources and degradation status. The contribution of THAA to DOC (THAA-C%) increased from 2.71% in the nearshore region to 5.09% in the offshore region. Influenced by the Changjiang River input, the nearshore region was dominated by terrestrial, highly degraded DOM, characterized by a lower THAA-C% and predominantly negative DI values. In contrast, the offshore region exhibited enhanced phytoplankton production and a higher proportion of fresh DOM, leading to an increased THAA-C% and a shift of DI values toward predominantly positive values. The increase in the D/L ratio of amino acids in bottom waters directly demonstrated the ongoing microbial degradation. Redundancy analysis further revealed that the composition and transformation of DOM are synergistically regulated by physical mixing, photodegradation, and microbial activity, and a significant coupling effect between photodegradation and microbial degradation was identified. These findings deepen the understanding of DOM biogeochemical mechanisms in estuarine areas from a multi-process coupling perspective and provide new basis for quantifying the fate of terrestrial carbon in coastal ecosystems.