Coastal sediments recycle large amounts of organic matter (OM), yet the fate of component-specific dissolved OM (DOM), whether degraded or preserved, and its role in hypoxia formation remain poorly quantified. We combined fluorescence spectroscopy, parallel factor analysis, component-specific benthic fluxes and hydrodynamic modeling to assess the sources, reactivity, and oxygen consumption of sediment-derived DOM in the Pearl River Estuary. We identified three humic-like and two protein-like fluorescent DOM (FDOM) components. Humic-like FDOM components accumulated during microbial oxygen consumption in coastal bottom waters, as indicated by positive correlations with apparent oxygen utilization (AOU). Conversely, protein-like components were degraded and declined with increasing AOU. Component-specific fluxes revealed that sediments contributed 16±13% of marine humic-like and 17±16% of tyrosine-like FDOM to coastal bottom waters, originating primarily from settled planktonic detritus, with largest contributions in hypoxic regions (31±9% and 38±8%, respectively). These patterns were sustained by oceanic intrusion and longer residence times in the coastal region. In contrast, the upstream river waters deviated from these trends due to stronger terrestrial inputs, enhanced mixing, and limited benthic influence. Overall, our findings highlight the critical role of sediment-derived FDOM in promoting oxygen depletion and hypoxia formation in coastal ecosystems.
 

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