Seasonal variability in lateral carbon exchange and dissolved organic matter exports from a salt-marsh tidal creek of the Yellow River Delta

Coastal wetlands are key components of the global coastal carbon cycle. However, their carbon sink capacity cannot be fully assessed without considering lateral carbon exchange through tidal creeks. Here, we quantified seasonal and within−tide lateral carbon fluxes in a salt−marsh tidal creek in the Yellow River Delta, and characterized the sources and composition of exported dissolved organic matter (DOM) using optical indices, EEM−PARAFAC, and stable carbon isotopes. The annual carbon budget indicated an overall net lateral export of -662.06 g C m−2 yr−1, but both the magnitude and direction of exchange varied markedly among seasons. Net export was strongest in summer and autumn, whereas winter showed net import under some tidal conditions, particularly for DIC, indicating pronounced seasonal shifts in hydrodynamic connectivity and biogeochemical processing. Exported DOM was strongly influenced by terrestrially derived, highly humified material: low fluorescence index (FI), high humification index (HIX), and predominance of humic−like PARAFAC components pointed to a substantial marsh contribution, especially during warm seasons. Stable carbon isotopes further supported a greater contribution of marsh−derived DOC in summer and autumn. High−frequency observations revealed pronounced low−tide carbon pulses, with elevated DIC, dissolved CO2, POC, and (to a lesser extent) DOC during ebb tide and at low water. Their relationships with turbidity and conductivity suggest that sediment resuspension and porewater drainage were important drivers of these short−lived export events. Overall, lateral exchange through tidal creeks is a major component of carbon cycling in this Yellow River Delta salt marsh; seasonal variability, DOM source characteristics, and low−tide pulsed export should therefore be explicitly incorporated into blue carbon assessments and coastal wetland carbon budgets.