A tidal marsh creek system acts as a main carbon source to adjacent coastal waters

The lateral carbon exchange between tidal flat wetlands and adjacent coastal water is an essential process determining whether a salt marsh is a carbon source or sink, yet current research predominantly focuses on vertical carbon exchange. A tidal creek in the Yellow River estuary salt marsh was monthly sampled from April 2024 to March 2025 to explore the spatiotemporal patterns, origins, and influencing factors of dissolved organic carbon, dissolved inorganic carbon, and particulate organic carbon across complete tidal cycles and seasonal variations. Both dissolved organic carbon and dissolved inorganic carbon exhibited pronounced seasonal variations, with lower concentrations in winter, whereas particulate organic carbon exhibited no pronounced seasonal variations. Different carbon concentrations exhibited a seaward‐to‐landward increasing gradient across the creek system and an inverse trend with water depth, except winter particulate organic carbon, with terrestrial humic‐like components making up about 66%. This indicates that salt marshes act as lateral carbon sources, primarily in the form of dissolved inorganic carbon (accounting for about 80% of the total carbon), exporting terrestrial carbon to adjacent coastal waters via tidal creeks. Carbon dynamics were seasonally controlled by shifting biotic and abiotic drivers. Temperature and phytoplankton regulated dissolved organic carbon in spring and summer, while tidal forcing dominated in winter; terrestrial inputs dominated year‐round. Tidal activity and biological processes primarily governed dissolved inorganic carbon, with enhanced nutrient‐driven effects in summer and winter. Particulate organic carbon was driven by biotic factors, especially phytoplankton and microphytobenthos, across seasons. This study provides foundational data for quantifying lateral carbon exchange in salt marshes, essential for assessing blue carbon contribution to climate change mitigation.