Sediment stratification is a ubiquitous feature in intertidal aquifers, yet its precise role in regulating dissolved nutrient dynamics remains poorly understood. This study examined two typical kinds of intertidal transects including the sandy beach and mudflat, each exhibiting distinct sediment stratification. We analyzed the multi-depth distribution patterns of dissolved nutrients (nitrogen, phosphorus, and silicon) and carbon in intertidal groundwater. We also identified the key drivers using stable isotope tracers (δ15N-NO3− and δ18O-NO3−) and multivariate statistics, and quantified the interface fluxes to the coastal ocean. The findings indicated that the distribution patterns of nutrients and carbon between fine- and coarse-grained sediments in both transects were synthetically influenced by hydrological dynamics and biogeochemical reactions. In the sandy beach, groundwater composition transitioned from being NO3−-dominated in the deep coarse-grained sediments, where attenuation occurred via denitrification, to NH4+-dominated in the surface fine-grained sediments due to organic matter mineralization. In the mudflat, organic-rich sediments and low water exchange rates favored NH4+ dominance, with fine-grained sediments accumulating high concentrations of NH4+, PO43−, and DSi in groundwater. Moreover, the sandy beach served as hotspots for groundwater discharge, delivering nutrient and dissolved carbon fluxes up to two orders of magnitude higher than the mudflat. The presence of a surface fine-grained sediments enhanced the export of groundwater-derived NH4+ and DIC, potentially exacerbating coastal eutrophication and acidification. These findings demonstrate that sediment stratification fundamentally alters the composition and flux magnitude of nutrients and carbon discharged into coastal waters, thereby influencing coastal solute budgets.
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