Controls on carbon and nutrient transport in a tidal creek across a coastal forest-marsh transition under sea-level rise

Sea level rise (SLR) and saltwater intrusion (SWI) drive ecological and biogeochemical changes in coastal forests that alter solute fluxes between marshes and estuaries. Tidal creeks mediate transport of carbon (C), iron (Fe), and nitrogen (N) during flood and ebb tides, yet quantifying elemental fluxes is challenging due to bidirectional tidal flow and frequent bank overtopping. We quantified surface-water transport of C, Fe, and N during spring tides and storm events (2022–2023) in a tidal creek draining a transitioning coastal forest along the Delaware Bay. Discharge was estimated using a novel rating-curve model that incorporates bank overtopping dynamics. Groundwater wells and redox probes along a marsh-to-forest transect monitored water table elevation, salinity, and redox potential. We observed net imports of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), dissolved Fe, and ammonium (NH4-N). DOC hourly loads were positively correlated with dissolved Fe loads, suggesting microbial dissolution of C-bearing Fe oxides during flood tides, and reprecipitation and C retention on ebb tides. Plant uptake and nitrification drove NH4-N and DON imports, as plant uptake during high tide could remove N from the water column. There was no import or export of nitrate (NO3-N), but NO3-N hourly loads were positively correlated with marsh redox potential, consistent with denitrification under reduced conditions. As the coastal zone responds to accelerating SLR, dissolution of Fe protected C will inhibit C sequestration and marsh loss to SLR could increase N delivery to coastal waters as plants uptake less N, with downstream implications for eutrophication.