Using Hydrological‐Biogeochemical Linkages to Elucidate Carbon Dynamics in Coastal Marshes Subject to Relative Sea Level Rise

Coastal marshes are an important component of the global carbon cycle, yet our understanding of how these ecosystems will respond to sea level rise (SLR) is limited. Coastal marsh hydrology varies based on elevation, distance from channel, and hydraulic properties, resulting in zones of unique water level oscillation patterns. These zones impact ecology and geochemistry and correspond to differences in carbon accumulation rates. These physical‐biogeochemical linkages enable use of a hydrological model to predict changes in marsh zonation, and in turn carbon accumulation, as well as groundwater‐surface water exchange under SLR. Here, we developed a calibrated hydrological model of a Delaware coastal marsh using HydroGeoSphere. We simulated three scenarios each of SLR, sediment accretion, and upland hydrologic response, and we quantified changes in the spatial coverage of different hydrologic zonations and groundwater‐surface water exchange. Results show that relative SLR reduces marsh area, carbon burial, and lateral water fluxes. However, the magnitudes of change are linked to the terrestrial groundwater table response as well as relative SLR. In scenarios where the upland water table does not change with SLR, the magnitude of decline in marsh area and carbon accumulation is reduced compared to scenarios where the upland water table keeps pace with SLR. In contrast, the reduction in lateral water flux is minimized in scenarios with an upland water table rise equal to SLR compared to scenarios where the upland water table is held at present‐day levels. This study highlights the importance of regional hydrologic setting in the fate of coastal marsh dynamics. Plain Language Summary Coastal marshes are efficient carbon sinks, but their location at the land‐sea interface makes them vulnerable to sea level rise. Through a web of complex interactions, coastal marsh hydrology, which is spatially variable, impacts carbon burial. This link between ecosystem processes enables the use of a physical, hydrological model to forecast changes in hydrology under variable relative sea level rise scenarios that can be related to alterations in carbon accumulation across the marsh. Our results show that both sea level and the water table on land impact coastal marsh hydrology. Results show a decrease in marsh area and carbon sequestration capacity with sea level rise, and variations in relative sea level rise and groundwater table response to climate change impact the magnitud