Salinity increases with water table elevation at the boundary between salt marsh and forest

Abstract

The migration of salt marshes into forests along coastal regions is nowadays well documented. Sea level rise and storms threaten coastal forests by increasing groundwater levels and salinity. Salinization is the main cause of forest conversion to salt marsh in North America. In this paper we study groundwater levels and salinity in two wells installed at the border between forest and salt marsh in the lower Delmarva peninsula, USA. The upper well is located in the regenerative forest, where recruitment is still possible, while the lower well is located in the persistent forest, where only mature trees survive. Groundwater in the upper well is fresh at the root depth, while in the lower well the mean salinity is 8 ppt. Our data suggest that rainfall has an instantaneous effect on salinity and groundwater levels, but it does not affect salinity and groundwater levels on longer periods (weeks to months). Groundwater levels and salinity reflect the hydraulic gradient toward the marsh (a proxy for outgoing water fluxes), the uphill hydraulic gradient (a proxy for incoming water fluxes) and temperature (a proxy for evapotranspiration). Salinity increases when groundwater levels are high. To explain this result, we put forward the hypothesis that a high water table favors the flux of surficial, fresh water to the marsh, and loss of freshwater by evapotranspiration. These losses are likely replenished by saltier water moving at depth.