PROCESSES CONTROLLING POREWATER SALINITY DISTRIBUTIONS IN A SOUTHEASTERN SALT MARSH

Coastal wetlands provide many important ecosystem services, which include carbon and nitrogen sequestration and transformations, the provision of habitats, and the reduction of erosion due to the prevalent vegetation. Plant distribution as well as plant productivity in coastal wetlands depends on inundation patterns and soil conditions, including porewater salinities. Therefore, as part of the Georgia Coastal Ecosystems Long Term Ecological Research project, an integrated modeling approach has been developed that simulates salinity distributions in wetland sediments by coupling a hydrodynamic flow model with a soil model.

We will present the development of the soil model, which is based on mass conservation for water and salt and links physical, hydrological, and biological processes that determine porewater salinity, including precipitation, evapotranspiration, salt exchange between surface and subsurface, groundwater exchange, and tidal inundation, with the lateral exchange controlled by marsh topography. The model is applied to the Duplin River marsh, Sapelo Island, Georgia and simulates porewater salinity and water content distributions in surface sediments across the Duplin River marsh domain. Model validation is performed by comparing simulated salinities to porewater salinity measurements in different vegetation classes and over a range of marsh elevations and years.

To identify the environmental factors that control marsh salinities, a sensitivity analysis was carried out that assesses the effect precipitation intensity, evapotranspiration, hydraulic conductivity, salt exchange, tidal salinity, and marsh elevation have on porewater salinities. Also, model-derived variability in porewater salinities was quantified over spring-neap tidal cycles, seasonal, and annual time scales, accounting for drought, normal conditions and years with excess rain.

Being able to accurately predict future porewater salinities provides a foundation to assess the impact of sea level rise on wetland soil conditions, including vegetation zonation and productivity. Future efforts will link the soil model to a plant model, which will further provide insight into the impact of climate change on carbon and nitrogen sequestration.