Modeling soil salinity distribution along topographic gradients in tidal salt marshes in Atlantic and Gulf coastal regions

Soil salinity plays a very important role in determining the distribution of vegetation, plant productivity, and biogeochemical processes in coastal marsh ecosystems. Salinity gradients and salinity–vegetation associations in salt marshes have often been observed but rarely explained. A quantitative and systematic study on the soil salinity distribution in salt marshes is not only important to the understanding of coastal marsh ecosystems but also to the development of a potentially useful ecological and environmental indicator. In this research, we developed a salt marsh soil salinity model based on an existing salt and water balance model with modifications to several key features to examine the impacts of tidal forcing, climate, soil, vegetation, and topography on soil salinity distributions of the Atlantic and Gulf coastal marshes. This model was calibrated and validated using field observations from the St. Marks National Wildlife Refuge (NWR) of northwestern Florida, USA. The results showed that the model had good agreement (r2 = 0.84, n = 15, P < 0.001) with field observations. We found that the mean higher high water (MHHW) level determines the location of the salinity maximum in a coastal salt marsh. Simulations indicate that tidal irregularity primarily controls the width of the salinity maximum band. Evapotranspiration, temperature, hydraulic conductivity, and incoming tidal salinity significantly affect the salinity maximum band, which may lead to the formation of salt barrens/flats when reaching a threshold level.