Modeling Soil Salinity Distribution Along An Elevational Gradient In Tidal Salt Marshes In Atlantic And Gulf Coastal Regions

Soil pore water salinity plays a very important role in determining the distribution of vegetation, plant productivity and biogeochemical processes in estuarine ecosystems. Pore water salinity gradients and salinity-vegetation associations in salt marshes have often been observed but rarely explained. A quantitative and systematic study on the pore water salinity distribution in salt marshes is not only critical to the understanding of the phenomenon itself but also to the use of the phenomenon as a convenient ecological and environmental change indicator. In this research, we developed a salt marsh pore water salinity model based on a salt and water balance model with modifications to several key features (e.g., applying the Penman-Monteith equation to calculate ET for different climate zones) to examine the impacts of climate, tidal forcing, soil, vegetation, and topography on pore water salinity distribution along elevation in the Atlantic and Gulf coastal regions. 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 along an elevational gradient, and the salinity maximum most likely occurs at an elevation approximately 25 cm above MHHW. Simulations indicate that tidal irregularity (defined as the standard deviation of tides in this study area) primarily controls the width of the salinity variation band (i.e., elevation range with soil salinity dramatically > incoming tidal salinity) along elevation. A standard deviation increase of 10 cm in tidal heights could result in an increase in salinity variation band by approximately 40 cm (mostly seaward). Moreover, ET, temperature, hydraulic conductivity, and incoming tidal salinity are the dominant factors determining the magnitude of the salinity maximum, which may lead to the occurrence of salt barrens/flats when reaching a threshold level (e.g., >70 ppt). Our analyses are important to understanding the effects of climate change and sea-level rise on the productivity and biogeochemical processes of salt marsh ecosystems by monitoring soil pore water salinity, an effective environmental indicator, over a salt marsh elevational gradient. Key words: Pore water salinity, Tide, Salt marsh, Elevational gradient, Model simulation, Atlantic and Gulf coasts