The Role of Groundwater Flow in Salt Marsh Ecology

Groundwater flow plays a role in both the spatial distribution of salt marsh plants and the exchange of nutrients between marsh pore waters and tidal waters.

Plant distribution and productivity are partly governed by the concentrations of salt, sulfide and nutrients in marsh pore waters. In South Carolina wherever marshes border extensive forested uplands upwelling fresh ground water flushes salt and sulfide from high marsh soils. As a result salt tolerant Salicornia is excluded from such areas, even at marsh elevations where it is otherwise observed. In contrast Salicornia is often found in high marsh areas where groundwater flow has a strong downward component, which thereby allows the infiltration of saline tidal water and its subsequent evapoconcentration.

Along creek banks pore water drainage during low tide flushes out dissolved sulfide and thereby enhances the productivity of Spartina alterniflora in this environment. Pore water drainage during low tide also transfers dissolved nutrients, and possibly radium isotopes, from the marsh soil into adjacent creeks. Because the concentrations of nutrients in marsh pore waters are typically one to two orders of magnitude greater than those in tidal waters, their supply via drainage can be as large as their supply via river runoff from terrestrial uplands.

Further understanding of the role of groundwater flow in salt marshes via numerical simulations is proving to be a major challenge because of the complexities introduced by tidal dynamics, evapotranspiration, seepage face formation, marsh soil heterogeneity and other factors.