Predicting Freshwater and Oligohaline Tidal Marsh Vegetation Communities in the Vicinity of the Savannah National Wildlife Refuge

Abstract

A large portion of the remaining tidal freshwater marshes left in the southeastern U.S. lies within the braided channels of the lower Savannah River deltaic marsh complex (Georgia, USA). These marshes occur upstream from the large shipping port of Savannah and have been subjected to a variety of hydrologic changes as a result of shipping channel modifications in recent decades. The Georgia Ports Authority and the U.S. Army Corps of Engineers funded the development of hydrodynamic and vegetation community models in order to predict impacts of future channel modifications on the sensitive freshwater/oligohaline marshes. The goal of this study was to document vegetation communities throughout the tidal freshwater and brackish range (roughly 0-7 parts per thousand (ppt)) of the lower Savannah River, identify the environmental conditions influencing their distribution, and predict community distributions based on the underlying gradients. Using a combination of classification trees, cluster, and indicator species analyses to identify community types and their environmental thresholds, our final model used average interstitial salinity of the prior growing season and soil percent organic content as the best predictor variables. The primary indicator species for each community were identified as Eleocharis spp., which dominated the interior marshes in areas with <1.0ppt salinity, while Zizaniopsis miliacea dominated areas with lower soil organic content at <1.0ppt. Scirpus validus was dominant between 1.0-3.5ppt, while Scirpus robustus and Spartina spp. occurred at >3.5ppt average growing season salinity. Using these results with hydrodynamic model predictions of river salinity and marsh interstitial salinities, changes in total acreage of freshwater/oligohaline and brackish marshes can be estimated for a variety of shipping channel modifications.