The general paradigm for the observed plant zonation in salt marshes is that a combination of abiotic stress and competition drive vegetation patterns within a single marsh, but there have been few studies investigating the bankside vegetation changes that occur along the longitudinal salinity gradient of estuaries. The main objectives of the research presented in this dissertation were to examine whether the same controls that explain the distribution of Spartina alterniflora in the salt marsh can be applied without modification to a longitudinal salinity gradient and to evaluate changes in Spartina distributions under drought conditions. Reciprocal transplant studies, greenhouse experiments, species removals in mixed Spartina stands, and vegetation surveys were conducted in the estuary of the Altamaha River, GA, where S. cynosusroides occurs upstream of S. alterniflora. In reciprocal transplant experiments, each plant survived and performed best in its natural habitat. The presence of conspecific neighbors slightly reduced S. alterniflora plant performance in the salt marsh whereas S. cynosuroides showed little response in either environment. The results of these and other experiments suggest the lower estuarine distribution of Spartina cynosuroides is controlled by abiotic conditions (salinity or sulfide concentrations). The upper estuarine distribution of S. alterniflora is not well understood, but seems to also be primarily controlled by abiotic factors (possibly a sulfate requirement). These outcomes challenge results from previous investigations of zonation controls in salt marshes and suggest that modifications to the salt marsh paradigm are necessary when describing vegetation distribution along an estuarine gradient. During an extended drought (2000-2002), Spartina alterniflora density increased to a greater extent than S. cynosuroides in mixed stands and the location where Spartina cover was 50 % S. cynosuroides and 50 % S. alterniflora shifted approximately 3 km upriver, suggesting that Spartina communities can respond rapidly to increasing estuarine salinity. These studies improve our understanding of the ecological linkages in estuaries and can aid coastal policymakers in making better management decisions and predictions concerning how changes in freshwater inflow might impact the distribution of estuarine organisms.
