Spartina Species Zonation along the Altamaha River Estuary

Changes in freshwater inflow can cause changes in the distribution and diversity of marsh vegetation in estuarine habitats. In the fall of 2002 bankside vegetation was surveyed along the 24 km length of the Altamaha River estuary (n= 14 sites). Sites were quantified for multiple plant and edaphic parameters, including plant density, height, and tiller diameter. In this paper we present the characteristics of the bankside marsh vegetation as they change along the estuarine salinity gradient, and evaluate the use of a proportional relationship between two marsh grasses, Spartina cynosuroides and S. alterniflora, as a way to identify a transition line between salt and brackish marsh communities. S. alterniflora densities were greatest at the mouth of the estuary and decreased upstream and S. cynosuroides densities showed the opposite pattern, but there was not a well defined transition between these two plant communities. The percent S. cynosuroides cover along the estuary is a potentially useful way to document the response of the estuary to changing amounts of freshwater inflow. INTRODUCTION Estuarine environments are adversely impacted by reductions in freshwater inflow, sea level rise, and coastal land submergence. Changes in water resource management, such as increased withdrawals from rivers, may cause changes in the distribution and diversity of marsh vegetation in estuarine habitats by influencing water quality (i.e. temperature, salinity, nutrient load, turbidity, dissolved gases, and mineral concentrations) downstream. Increases in surface water withdrawals are anticipated in Georgia as groundwater use becomes more restricted, and this may in turn cause changes in habitat structure and function in estuaries due to increasing salinity intrusion into freshwater and brackish habitats. The current challenge is to develop policies that can be used to allocate available groundwater and surface water resources. These policies must be designed to maintain adequate freshwater inflows to the coastal environment so as to protect estuarine communities and their environmental functions. Figure 1. Sampling locations in the Altamaha River estuary, Ga., October 2002. A lta m ah a So un d/ A tla nt ic O ce an Salinity is often considered a key predictor for the distribution of habitats and organisms along an estuary. Temperate riverine estuaries are characterized by distinct vegetation along the salinity gradient, with salt marsh vegetation in the polyhaline range of the system (salinities greater than 18 (practical salinity units, psu), brackish marsh plants in the oligoand mesohaline ranges (between 0.5 and 18 psu), and tidal freshwater plants furthest upstream (less than 0.5 psu) (Odum, 1988). A crucial component of evaluating if there are adequate freshwater inflows into estuarine environments is to link biological indicators with actual salinity levels along an estuarine gradient. This biological connection may then be used as a “benchmark” in the determination of how much water flows can be reduced upstream without causing unacceptable salinity changes within the estuarine environment downstream. Previous research in the Suwannee estuary in Florida successfully identified a relationship between the relative abundances of freshwater and salt tolerant vegetation and the maximum salinity that vegetation experienced (Clewell et al., 1999). The shift in relative abundance from the more salt tolerant rush, J. roemarianus, to the less salt tolerant sedge, C. jamaicense, occurred between 5 and 10 psu in the Suwannee estuary. The Altamaha River estuary is considered a piedmont estuary with an extensive watershed and substantial freshwater discharge (Dame et al., 2000). The tidal marshes of the Altamaha River estuary consist of salt marsh communities (predominantly S. alterniflora with some Distichlis spicata) found up to 6 km from the mouth, brackish marsh communities (predominantly S. cynosuroides with S. alterniflora near the creek bank) between 6 and 16 km upriver (although there are patches further downstream near 3 km), and freshwater marsh communities (wild rices, Zizania and Zizaniopsis) upstream of 16 km (Smith et al., submitted). Although salinity explains the general distribution of tidal marsh vegetation, there is also a more dynamic, local scale variation that results in changing border positions between vegetation types (Smith et al., submitted). The research presented in this paper focuses on describing the distribution and plant characteristics of two Spartina species (S. alterniflora and S. cynosuroides) in relation to the salinity gradient of the Altamaha River estuary. A vegetation survey was completed to evaluate connections between the distribution of plants and their location along the length of the estuary and to assess whether there is a distinct transition point where salt tolerant S. alterniflora communities shift to freshwater/brackish S. cynosuroides communities that could serve as a benchmark for freshwater inflow assessment in this system.