QUANTIFYING VERTICAL ACCRETION AND ELEVATION VS. SEA-LEVEL IN SALT MARSHES ALONG THE NORTHWEST COAST OF FLORIDA

A major problem in many northeastern Gulf of Mexico coastal wetland areas has been the insufficiency of sediment supply to enable wetland elevation to keep pace with sea level rise. Mean sea level is said to be rising at a mean global rate of 1.2 mm/yr and 2-3mm/yr locally. With increasing sea level compounded by marsh subsidence, coastal wetland elevation can only be increased or sustained through sediment input and/or plant growth. The long-term health of salt marshes is determined by a multitude of factors, including: the amount of sediment and nutrient delivered by rivers, the rate of local sea-level rise, storm events, tidal range, wave energy, and subsidence of the marsh substrate. Three locations along the Florida panhandle coast were selected for this study. They include the coasts of Apalachicola, Ochlockonee, and St. Marks Rivers. These sites were selected due to their varying amounts of sediment input from river sources. Each site has been monitored for deep subsidence, shallow subsidence, accretion, sea-level rise and the influence of nutrients on elevation. In the course of this study several techniques have been utilized to quantify the processes that control the elevation of the coastal marsh environment. Deep and shallow SET (Sediment Elevation Table) devices were used to quantify elevation rates. Repeated cryogenic coring of feldspar marker horizons was used to quantify sediment accretion. Additionally, fertilized and unfertilized SET plots were used to assess the contribution of root biomass to the overall stability of the marsh with respect to sea level. Due to the current rate of sea level rise and the low level of sediment input into this region of the NW Florida coast, marsh elevation measurements show that, while most of the marshes are increasing in elevation, it is at a slower rate than sea level rise. If continued, such a situation will ultimately result in the deterioration of these coastal environments.