Wetland and tidal channel evolution affecting critical habitats at Cape Sable, Everglades
National Park, Florida
Brigitte Vlaswinkel 1 and Harold Wanless
2
1
Rosenstiel School of Marine and Atmospheric Science, University of Miami, FL
2 Department of Geological Sciences, University of Miami, FL
Tidal wetlands and marshes are extremely heterogeneous environments influenced by a range of variables that act across several spatial and temporal scales. The organic/carbonate coastlines of southwest Florida include vulnerable ecosystems that provide valuable information on the nature and dynamics of coastal change in anthropogenically modified systems. In these complicated systems, subtle changes in boundary conditions (creek attributes, sea level) appear to drive large hydrodynamic and morphological changes. The purpose of this research is to 1) document historical patterns and rates of change of the various subenvironments in the Cape Sable area; 2) define the relative roles of day-to-day processes and major storms in geomorphologic and ecosystem evolution; 3) establish principles for system dynamics to be used as a protocol for the evolution of other sensitive channeled mangrove-to-freshwater wetland complexes. This project integrates analysis of aerial photography and high-resolution satellite imagery, and links sedimentologic core data to hydrodynamic measurements (current velocities, discharge and suspended sediment concentrations).
Portions of Cape Sable, a large coastal wetland complex in Everglades National Park, have undergone rapid changes over the past 80 years. Subtle human modifications, such as canals dredged in the 1920s, act in concert with changes driven by recent sea-level rise. Before the
1920s, there was no connection from a large (~ 20 km
2
) inland freshwater lake and marshes to the marine environments of Florida Bay and the Gulf of Mexico. Since the opening of the canals, the freshwater lake has transformed into an estuary. Sediments have been deposited in the estuary, filling ~ 25% of its area from 2 meters depth to supratidal levels. The rapid rate of widening of the canals (up to 4 ft/year) and natural creeks reflects a system out of equilibrium in which the channels are still seeking to accommodate the increased tidal prism.
Especially critical is the incursion of marine waters across a bounding marl ridge that used to separate the interior freshwater marsh from the marine environment. With saline water intrusion, large areas of the freshwater marsh have died out. The organic matter has oxidized and disappeared, which has resulted in the collapse of the marsh and conversion to a shallow subtidal water body. With continued relative sea level rise and saline water intrusion, the loss of freshwater marsh and collapse of the wetland into an aquatic environment is extending further into the interior of Cape Sable.
Detailed hydrodynamic studies have been carried out in the summer and winter of 2003-2004.
Preliminary results reveal that with each tidal cycle, approximately 200 x 10
3
kg of sediment is redistributed from former interior freshwater wetlands to the marine environments of the estuary.
The results illustrate the complicated connectivity of processes and products in low-energy tidal wetlands.
Contact information: Brigitte Vlaswinkel, Marine Geology and Geophysics Division, School of
Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami,
FL 33149. Phone: 305 361 4918, Fax: 305 361 4632, Email: bvlaswinkel@rsmas.miami.edu