EVOLUTION OF COASTAL WETLAND GEOMORPHOLOGY AND COMMUNITY COMPOSITION IN RESPONSE TO SEA-LEVEL RISE, SOUTHWEST FLORIDA

Fluctuations in sea level during the Holocene have driven changes in vegetative communities along Southwest Florida’s coastline. Predicted rates of current sea-level rise (SLR) for this region vastly exceed rates experienced over the last 5,000 years. Tidal pond formation appears to correspond with the degradation of natural wetland types as a reaction to SLR, but the specific mechanism for these transformations has not yet been described. Data on whether certain precursor habitats are preferentially degraded into tidal ponds, and whether such degradation follows a predictable sequence, are lacking. Thus, the key objectives of this research were to 1) seek and identify a mechanism of floral succession within tidal fresh and brackish water marshes in relation to accelerated SLR via field analyses, and 2) quantify the evolution in the distribution and aerial coverage of tidal ponds over time in response to SLR using GIS methodologies.

Five transects perpendicular to the tidal gradient and crossing through numerous wetlands and ecotones were established, three within the Ten Thousand Islands National Wildlife Refuge and two within the Fakahatchee Strand Preserve State Park. One to two meter sediment cores were extracted in pairs––one within a marsh community, and one within an adjacent tidal pond––at each unique vegetative environment along each transect. Facies were described based on sedimentologic, faunal, vegetative, taphonomic, and histologic characteristics, and their representative paleoenvironments interpreted. Stratigraphic analyses thus far indicate a transition from an upland environment to an intermittently flooded, short hydroperiod wetland, and finally to longer hydroperiod marshes or ponds representing the current modern community types.

ArcGIS 9.3 software was utilized to quantify surface area and distribution of tidal ponds within twelve randomly selected marsh environments within the Ten Thousand Islands via analysis of aerial imagery from 1940, 1953, 1962, 1973, 1984, 1995, and 2009. Marsh locations were uniform in size, and chosen from inspection of both 1953 and 2009 images to reduce bias. Mean tidal pond area was statistically compared between each of the time frames. Results indicate an overall increase in the aerial extent of tidal ponds across the landscape over the past 60 years.