HISTORIC DISTRIBUTION OF REEF-BUILDING ORGANISMS AND THEIR CONTROL ON COASTAL GEOMORPHOLOGY IN THE EVERGLADES

A distinct change in coastal geomorphology exists in Southwest Florida between the Ten Thousand Islands (TTI) and the Everglades Estuarine Tract (EET). It has been documented that the geomorphology of the TTI is the result of vermetiform gastropods and the American oyster, Crassostrea virginica, producing loci of increased sedimentation to form intertidal reef structures that provide substrate for red mangrove settlement. This succession produces mangrove islands typical of the TTI, with a morphology that mimics that of the relict oyster reefs upon which they are founded. South of the Lopez River this geomorphology is replaced by a series of large landmasses separated by channels connecting a similar interior bay complex to the gulf, suggesting that the role of reef building in coastal geomorphic development is different within the EET.

Four core transects, each containing 4-5 cores, were completed, from north to south, from the Fakahatchee estuary (TTI), Chatham River (transition region), Lostmans River (EET), and Broad River (EET). Cores taken from Fakahatchee estuary show a stratigraphy typical of the TTI with progradational development of vermetiform gastropods and oyster reefs overlying transgressive submarine sediments. Preliminary results from the transects along both Lostmans and Broad River show no transgressive submarine sediments, but rather the persistence of mangrove environments suggesting the rate of mangrove production kept pace with sea-level rise producing this EET geomorphology. Furthermore, cores within the EET show that the geographic distribution of oyster reefs has been limited to the estuary mouths. This, coupled with the absence of living oysters on mangrove prop roots upstream of the mouths, suggests the EET morphology did not depend upon reef development. Chatham River, located mid-way between the TTI and EET, exhibits features in common with both geomorphic regions. Subsequent work will test alternative mechanisms for the formation of the EET. These results have profound implications for freshwater management and restoration of the Everglades. If our assessment is correct, reduction of freshwater input to the EET could promote ideal conditions for oyster-reef development and a very different sedimentary regime from one that produced its geomorphology.