LATE HOLOCENE VERMETIFORM GASTROPOD AND OYSTER REEF DEVELOPMENT AND THEIR RELATION TO COASTAL EVOLUTION IN SOUTHWEST FLORIDA

The coastal system of Southwest Florida is composed of an inshore mangrove forest, an intervening network of inner islands and bays, and a distal, more offshore complex of mangrove-forested islands, known as the coastal bay complex (CBC). Previous research has shown that this complex coastal geomorphology is the product of both transgressive and regressive sedimentation within the overall late Holocene transgression. Shifts between transgression and regression are controlled by changes in the overall rate of sea-level rise relative to sediment production and aggradation. The islands of the CBC are underpinned by Holocene molluscan reef structures built by the American oyster, Crassostrea virginica, and two vermetiform gastropods, Vermetus nigricans and Vermicularia knorri, which produce evolute conchs that resemble serpulid worm tubes. The coastal geomorphic evolution of this system depends critically on the development of these reefs.

Vibracore transects through both protected estuarine and open coastal reef structures in the 10,000 Islands and Estero Bay, Southwest Florida show that fossil vermetiform reefs are restricted to currently more open coastal positions. Oysters cap vermetiform reefs and dominate modern reef growth in more protected estuarine settings. These reefs contain a complex history of succession and alternation between oyster and vermetiform gastropod-dominated frameworks. Preliminary C14 dating demonstrates that vermetiform reef development initiated during a period of rapid sea-level rise prior to 3200 ybp.

Ongoing research will test the hypotheses that vermetiform gastropods produced framework structures during times of rapid sea-level rise in unprotected coastal settings, and that they created protective barrier reef environments that provided estuarine-like conditions favoring the development of oyster reefs.