CHANGES IN LATE HOLOCENE COASTAL PALEOENVIRONMENTS IN SOUTHWEST FLORIDA AS REFLECTED IN REEF-ASSOCIATED MACROFAUNA

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 the two vermetiform gastropods, the species Vermetus nigricans and Vermicularia knorri, both of which produce evolute conchs that resemble serpulid worm tubes. The coastal geomorphic evolution of this system depends critically on the development of these reefs.

During the initial Late Holocene transgressive phase, the reef-associated fauna was a mixture of species with different salinity tolerances. As the transgression continued, the fauna became characteristic of the open ocean shoreface rimming the Gulf of Mexico. Vermetiform gastropods appear in the reefal structures after the fauna had stabilized and during a time of rapid sea level rise replacing, in several cases, the oysters that had initiated reef growth. However, during a time of slower sea level rise, oysters replaced the gastropods within the reefal structure. During the time of active reef building, biological activity caused a local regression. An understanding of the ecological conditions which affect the growth and development of the molluscan reef-builders will help predict the stability of the region’s coastal geomorphology in the face of changing ecological conditions.