Abstract
Sea-level rise (SLR) rates have accelerated dramatically in the past century, raising concerns about the ability of the natural coastal systems in regions such as Southwest Florida to continue offering a protective barrier. The coastal geomorphology of Southwest Florida is driven largely by reef-building vermetiform gastropods (Vermetus nigricans and Vermicufaria knorri) and American oysters (Crassostrea virginica), all of which provide a foundation for the estuarine islands along the coast, and which rely on relatively slow rates of S LR. In order to predict the response of these organisms to changing environmental conditions in the future, it is important to understand the driving mechanisms which have led to their success in the past. This study identifies the timing of reef formation and the alteration of paleoenvironmental conditions through time in Southwest Florida by considering stratigraphic, paleontologic, taphonomic, and isotopic evidence from oyster and vermetiform gastropod reefs in Estero Bay and the Ten Thousand Islands. The primary objectives were to approximate the onset of estuarine conditions in these two locations, to determine if the reef-building organisms in these areas exhibit autogenic or allogenic ecological succession through the late Holocene, and to investigate the role of reef building Crassostrea virginica and vermetifom1 gastropods in the development and maintenance of estuarine environments. Two sediment packages were described: first was a transgressive systems tract which consisted of a transgressive basal peat that began to form approximately 6,000 ybp, to a subtidal marine facies; second was a regressive high stand systems tract which progressed from a vermetiform gastropod reef facies beginning at approximately 2,700 ybp, to an oyster-dominated reef facies beginning at approximately 500 ybp. The shift in dominant reef-building organisms from stenohaline vermetifom1 gastropods to euryhaline oysters was supported by a concomitant shift in the associated molluscan fauna. A trend toward more brackish water conditions over the period of reef development was further demonstrated by more negative mean 813C values up-core in both locations and more negative mean 8180 values up-core in Estero Bay. Increased seasonal variation was also apparent for Estero Bay based on the high and low 8180 and 813C values. Sedimentary evidence for Estero Bay illustrates a fining upward trend in dominant grain size with poorer sorting through time, demonstrating that reef development was coupled with a transition to an environment with more variable energy conditions. The data compiled during this study suggest that autogenic succession occurred within the reefs from both study locations. However, radiocarbon dates acquired from a barrier island bordering Estero Bay to the west suggest that these structures likely played a significant role in the transition to estuarine conditions at this location, providing an allogenic mechanism for environmental change. The combination of approaches used in this study provides compelling evidence that reef-building organisms are critical players in the successful development of coastal estuarine environments in Southwest Florida. Further, it can be surmised that the maintenance of these coastal systems is intimately linked to the health and perpetuity of these organisms, particularly in light of increasing sea-level rise rates.