Sea level has been rising globally since the end of the last ice age, 12,000 years before present (ybp), though not at a consistent rate. At the onset of the late Holocene, approximately 5000 ybp, sea-level rise rate decelerated considerably. During times of modest sea-level rise, coastal regions may begin to build seaward or prograde despite the rising seas. It is during the late Holocene, consequently, that our present-day estuaries developed through coastal progradation and when Southwest and South Florida developed its outer barrier islands and estuarine interior mangrove-forested islands. Progradation in Southwest Florida occurs through the development of oyster reefs at intertidal depths; mangrove propagules will then settle on a reef and eventually transform into a mangrove-forested island. As long as the rate of reef and mangrove sedimentation exceeds the rate of sea-level rise, the coast continues to build seaward.

Though sea-level rise rates have been measured from 5-10 cm / 100 years during the previous 3500 years throughout our coastal region (for Estero Bay, Henderson Bay, and the Ten Thousand Islands), the rate has accelerated by an order of magnitude since the beginning of the industrial revolution, 150 ybp. There has been much debate as to whether this recent acceleration is due to natural perturbations or to human-induced global warming. Regardless of the cause, if the rate persists over a centennial time scale, our coastal system, as well as estuaries worldwide, will degrade or at least drastically change in character. For Southwest Florida, this means a fundamental shift in our coastal and freshwater wetland distribution and ecology and major impacts to human development.

The study of sea-level rise and coastal response requires field work and sediment coring. Sediment cores have been taken using a vibracore device from 4 subregions in Southwest Florida; from north to south, the estuarine systems are: the Caloosahatchee, Estero, Rookery Bay, and the Ten Thousand Islands. Cores of the late Holocene sedimentary package, which sits unconformably on top of either Pliocene or Pleistocene bedrock, may reach 5 meters in length. Cores are split, and facies (packages of sediment that possess unifying characteristics that are indicative of particular paleoenvironments) are described based on the sedimentologic, paleontologic, and taphonomic characteristics.

This research program attempts to accomplish 3 objectives. First, how have variable sea-level rise rates affected Southwest Florida’s coastal geomorphology during the late Holocene? It appears that the inner coastal margin, i.e., the mangrove-forested fringe that lines the eastern interior of the bays, has remained static for 3000-3500 years. Yet during this time interval, the mangrove-forested islands developed within the interior of the bays. Second, how has the development of oyster reefs influenced coastal morphology? Oyster reef development appears to be critical to coastal evolution. Reefs can have fairly lengthy histories, persisting for up to 3500 years. In many reefs, a community succession appears with early reef history dominated by vermetiform gastropods (worm-like sessile snails belonging to the families Turritellidae and Vermetidae), followed by oyster-dominated assemblages. Modern vermetiform gastropods require higher salinities and can withstand greater environmental energy than the American oyster, Crassostrea virginica. This suggests that vermetiform reefs may have formed an initial, more Gulf-exposed barrier that eventually created the more protected estuary within which oysters later flourished. Third, can these late Holocene results help predict coastal response in the future? Present-day sea-level rise rates are comparable to those Southwest Florida experienced prior to 4500 ybp. Oyster and vermetiform reefs are not well developed in this time frame. There are occurrences of oyster reefs, but they are short-lived and not very extensive. This suggests that if accelerated rates persist, oyster reef development will decline. The Southwest coast is exhibiting signs of incipient change. These include: a set back of the previously stable inner bay margin; the transgression of mangrove forest and concomitant consumption of freshwater and brackish water prairies; and a transgressive shift of the loci of most prolific oyster reef development.