GEOMORPHOLOGICAL RESPONSE TO LATE HOLOCENE SEA-LEVEL RISE IN COASTAL SOUTHWEST FLORIDA

The Southwest Florida coast has prograded seaward during the last 3500 years despite rising sea level. Regression occurs as oyster reefs grow into intertidal depths, thereby permitting the recruitment of mangrove propagules and the development of inter- to supratidal wetlands. Coastal response is an interplay between sea-level rise (SLR) rate and rate of sedimentation associated with oyster reefs and mangrove forests. It is hypothesized that once SLR rates exceed 15-20 cm / 100 years, this system of reef to mangrove progradation will degrade. SLR rates have clearly exceeded these critical values since the beginning of the Industrial Revolution, yet no obvious shift in reef development has occurred. Our work, principally within estuarine environments of Estero Bay and the Ten Thousand Islands, employing standard practices of stratigraphic, sedimentologic, and paleontologic analysis of vibracored sections, demonstrates that coastal progradation is dependent upon low rates of SLR and that a subtle, but clear coastal response to accelerated rates is evident.

Extensive vermetiform gastropod and oyster reef development began approximately 2700 ybp during intervals where SLR rates ranged between 5-10 cm / 100 y. These reefs typically exhibit a paleoenvironmental and faunal succession indicative of SLR and a progressive development of estuarine conditions. Oyster accretion rates are slightly higher than vermetiform gastropods with the former less than 18 cm and the latter less than 12 cm / 100 y. An earlier, but failed interval of reef development occurred between 4500-5500 ybp during times of SLR that exceeded rates of 15 cm / 100 y, suggesting that this is close to the rate required to sustain reef growth. Although recent reef degradation is not obvious, there are a number of indicators of geomorphic change. First, the estuarine region of greatest oyster productivity has moved up into the lower rivers. Second, the inner margins of the inner bays are stepping landward, despite their persistence for the previous 3000 years. Third, intertidal wetlands and tidal ponds are consuming brackish and freshwater grass prairies, indicating that the brackish water ecotone is transgressing within the coastal mangrove forests.