THE ROLE OF SEA-LEVEL RISE AND STORMS IN FORMATION AND RESILIENCE OF FLORIDA BAY ISLANDS, EVERGLADES NATIONAL PARK

Cores collected in 2014 on four islands in Florida Bay have provided information on the mid-late Holocene history of geomorphic and ecologic changes to the southern coastline of Florida. Based on high-resolution age models, stable carbon isotopes, pollen, and molluscan assemblages, the modern Florida Bay estuary was a freshwater wetland in the mid-Holocene. The shift to mangrove coastal forests began ~4.0 to 3.6 ka and to estuarine environments ~3.4 to 2.7 ka. This inundation of the coast occurred despite the relatively low rate of sea-level rise (SLR) during this time (~0.67 to 1.6 mm/yr, compared to current 2.4 to 3.7 mm/yr), which raises important questions regarding the future of Florida Bay islands and the southern Florida coast under projected SLR scenarios for the next centuries. Records from other locations suggest this was a period of increased climate variability and anomalously wet, stormy conditions. After submergence during the late Holocene, the islands began to emerge ~2.5 to 0.5 ka. Recent fieldwork following the passage of Hurricane Irma in 2017 provided additional insights into the role of storms in overstepping coastal boundaries. All studied islands contain central barren mudflats that lie below sea level, surrounded by mangrove berms at higher elevations. Hurricane Irma eroded the berms and caused significant damage to the mangroves that anchor the berms. Two islands have already been breached along their perimeters and regularly exchange water with the Bay during tidal cycles. Loss of mangroves from storms, coupled with a subsequent period of increased wave action on the berms, could cause overstepping of coastal boundaries (documented by Smith et al. 2009). These events might be associated with periods of heightened storminess and could be the mechanism for the earlier inundation of the coastline from 3.4 to 2.7 ka. Given the projected rate of SLR over the next century and the uncertainty surrounding future tropical storm intensity, the islands of Florida Bay could be submerged again within this century. These data on the role of external forcing factors in shaping Florida Bay islands and the coastline provide important information to resource managers to support decision-making on resource allocation to mitigate impacts of future change.