Reef-Building Threatened by Ecosystem Decline: A Case Study from Buck Island, U.S. Virgin Islands
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Coral reefs are critical barriers to coastal erosion and flooding throughout tropical regions, but their role in coastal protection is threatened by declining coral populations and sea-level rise. Climate change is causing widespread losses of live-coral cover on Caribbean reefs, and is reducing the capacity of reef-building to keep pace with sea-level rise. Reefs that lag behind rising sea level will struggle to maintain their role as protective barriers, as rising water-levels over shallow reef-crests result in a reduction of the dissipation of wave energy. A clear understanding of reef-building processes in the Caribbean will be a critical component in regional analyses of coastal hazards under sea-level rise. The objective of this study was to investigate the spatial variability in contemporary reef-building processes on a bank-barrier reef in Buck Island National Monument, a marine protected area in the U.S. Virgin Islands. Reef-building processes were evaluated using a census-based carbonate-budget model, which estimates rates of contemporary geomorphic change based on ecological data. The model reveals that in 8 of the 9 shallow-water reef habitats studied, reef framework is being eroded at a faster rate than new framework is being produced. Reef habitats that grew at an average rate of +3.1 mm yr-1 in the late Holocene are now in a state of net erosion, with reef surfaces poised to lose 8–25 cm of elevation by the year 2100. The contemporary state of carbonate production on Buck Island reefs is a result of the relationship between biological erosion and declining rates of carbonate production, due to recent coral mortality. The erosion of the reef framework will amplify the rate of relative sea-level rise over shallow-reef crests surrounding Buck Island, which will have negative consequences for the long-term viability of the ecosystem services that the reefs provide. The results of this case study indicate that recent ecological changes will have long-term geomorphic consequences for reefs across the Caribbean region.