A window of opportunity to build coastal resilience: how to optimize coral restoration to reduce impacts of sea-level rise

By Coastal and Marine Hazards and Resources Program May 1, 2023

Restoration efforts are being implemented on coral reefs around the globe, and science is needed to ensure their success. A new U.S. Geological Survey led study in Buck Island Reef National Monument shows what it would take for coral restoration to increase reef growth enough for them to keep pace with projected sea-level rise and mitigate future storm-driven flooding on reef-lined coasts.

Sound Waves Newsletter - May-June 2023

Coral reefs serve as natural infrastructure that protect our Nation’s shorelines from coastal hazards such as erosion and flooding. The ability of reefs to continue providing these benefits into the future depends on their capacity to grow and keep pace with sea-level rise. To revitalize reef growth rates and combat their recent declines, numerous organizations have turned to coral restoration. An increasing number of these programs are aspiring to scale-up their efforts to a level that would improve not just the amount of living coral, but also important reef functions like coastal protection.

Map of a reef-lined island. Many red dots show reef erosion hotspots, a few blue dots indicate areas of net reef growth — Buck Island Reef National Monument (BIRNM) is a culturally and ecologically important location in St. Croix, U.S. Virgin Islands, that, like many coral reefs in the western Atlantic, has begun experiencing net reef erosion rather than reef growth in recent decades. The island’s beaches provide habitat for several threatened and endangered species like sea turtles and the St. Croix Ground Lizard and serve as nesting habitat for migratory shorebirds. Its reef provides protection for these shores and in the words of President John F. Kennedy is, “one of the finest marine gardens in the Caribbean Sea.” In addition to its ecological functions, the park’s rich history was also instrumental in the decision to make it the first marine protected area in the United States.

Research conducted by the USGS is helping to guide these efforts and provide specific objectives that will be most effective at reducing the effects of coastal hazards. The bottom line—it will take aggressive action in the near-term to combat the effects of low to moderate levels of sea-level rise expected by the end of the century. It’s possible, but with ongoing impacts like coral disease and ocean warming the window of opportunity may be closing.

Led by Dr. Lauren Toth, this paper in Nature Communications is the first study to integrate estimates of coral-reef growth (using a method called carbonate budget modeling) with wave and water level modeling to define restoration targets for reducing the risks of sea-level rise and storms on coral-reef-lined coasts. USGS and partners used those models to estimate end-of-century coastal-flooding impacts with and without restoration at Buck Island Reef National Monument (BIRNM).

USGS research has shown that focusing coral-restoration efforts on reef-building corals such as Acropora palmata, the elkhorn coral, may help revive important processes like reef growth. This fast-growing, branching coral was historically dominant in the shallowest areas of Caribbean reefs known as the reef-crest zone. When coral cover is high enough, elkhorn colonies are effective at helping break waves and dissipating wave energy away from the coastline.

The new study by Toth and colleagues provides important guidance on the extent of short-term restoration efforts that are needed to have a sustained impact on coastal protection. The authors found that most of Buck Island’s reefs are currently eroding away, therefore decreasing their ability to help protect the coast. The research also shows that aggressive coral restoration of A. palmata (planting between 1-3 individual colonies per square meter of reef by 2030) could, if successful, promote increases in coral cover and reef growth that might help mitigate the worst effects of sea-level rise on coastal flooding by the end of the century. If restoration efforts can increase coverage of A. palmata to ~36% cover on BIRNM reefs (current status: 7% cover), the resultant growth rates could be high enough for the reef to keep pace with moderate projections of end-century sea-level rise (~0.5 m) and decrease the potential flooding associated with major hurricanes (Category-5) to levels projected for weaker tropical storms. Alternatively, if no restoration efforts are made, the reef will likely continue to erode by as much as 0.45 meters by the year 2100, and impacts of sea-level rise and future storm impacts will worsen.

The bottom line: while coral restoration could help minimize the impacts of climate change on coastal hazards, allowing reef erosion to continue unchecked could amplify them.

plot shows how more aggressive restoration of A. palmata coral can help keep pace with low to intermediate sea level rise — If restoration efforts can increase coverage of *Acropora palmata* by ~30% cover at Buck Island Reef National Monument, resulting reef growth rates could be high enough for the reef to keep pace with moderate projections of sea-level rise (~0.5 m) by the end of the century. This reef recovery would decrease the potential flooding associated with major hurricanes (Category-5) to levels projected for more minor tropical storms.

Climate change is predicted to not only increase sea levels and cause stronger tropical storms but is also expected to continue causing declines in coral populations. This study acknowledges that mitigation of climate change may be essential for coral restoration to be effective enough to protect coasts. While coral restoration is often considered a stop-gap solution until larger climate change challenges can be addressed, this study suggests instead how restoration can be incorporated into the portfolio of solutions to reverse coral-reef decline and preserve the benefits reefs provide for coastal resilience.

This case study can not only be used to inform restoration targets within BIRNM to help the island adapt to rising sea levels but is also useful for informing restoration strategies worldwide. Though every reef is unique, aggressive action now could help combat the effects of sea-level rise on reef-lined coasts into the future. This key message offers hope that coral restoration could truly help build reefs back better.

Read, "The potential for coral reef restoration to mitigate coastal flooding as sea levels rise" in Nature Communications.

The team included Drs. Lauren Toth, Curt Storlazzi, Ilsa Kuffner, and Anastasios Stathakopoulos from the U.S. Geological Survey along with several partners at the National Park Service, Deltares, IHE Delft Institute for Water Education, and the Florida Institute of Technology.

This research was supported by an award to Ilsa Kuffner and Curt Storlazzi from the U.S. National Park Service's Natural Resource Stewardship and Science program, titled “Mitigate Coral Reef Degradation at Buck Island Reef NM” (PMIS 229486) and funding from the U.S. Geological Survey through the Coastal and Marine Hazards and Resources Program.