For decades, coral-reef science has been dominated by tales of doom-and-gloom. The effects of climate change and other human impacts have caused unprecedented declines in coral populations that have left the world’s reefs in a highly degraded state. Our study site at Buck Island Reef National Monument, St. Croix, in the U.S. Virgin Islands, is unfortunately no exception to this trend. Once considered “one of the finest marine gardens in the Caribbean Sea” (by U.S. President John F. Kennedy in 1961), our reef surveys in 2016 showed that coral losses at Buck Island over the last half a century have been so extreme that the reef is now eroding away. 

How it started vs. how its going. Like most reefs around the Caribbean, Buck Island’s reef was once dominated by large thickets of the elkhorn coral, Acropora palmata (left). Now most of the A. palmata at Buck Island has died and the structures created by those corals are starting to erode (right). Source: Public domain (left-National Park Service; right-https://www.usgs.gov/media/images/dead-elkhorn-coral-buck-island-reef-national-monument-st-croix-us-virgin-islands).

We measured the balance between the growth (or “accretion”) and erosion of Buck Island’s reefs using an approach known as carbonate-budget modeling. These models provide estimates of a reef’s accretion potential by combining information about the abundance and growth of corals and calcifying algae that build reef structures, while also accounting for the effects of parrotfish, urchins, sponges, and microbes that break those structures down through bioerosion. It’s important to note here that carbonate budgets are a simplified representation of the very complex process of reef accretion, so they are never fully balanced (hence the word “potential”), but they nonetheless provide a powerful tool for generating real-time estimates of the geologic-scale process of reef accretion.

Click to play video

Source: Public domain (https://www.usgs.gov/media/videos/carbonate-budget-animation).

Over hundreds to thousands of years, reefs form a complex, three-dimensional architecture, which serves as the literal foundation for a variety of important ecosystem services. In addition to providing habitat that supports biodiversity and fisheries, reefs can act as underwater barriers that buffer wave energy and reduce coastal flooding and erosion during storms. The persistence of that essential function in the future, will depend on whether reefs can continue to grow on pace with the 0.2‒2 meters of sea-level rise predicted by the end of the century.   

Our carbonate-budgets showed that at Buck Island, erosion, rather than accretion, is now the dominant process on most (91%) of the reef. Our wave and water level models showed that by increasing water depth and decreasing wave breaking over the reef, reef erosion will amplify the already significant coastal flooding and erosion of Buck Island during storms predicted by 2100. With more and more reefs around the world losing their capacity for continued reef accretion, these results portend a dire outlook for the resilience of the world’s reef-lined coasts.  

Buck Island’s reef is eroding. Summary of the results of our carbonate-budget models, which indicate that most of the reef at Buck Island is eroding, rather than growing. Only the shallow reef-crest habitat in the south, where Acropora palmata is still present, still has a positive carbonate budget. Source: Public domain (https://www.usgs.gov/programs/cmhrp/news/a-window-opportunity-build-coastal-resilience-how-optimize-coral-restoration).

Most of the time, that gloomy outlook is where the story ends, but coral restoration is starting to change the narrative for some of the world’s reefs. Instead of simply continuing to document declines, coral restoration presents an opportunity for scientists to take a more active role in determining what the future of coral reefs could look like. In recent years, not only have coral-restoration efforts increased in scale, but the goals of restoration programs have expanded from simply supplementing coral populations to improving essential reef functions like coastal protection.

Wave breaking over the reef crest at Buck Island. Although corals can’t stop sea-level rise, Acropora palmata is an ecosystem engineer, that when present in large enough numbers, can increase the vertical growth of the reef crest to help the reef keep pace with rising water levels. Healthy reef crests covered in A. palmata coral can break waves on the surface and help reduce wave-driven flooding events on nearby shorelines. Source: Public domain (https://www.usgs.gov/media/images/wave-breaks-over-degraded-reef-crest-buck-island-st-croix).  

Ours is the first study to really dig into what it could take for that aspiration to be realized. We focused on the potential effects of restoring the elkhorn coral, Acropora palmata, because it was the dominant shallow-water species in the Caribbean historically, and is, therefore, the coral that can contribute most to coastal protection. We started by creating a simple population model for A. palmata, based on our measurements of its rapid growth rates at Buck Island, survival rates documented by other researchers, and a range of the number of restored A. palmata colonies (“outplants”) based on real-world efforts by western Atlantic restoration practitioners. We also considered the possibility of things getting better or worse for A. palmata populations over time, by varying their survival rate in the model. We then input the range of outcomes from that model into our carbonate-budget models to see how restoration could change the ability of reefs to keep pace with sea-level rise and maximize their coastal protection function.

A new beginning with coral restoration. This photo from Buck Island Reef National monument showing small colony of Acropora palmata in the foreground with a large, relict colony in the background. Source: Public domain (https://www.usgs.gov/media/images/elkhorn-coral-colonies).

Our models showed that if A. palmata cover at Buck Island could be increased by 25% or more to 2100 (to at least 36% total cover), the reef crest could keep pace with the ~0.5 m of sea-level rise that is predicted for the end of the century with moderate-to-aggressive carbon emissions reductions. Sea-level rise would still increase the flooding potential of Buck Island, but this outcome would allow the reef to continue to break waves during storms and, therefore, prevent the worst outcomes during future storms. In fact, we showed that successful restoration could reduce the potential flooding associated with major hurricanes (Category-5) to levels projected for weaker tropical storms.

How coral restoration could help Buck Island’s reef keep pace with 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. Source: Public domain (https://www.usgs.gov/programs/cmhrp/news/a-window-opportunity-build-coastal-resilience-how-optimize-coral-restoration).

What would it take for restoration to be most successful at preventing coastal flooding? It is true that increasing A. palmata cover by 25% or more would be no small task. Our model suggested that one way to reach that goal would be to outplant 500,000 A. palmata colonies (~3 per square meter) by 2030. Even then, the success of that effort would require some increase (≥5% per decade) in the survival of A. palmata over time: either through direct climate change mitigation, increased acclimatization or adaptation, or by jump-starting sexual reproduction. While this, optimistic, best-case scenario may be challenging to achieve, it’s important to emphasize that any increase in reef-accretion potential through restoration would have the positive impact of reducing erosion and coastal flooding potential. Put another way, while coral restoration could help minimize the impacts of climate change on coastal hazards, doing nothing and allowing erosion to continue unchecked will amplify them.

I have spent my career telling stories about the decline of coral reefs and, frankly, I’ve grown weary of the doom-and-gloom. It was so refreshing and uplifting for me to be able to share this study that has the possibility of a happier ending. I would be remiss if I didn’t mention that this research was truly a collaborative effort that would not have been possible without the help of our inter-disciplinary science team and the support of our partners at the National Park Service. I know that coral restoration isn’t the panacea for coral reefs, especially as climate change continues to turn up the dial on global temperatures, but I believe that it could be an important part of the solution. Overall, the potential impact of coral restoration gives me hope that the story of coral reefs like the one we studied at Buck Island is not yet fully written.