New USGS-led research shows despite unprecedented declines in reef-building corals in recent decades, real-world coral restoration efforts could bring coral reef growth back to historic levels—as high as they were 7,000 years ago.
Can coral restoration reverse long-term declines in coral reef growth?
The complex, three-dimensional reefs built by corals over hundreds to thousands of years provide invaluable ecosystem services to society—contributing billions of dollars per year to the global economy through shoreline protection, tourism, and habitat for biodiversity and fisheries. Growth of these reefs is increasingly threatened by climate change and other disturbances, which have caused global-scale reductions in reef growth and increases in reef erosion. New research by Dr. Lauren Toth and colleagues shows that despite these declines, and the likelihood of continuing environmental stressors, the future of some of Florida’s reefs actually looks pretty promising.
Cover photo: Concrete and stainless-steel marker photographed 17 years after being installed into the reef in 1998 by Harold Hudson, NOAA. When installed, the top of this fixture was even with the surface of the coral reef. The separation between it and the surface of the reef today demonstrates the amount of erosion that has taken place since. This photograph was taken during USGS research in the Florida Keys National Marine Sanctuary at Hen and Chickens Reef Sanctuary Preservation Area under scientific collection permit number FKNMS-2013-024-A2.
First, let’s talk budget—a carbonate budget
A carbonate budget is kind of like a financial budget—except instead of keeping track of our bank account balance we’re tracking the amount of reef material. To measure a carbonate budget, scientists measure the balance between two processes that control reef growth: carbonate production, or how much coral skeleton is produced, and bioerosion, or how much coral skeleton is eaten away by organisms living on the reef. In both cases, if the amount being added exceeds what’s being lost, we can expect to see growth—a pretty good place to be if you are a reef. However, the opposite case can be pretty dire—for both our bank accounts and for a coral reef.
The missing pieces: there are other suspects for reef erosion that are more difficult to measure. Although this study measured bioerosion, other groups at the USGS are studying broad-scale seafloor elevation change, the effects of ocean acidification, sediment movement, and biogeochemical processes that can help refine these estimates.
A history of coral reef decline: blowing the budget
Dr. Toth and her team used historic reef-survey data from the Florida Fish and Wildlife Conservation Commission and the National Oceanic and Atmospheric Administration (NOAA) to measure the carbonate budget of 46 reefs in the Florida Keys from 1996‒2019, and compared those contemporary reef growth estimates with Toth’s previous reconstruction of reef growth across the last 8,500 years. Earlier in the Holocene (7,000 years ago), coral reef growth was at its peak, but Florida’s reefs stopped growing around 3,000 years ago making them geologically senescent. The unprecedented loss of reef-building corals over the last two decades—due to climate change, extreme hot and cold events, disease outbreaks, and many other stressors—have resulted in significant, region-wide declines in carbonate production. This means coral reef growth can’t keep pace with erosion taking place in the Florida Keys (think: "blowing the budget"). In fact, the authors found that about 85% of reefs in the Keys are now eroding rather than growing. Without active restoration of reef-building corals, the persistence of key reef functions in this region may be in jeopardy.
A surprising finding of the paper was that patch reefs in the lower Keys have maintained positive carbonate budgets despite the suite of disturbances in recent decades. This is likely due to low water clarity of inshore environments helping to protect the reef from sunlight, and therefore additional heat stress—making them more resilient to temperature extremes. Similarly, recent findings by Dr. Ilsa Kuffner, Toth, and others show that these kinds of low-visibility environments may act as “oases” where corals are more likely to survive and outgrow their neighbors. These findings are important for managers to consider when making decisions about where to focus restoration efforts.
Restoration provides hope for the future
The study suggests that although the present state of Florida’s reefs may seem dire, it may still be possible for coral restoration to revive reef growth in some locations. Toth and team showed that if the specific restoration goals set by NOAA’s Mission: Iconic Reefs initiative are met, reef growth in some locations could potentially be restored to levels comparable to 7,000 years ago—the peak of reef-building in the region. Growth rates this high could even be enough for coral reefs in the Keys to keep pace with mid-range projections of future sea-level rise.
But this is only one piece of the puzzle. Local restoration efforts can only be successful if the global stressors causing reef declines are addressed. Scientists predict that without climate change mitigation strategies, coral reefs will continue to suffer, and restoration efforts may face an uphill battle.
Long story short: although catastrophic losses of reef-building corals have caused the already strapped carbonate budgets of Florida’s coral reefs to “fall into the red” of reef erosion in recent decades, there is hope for the future. This research shows that restoration really does have the potential to rebalance the budgets for Florida’s reefs, revive the process of reef growth, and maintain the key ecosystem services it supports into the future.
This paper was published in the journal Global Change Biology and was led by Dr. Lauren Toth of the U.S. Geological Survey (USGS) St. Petersburg Coastal and Marine Science Center, Travis Courtney of the University of Puerto Rico, Selena Kupfner Johnson of the USGS St. Petersburg Coastal and Marine Science Center, and Michael Colella and Robert Ruzicka of the Fish & Wildlife Research Institute, Florida Fish & Wildlife Conservation Commission.