After three thousand years of precarious stability, when reefs in the Florida Keys managed to grow as much as they deteriorated, scientists now know the only inshore reef in the continental U.S. has passed that tipping point.
In a new study, the U.S. Geological Survey found that most of the reef is now shrinking, with coral unable to fend off rising temperatures and other threats.
“It's not that erosion is actually increasing. It's just that there are fewer corals out there now that are building that structure,” said lead author and USGS research scientist Lauren Toth. “Erosion is now the dominant process.”
Over the last few decades, corals on the reef have been hammered by warmer water and ocean heat waves, disease and hurricanes. In the late 1990s, a global bleaching event slowed coral growth on about 75 percent of the reef tract. Then in 2010, a historic freeze descended on South Florida. Four years later, a stony coral disease that attacks reef-building boulder corals was discovered off Virginia Key and rapidly spread.
The estimate is likely overly conservative, the researchers said, because they did not model impacts from climate change or reef-crushing hurricanes.
If the U.S. loses its reef tract, it loses a powerful defense against more intense hurricanes fueled by climate change. A 2019 estimate put the value at nearly $2 billion, although Toth said USGS researchers are working on updating the amount.
Of the 49 reefs examined in the Keys, home to two-thirds of the tract, Toth said only a handful of inshore patch reefs seem to be hanging on, likely helped by turbulent, cloudy water that can shade corals.
The finding comes after nearly a decade of trying to understand the reef’s millenniums-long history and how it grew at various times in its ancient history, she said.
“We started out looking at these geologic time scales where we were studying the growth of reefs over thousands and thousands of years,” she said. “We're all interested in what's going to happen to reefs in the future. But we realized to answer that question, we needed the piece of time that the geologic record doesn't capture, so that recent history of reefs.”
The busiest, healthiest time for the reef occurred 7,000 years ago when the fastest growth occurred. During the Bronze Age, about 3,000 years ago, it leveled off, reaching a kind of equilibrium.
“They've kind of been balanced at this tipping point where there were still coral there, but it wasn't enough to produce a lot of new reef structure,” she said.
To calculate the rate, the researchers looked at what they call the carbonate budget.
“It works similarly to a financial budget,” Toth said. “You have a positive side and a negative side of the equation.”
Positive is obvious: the amount of coral growing. The negative erosion side is determined by the amount of reef-eroding organisms are present, including sea urchins, sponges and parrotfish. While this marine life is needed for a healthy reef, when corals grow too slowly, they can strip a reef.
"None of these eroding species are a bad thing on the reef,” she said, “as long as their eroding actions are in balance with coral growth.”
As part of the study, the team also looked at restoration efforts now underway.
Three years ago, the National Oceanic and Atmospheric Administration unveiled a 20-year plan to restore seven reefs in the Florida Keys. The first phase is expected to cost about $97 million, last five to seven years, and focus on replanting fast-growing staghorn and elkhorn coral grown in nurseries and apparently immune to stony coral disease.
When the researchers modeled the targets set by the plan with their erosion data, they found the work reversed the downward spiral.
“They would potentially grow at rates as high as we saw 7000 years ago,” Toth said.
What’s standing in the way is climate change and disease.
“Restoration, of course, is only going to be one piece of saving coral reefs. We have to address those bigger threats. We have to address climate change and all the other things that cause coral mortality,” she said. “But restoration provides us a way of starting to reverse that trend of coral loss and declining reef growth in the very near future.”
