The blistering summer heat wave that hammered South Florida’s coral reefs last year wiped out the last wild stands of its iconic elkhorn coral in the Lower Keys, the National Oceanic and Atmospheric Administration announced this week.
The deaths amounted to a 77% loss in genetic diversity needed to help sustain a vanishing coral that once blanketed Florida reefs.
That’s left scientists who have been working for decades to restore the antler-shaped coral struggling with what to do next.
“The community wasn't expecting this kind of off-a-cliff catastrophe,” said Ilsa Kuffner, a research biologist with the U.S. Geological Survey. “There were already declines that were pretty pronounced in the last 20 years. So this was a big ratcheting down of that.”
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When the heat wave hit, scientists were already struggling to keep up with efforts to grow and plant coral on an ailing reef under attack from a new disease that cropped up in 2014 near Miami. That disease had intensified restoration efforts and focused work on breeding a new resilient coral that could both tolerate heat and disease. The unexpected heat wave was like a fire alarm.
By July, prolonged heat that began rising three months earlier was wilting coral, causing them to begin expelling their life-sustaining algae, turn white and die.
A race to save the coral
Across the Keys, scientists and divers raced to retrieve coral from offshore nurseries and tend to wild coral on the reef, often showing up too late. In the Dry Tortugas, Kuffner and her team mounted screens to shade elkhorn planted in a small nursery in 2018 in an effort to revive the population. They also installed lights to draw more plankton at night to fee the coral. Ultimately, the elkhorn died.
“We weren't expecting this kind of temperature extreme this soon,” she said. In a 2015 study, USGS scientists warned annual bleaching could occur in 2030, which at the time was an urgent warning to speed up work.
While scientists have worried that time was running out, they also hoped they could make advances in breeding more resilient coral. Scientists can now coax coral in labs into ingesting more heat tolerant microscopic algae, the nutrient partners they need to survive. They also mastered tricking coral into spawning in labs, so that they can target reproduction work on coral they know can withstand hotter water or better fight disease.
Earlier this summer, University of Miami Rosenstiel scientists retrieved about a half dozen colonies of elkhorn from Honduras, where they grow in hotter water, to crossbreed with Florida coral. Some of the elkhorn spawned at the Florida aquarium over the summer. The next step is to produce more resilient Florida babies. And the Keys national marine sanctuary is in the midst of a 20-year $100 million restoration effort.
But work on the reef, and the facilities needed to breed coral, have been increasingly outpaced by warming waters. Over the summer, as they raced to rescue coral, scientists complained they were running out of space in labs.
The heat wave, Kuffner said, was a reminder of how perilous conditions have become.
“I think it's easy for people to look at the global trends in ocean surface temperature and assume that it's going to be this linear gradual increase,” Kuffner said. “But that's not what drives extinction events. It's these episodic disturbances, these crazy, all of a sudden, oh my gosh, it's 90-plus degrees Fahrenheit on the reef.”
And that’s moving elkhorn closer to the brink.
“Reefs don't respond to mean temperature,” she said. “They respond to the maxes and the minimums.”
Heat thwarts Endangered Species listing and a rescue plan
Elkhorn, along with staghorn, were the first species to be protected under the Endangered Species Act when they were listed as threatened in 2006. On the reef, the branching coral grow at the top, making them a powerful weapon in fighting storm surge but also vulnerable to higher temperatures. The two corals were also the first to have climate change included in the risks threatening their survival.
Because of that listing, federal wildlife managers worked on a rescue plan. That included the efforts in the Dry Tortugas, a necklace of islands about 70 miles west of Key West. In 2018, researchers transplanted elkhorn from the Upper Keys on cinder blocks that represented five different genetic lines, Kuffner said.
“They were flourishing,” she said, reaching two feet in diameter. “Probably of reproductive size.”
But the heat wave drove temperatures near the elkhorn nursery into the 90s, she said. It was the highest recorded since the team began recording at the site in 2009.
Before the heat wave, NOAA scientists had identified at least 160 genetically unique wild elkhorn across Florida’s reefs. Afterwards, NOAA said in a statement, they could locate only 37 different genotypes on 16 reefs in the Upper Keys and north to Miami-Dade and Broward counties. That means the majority of genetic diversity now exists in labs — where rescued elkhorn represent nearly 200 different genotypes — and leaving wild elkhorn to endure with an ever more shallow gene pool.
Populations are also now so spread out, NOAA officials said, that natural reproduction — where corals release eggs and sperm into the water to spawn — is nearly impossible.
“Basically we’re starting from scratch,” Kuffner said. “So the more diversity the better.”
NOAA is asking anyone who sees elkhorn to report it. While scientists have documented long-standing colonies of elkhorn, they want to be sure they haven’t overlooked any. Details and photos can be emailed to NOAA reef recovery coordinator Jennifer Moore at Jennifer.Moore@noaa.gov.
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