After a searing ocean heat wave in 2023, two of the most historically important coral species in Florida are functionally extinct from the state’s reef, scientists have found.
Elkhorn and staghorn coral, known for their branching limestone arms that create incomparable reef habitat, have all but vanished everywhere except the northernmost sections of the reef, according to research published Thursday in the journal Science. Those that remain are so diminished that they are not able to contribute to the ecosystem in a meaningful way, the authors said.
The findings are a bracing example of the toll that climate change is wreaking on the world’s coral reefs, which support an estimated quarter of marine species, including fish that provide crucial protein and income in coastal communities. Reefs also protect coasts from storms, and elkhorn coral’s tall branches are especially important in breaking the energy of waves.
“Scientists have been warning of this for quite literally decades,” said Andrew Baker, a professor of marine biology at the University of Miami and one of the authors of the study. “The surprise is that it happened so fast. And it wasn’t more deaths by a thousand cuts. It was a sudden, final kind of guillotine.”
That guillotine was a record-breaking marine heat wave that hit Florida in 2023. At the time, conservationists raced to pull corals out of the water and house them in tanks in a desperate effort to save some genetic diversity.
It was the ninth known mass bleaching to hit Florida’s coral reef; the first occurred in 1987. When corals are stressed, they lose the algae they rely on for survival and will die unless conditions improve quickly. In 2023, corals there experienced heat stress that was two to four times greater than any year on record. Temperatures were so high that some died even before bleaching, their tissue sloughing off.
One of the most biodiverse ecosystems on the planet, coral reefs are sometimes called rainforests of the sea. But they are sensitive to changes in temperature and have limited ability to migrate. As humans raise the planet’s temperature by burning fossil fuels and destroying forests, the world’s oceans have absorbed some 90 percent of the additional heat.
Elkhorn and staghorn coral, which are native to southern Florida, Mexico and the Caribbean, were already classified as critically endangered by the International Union for Conservation of Nature’s Red List. Since the 1980s, they have been devasted by marine heat and disease, along with other stressors like pollution and overfishing.
On Florida’s coral reef, the incidence of elkhorn and staghorn had declined by 90 percent or more before 2023, said Ross Cunning, a coral biologist at the Shedd Aquarium in Chicago and one of the study’s lead authors.
But the corals could still be found throughout the reef, bolstered by the work of restoration groups that grew and planted them.
However, in the Florida Keys and Dry Tortugas, the 2023 heat wave killed off 98 to 100 percent of the elkhorn and staghorn that remained, the study found. Farther north, off southeast Florida, the water temperatures were not as high and the corals fared better; 38 percent of elkhorn and staghorn died.
“The loss of the staghorn and elkhorn is so significant because those species have been the dominant reef builders on Florida’s reefs for the last 10,000 years,” Dr. Cunning said. “They have the fastest growth rates of Caribbean corals and therefore they grow and build three-dimensional structure more rapidly.”
Other kinds of coral, like brain and boulder coral, are still prevalent in Florida’s reef, and the researchers are currently analyzing the heat wave’s effect on those species. Early results appear to indicate that, while up to 90 percent of them bleached severely in 2023, they seem to have recovered relatively well, Dr. Cunning said.
Because those species build limestone mounds as opposed to branches, he compared Florida’s reef to a forest that has lost its tall trees.
The study published on Thursday had 47 authors and was based on diver surveys at more than 52,000 colonies of staghorn and elkhorn coral across 391 sites.
Dr. Cunning described what he saw on a diving expedition in September 2023 as “carnage.”
“We all kind of went into shock a little bit, just to cope with what we were seeing,” he said.
Large thickets of staghorn coral, healthy just three months earlier on a previous trip, were dead or dying. All the team could do, he said, was take pictures and collect data so that others would know what was happening.
Mark Hixon, a professor of marine biology at the University of Hawaii at Manoa who was not involved with the research, said that the study was by well-respected coral scientists and “forthrightly” reported the functional extinction of elkhorn and staghorn corals in Florida.
“Unfortunately for all of us, this disaster was expected after ocean temperatures surged in 2023, yet another tragic example of nations failing to address climate disruption,” Dr. Hixon said. “I personally witnessed the death of my favorite coral reef in the Bahamas during the first global coral bleaching event back in 1998. It was weird shedding tears into my face mask.”
The planet is currently in its fourth global bleaching event, which is the most widespread on record.
This month, a group of 160 scientists asserted that extensive coral reef die-offs meant that the planet had crossed its first climate-related tipping point, “threatening the livelihoods of hundreds of millions of people.”
Scientists expect some coral species to fare better than others.
Coral restoration groups continue to grow and transplant elkhorn and staghorn to Florida’s reef, and efforts are underway to help the species withstand higher temperatures.
For example, scientists have crossed Florida elkhorn with a variety from Honduras that thrives in higher temperatures. In July, for the first time, these hybrid “Flonduran corals” were affixed onto the Florida reef. It’s early days, and scaling such efforts would require tremendous resources. So far, despite more high ocean temperatures this summer, they look healthy.
But even if such interventions succeed, they alone will not be enough to save reefs, scientists emphasized.
“If we don’t stop runaway climate change, no amount of intervention and restoration is going to work,” Dr. Cunning said.
Catrin Einhorn covers biodiversity, climate and the environment for The Times.
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