By Daniela Ghelman, FIU News
Once a picturesque garden beneath the sea, some stretches of the Florida Coral Reef have now become graveyards. Unburied coral skeletons stand as haunting remnants of the catastrophic marine heat wave that swept through in the summer of 2023, leading to the total functional collapse of endangered elkhorn and staghorn corals.
Florida International University researchers at Mission: Iconic Reefs — a National Oceanic and Atmospheric Administration-led coral restoration initiative — documented the devastating toll. Their findings, recently published in Science, detail the dreadful aftermath: When the waters cooled, not a single coral of these species had survived.
The functional loss of elkhorn and staghorn corals strikes at the very heart of the reef. As architects of reef structures, these corals provided shelter for a variety of marine species and softened the impact of waves on Florida’s shores, helping prevent flooding and coastal erosion. Species diversity signaled the ecosystem’s function and health — and now, much of that is gone in the Florida Keys and Dry Tortugas.
“This loss makes the reef overall less diverse and less resilient,” said FIU research scientist Kathryn Lesneski, who also serves as research and monitoring coordinator at Mission: Iconic Reefs.
As one of the initiative’s research partners, FIU’s Institute of Environment supports the study of both restored and wild corals to track their health, providing scientific data to understand the impact of heat waves and guide restoration.
“To protect our coastal communities, we have historically relied on that natural reef structure,” she said. “But some corals are no longer growing, they’re breaking down. And that leads to further loss of habitat and of the natural barrier along the shoreline.”
For visitors drawn to the Keys by the promise of thriving underwater landscapes, the loss of these corals is a bitter disappointment.
“The elkhorn coral is really beautiful, it’s iconic,” Lesneski said. “People these days might expect to see the imagery that’s used in advertising the Keys. Unfortunately, the size and extent of the corals in that imagery just don’t exist anymore.”
Amid the devastation, not all is lost. Researchers are currently studying the differences among the reefs and why some colonies survived the heat wave — the hottest July-August period in more than 150 years for the Florida Coral Reef. In the lab, scientists are exploring potential interventions, such as bringing genetic material from reefs in other regions, like Honduras, or manipulating symbiotic algae — the primary source of food and energy for these corals, which also gives them their signature orange-brown glow.
“Perhaps we can culture symbiotic algae from Honduras and inoculate Florida corals with them,” Lesneski said. “Some are known to have higher heat-stress resistance. Or we can explore other ways to help the algae provide the coral host with the most thermal tolerance possible.”
For Lesneski, the 2023 heat wave is a learning opportunity.
“If we really focus our time and effort into examining the survivors, where they’re located, the habitat they’re in, the technique used to outplant them and their genotypes, we can get a complete picture of why they survived. That can help us better plan upcoming restoration efforts,” she said.
It’s not about pining for the faded corals, but, according to Lesneski, about using science to make intentional choices toward saving the reef and its fragile ecosystem.
This piece was originally published at https://news.fiu.edu/2025/catastrophic-heat-wave-wiped-out-2-endangered-corals-in-the-florida-keys-now-what. Banner photo: Bleached elkhorn corals at Horseshoe Reef, Key Largo, in July 2023 (Photo by Kathryn Lesneski).
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