Published Feb. 16
Record-warm ocean temperatures off the coast of Florida last summer stressed an already vulnerable coral reef system, prompting a massive die-off across the only living barrier reef in the continental United States.
Researchers now know it was the longest-lasting marine heat wave recorded in three decades, with average South Florida sea temperatures peaking 4.5 degrees above the norm in July.
They have said the coral bleaching was the worst in state history and they mounted successful efforts to save the corals by harvesting them from reefs.
Throughout the summer, scientists warned mortality rates would likely soar. Early data now offers a snapshot of the toll that unprecedented heat took on Florida’s reefs.
A team of researchers with the National Oceanic and Atmospheric Administration’s Mission: Iconic Reefs program surveyed 64 locations at five reefs across the Florida Keys. Here’s what they found:
- Less than 22% of approximately 1,500 staghorn coral surveyed remained alive.
- Only the two most northern reefs surveyed, Carysfort Reef and Horseshoe Reef, had any living staghorn coral.
- Live elkhorn coral was found at only three sites: Carysfort Reef, Sombrero Reef in the middle Keys, and Eastern Dry Rocks off Key West.
- No live staghorn or elkhorn corals were observed at surveyed areas at Looe Key Reef in the lower Keys.
There are a few caveats: The survey focused solely on corals that had been “outplanted” by researchers, which involves planting coral fragments grown in nurseries back onto reefs. The team did not survey any natural corals living on reefs.
Researchers also centered their attention on elkhorn and staghorn corals, which are some of the most vulnerable — and iconic — among reefs.
Part of the acropora family of corals, their distinct, branching arms provide habitat for hundreds of species. They’re particularly sensitive to heat because they have especially thin tissue that houses the algae that produces their food.
When corals are stressed by warm waters, they expel their algae and begin to starve. After enough of this stress, corals turn to a pale, “bleached” color.
About 90% of elkhorns and staghorns surveyed in August showed signs of bleaching, according to an update from the Mission: Iconic Reefs program.
Jason Spadaro, manager of Mote Marine Laboratory’s Coral Reef Restoration program, said what the team saw Wednesday wasn’t a big shock.
“It was in line with what we were expecting to see at those locations,” he said. “But given the conditions that we saw this summer, it’s phenomenal anything of the acropora — it’s phenomenal that they survived. It confirms that there are resilient traits in those populations.”
Looe Key, one of the most popular dive spots in the Keys, was hit hardest, the survey found.
“It is one of the truly iconic reefs in the Florida Keys,” Spadaro said. “And even just over the last 10 years, it has changed so dramatically that it doesn’t surprise me that there wasn’t any acropora found living there.”
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Explore all your optionsWhile he hasn’t tested his hypothesis yet, Spadaro thinks the unique flow of water around the Seven Mile Bridge has a dramatic effect on waters surrounding Looe Key. Spadaro said it’s likely that Looe Key is “very unfortunately located” in an area of Florida Bay where temperatures can vary wildly. It would explain why corals there suffered worse than on other nearby reefs.
“It is possible that we got extremely hot water that hit Looe Key,” he said. “I mean far and beyond most other places along the reef tract.”
The researchers also suspect that some hard coral species, including boulder, massive and brain corals, fared better than fragile staghorn and elkhorns, but rough weather conditions Wednesday prevented the team from taking a closer look.
Erinn Muller, a senior researcher at Mote who studies coral resilience, said researchers have known for years that these species are more tolerant to heat, but last summer tested their resilience to a new extreme.
“They fared better, above-and-beyond what I would have anticipated,” she said.
Muller said researchers are also trying to brace reefs for future heat waves on a warming planet. She said climatologists are predicting another summer of higher temperatures this year that Muller says could trigger a repeat of last year’s coral die-off.
“We don’t expect things to get necessarily better as climate change continues,” she said.
Muller said the next steps for researchers will be to carry out more surveys of reefs to get a clearer picture of which corals survived and why.
A decade of research has shown scientists which traits in corals are more heat tolerant. Muller said the most recent survey confirmed what her team had hoped: The corals they planted years ago were better equipped to handle warm waters.
“Those are the ones that actually survived. Those are all our babies, like literally babies that we created within Mote,” she said.
“Yes, it’s absolutely difficult to see this loss of corals but it’s giving credence to all the efforts that we put in and we need to just do more of that moving forward.”
Times staff writer Max Chesnes contributed to this report.
