In the summer of 2023, an unprecedented marine heat wave filled national headlines with apocalyptic projections for the future of Keys reefs. Scorching water temperatures triggered massive coral bleaching events, in which living corals lose their color after expelling the algae within their tissues upon which they rely for nutrients. Some went as far as to paint a picture of a “coral collapse” on “completely decimated” Keys reefs.
But a new paper released at the close of 2024, written by six Keys investigators led by research scientist Karen Neely with Nova Southeastern University’s National Coral Reef Institute, is one of the first peer-reviewed publications to investigate the eventual mortality of heat-stressed corals once the dust settled after the now-infamous summer – with a somewhat brighter conclusion.
Building on a program previously put in place to monitor the progression and treatment of stony coral tissue loss disease (SCTLD), another deadly ailment first reported off Florida’s coast more than a decade ago, the study tracked the fate of more than 4,200 reef-building brain and boulder coral colonies at nine inshore and offshore reef sites from Sand Key off Key West to Carysfort Reef off Key Largo.
In 2023, national headlines zeroed in on the demise of staghorn and elkhorn corals, two species found in Keys nurseries and outplanting sites as the center of many coral restoration efforts at the time.
However, shifting its focus to other species already under the microscope due to their susceptibility to SCTLD, the new study found that at seven of the nine sites, no more than 2% of the tracked brain and boulder corals died. At the two worst sites, meanwhile, 43% and 30% of the monitored corals died, with brain corals suffering a more severe blow than boulder corals.
“The heat wave was really catastrophic for staghorn and elkhorn corals, and particularly nurseries and outplants,” co-author Karen Neely told the Weekly. “That story doesn’t change, but those are a small component of what corals are out there in the wild, and we were able to show that bigger picture.”
In February 2024, a release from NOAA’s Mission: Iconic Reefs program provided similar observations from research cruises to quantify the impact of the 2023 heat wave. Surveying 64 locations across five iconic reef sites, the cruise’s preliminary data showed less than 22% of surveyed staghorn corals remained alive, with multiple reef sites showing no remaining staghorn or elkhorn corals.
But while researchers on the NOAA mission said weather conditions prevented them from conducting a more scientific survey of other species, they noted anecdotally that boulder, massive and brain coral outplants – the types described in the NSU paper – fared better than their branching coral counterparts.
Although the NSU paper acknowledges heat stresses leading to 100% coral bleaching at many sites in the new study, an important clarifier may bridge the gap between the new data and national headlines: A bleached coral is not necessarily a dead coral.
A stark white piece of stony coral may fit the bill for what an untrained observer would call “dead” coral. But when corals lose their color by expelling their symbiotic algae, it’s not an immediate death sentence. Rather, it’s an extreme “calorie cut,” of sorts, one that Neely said most corals can survive for weeks or even months. And in many cases, the corals eventually regain their color and make a recovery.
The sites matter
While the study found minimal coral deaths despite 100% bleaching at its offshore sites, inshore corals at two of the four sites weren’t as lucky, with the highest mortality at Newfound Harbor off Big Pine Key (43.1% of monitored colonies died) and Cheeca Rocks off Upper Matecumbe Key (30%).
Almost equally concerning for Neely was how quickly corals at these sites reacted to the heat wave.
“We were seeing these corals die before they ‘should have,’” she said, reaffirming that corals typically perish weeks or months after a bleaching event due to a lack of nutritional resources formerly provided by their symbiotic algae.
“We were seeing substantial mortality at Newfound Harbor when corals hadn’t been bleached for very long,” she said. “And so we don’t think that’s resource depletion – we think they were just boiling, and they were too hot to survive. That’s a different type of mortality, and as far as we know that’s not a type of mortality that’s been seen in the wild.”
In-water temperature loggers recorded a maximum of 34.06° C (93.3° F) on the seafloor at Newfound Harbor on July 10, the paper states. And in the week before reaching that maximum, water temps skyrocketed, more than quadrupling the average daily increase seen in the preceding days.
Even after bleached corals regained their color, Neely’s team noted unusual lesions of an unknown origin on mountainous star coral colonies, in some cases causing significant tissue loss over a few months before the lesions disappeared.
Forward to the future
Though the NSU team’s data helps paint a more complete picture, with a slightly brighter tone, of the fallout from 2023’s heat wave, the event could be a glimpse behind the curtain for almost-certain future events.
“I saw something that said, ‘This might be the hottest summer of your life, but it may be the coolest summer for the rest of your life,’” she said. “I think the reflection done by the restoration community after the 2023 event was really valuable, and some of them are completely rethinking what corals they use in their restorations. But we’re kind of getting a sneak peek into what species might do well 10 years or 100 years from now, and I hope that this can help inform those discussions.”
Beyond “a happy story, in showing that not everything died,” Neely said the monitoring served as a reminder that “there are a lot of corals out there that have been around for a really long time, and survived hundreds of years of problems.”
“We have to keep them alive, because it’s way easier to keep a coral alive than it is to replace it with new ones,” she concluded. “We still need to be thinking about these wild corals, what they’re susceptible to, what they’re resilient to, and how we can help keep them.”
