During the summer 2023 heatwave, reefs in the Florida Keys weathered unprecedented water temperatures and heat stress, which led to 100 percent coral bleaching. While the bleaching event sent shockwaves of possible gloom and doom for coral populations, a team of Nova Southeastern University researchers found an incredible resilience among most of the coral impacted by the marine event. Among their eye-opening findings:
- Despite 100 percent bleaching rates, bleaching-related mortality of brain and boulder corals across the Florida Keys was minimal during the bleaching event.
- The few sites where bleaching-related mortality on brain and boulder corals did occur also experienced extreme cumulative heat stress.
- Brain corals fared substantially worse than boulder corals at the few sites that exhibited substantial mortality.
NSU research scientist Karen Neely, with the Halmos College of Arts and Sciences’ disease intervention laboratory, and her team monitored more than 4,200 brain and boulder coral colonies across five offshore and four inshore reefs for mortality during the bleaching event. At seven of the nine sites, only 0 to 2 percent of the colonies experienced any mortality, according to data documenting her findings in the research publication Frontiers in Marine Science.
“We set up this monitoring program in 2019,” she said. “When we bring a new coral into our monitoring program, we put a tag the coral, take photos of the coral, and we map them all.” The brain and boulder coral species studied by Neely are all reef-building species that can grow larger than cars and live for hundreds of years. They provide refuge, nursery, and feeding areas for many marine organisms. They also provide coastal protection from waves and storms, and support fishing and scuba diving industries.
Corals have a symbiotic relationship with algae, which give corals their color and also provide them a lot of their nutrition.
“It’s like having an apple tree growing inside you,” Neely said.
When coral get too stressed – such as during a bleaching event – that symbiosis breaks down, leaving the coral without its nutritional intake. That can essentially cause the coral to starve to death.
With cumulative heat stress, it’s not just how hot the water is but how long the water is hot, Neely says. In 2023, the water warming started earlier than it ever had before.
However, Neely’s team documented something unusual in 2023.
“One of the things we found that was surprising was that we were seeing mortality of corals as early as July when we didn’t have a lot of cumulative heat stress,” Neely said about the most heavily impacted site.” “It wasn’t months after bleaching started when they died, so it seems more likely that they died because it was just too hot, not because they starved to death. As far as we know that has never been seen in the wild before” Neely said.
Neely and her research team were also able to show conclusively that some coral species perform better than others under the same thermal conditions, and that at the affected sites, brain corals had higher mortality rates than boulder corals.
Neely is based out of Big Pine Key and her team of researchers monitor corals from Key West through Key Largo. Her fate-tracking project was funded by the state of Florida though the Department of Environmental Protection, allowing the researcher to rent boats to aid in their monitoring.
While the newly published paper shows that most large corals survived the marine heatwave better than expected, the sites that experienced the highest temperatures and mortality rates are still concerning for the future of the coral reef system.
“One of the big problems is we don’t really know what those thresholds might be for these different corals at different sites, so we might start hitting temperatures that are going to kill coral much more quickly than we’re used to from bleaching,” she said.
South Florida’s coral reef system is an intricate part of the state’s tourism industry and blue economy. Florida’s coastline grosses more than $400 billion annually. In Florida, the blue economy accounts for more than 800 billion dollars in economic impact, or almost 80 percent of the state’s GDP. That includes everything from recreational divers, fisheries and boating industries to tourism, transportation, environmental safety and research.
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