Hot ocean waters around South Florida have scientists worried about a looming repeat of a prolonged marine heat wave that in 2023 killed coral throughout the Florida Keys.
During the first week of August, temperatures spiked, with the most extreme temperatures popping up from Elliott Key, off Cutler Bay, south to eastern waters off Key West.
The only other time scientists say they’d seen surface waters get that hot on reefs between Big Pine and Key Largo was two years ago, when the heat wave killed the last remaining stands of wild elkhorn in the Lower Keys and triggered widespread bleaching from the Dry Tortugas to Miami.
Since mid August, shifting winds cooled waters, but conditions remain treacherous for Florida coral dealing with persistent heat as climate change drives up ocean temperatures.
READ MORE: Scientists transplant crossbred corals to help save Miami's reefs from climate change
”Right now, it's just a waiting game,” said Katey Lesneski, the research and monitoring coordinator for the Florida Keys National Marine Sanctuary’s Mission: Iconic Reefs, a restoration effort expected to cost $100 million to save seven reefs. ”We've had a handful of windy days come through that really do help … We’ll see later this week where it goes.”
A new array of buoys deployed after the 2023 heat wave is giving scientists a better idea of conditions in deeper waters where coral live. Satellites only skim the surface. That data reflected a promising drop by mid month, but still above the threshold for bleaching.
Of the 15 reefs and coral nurseries monitored in the Florida Keys National Marine Sanctuary, half remain on a Level 1 alert, meaning the heat is beginning to stress coral with a risk of reefwide bleaching. And all remain above historic averages for water temperatures. At Level 2, more heat sensitive coral, including staghorn and elkhorn that grow at the top of the reef, can begin dying.
Restoration work, where divers plant lab-bred coral on reefs to help shrinking reefs, was halted Aug. 7 because of the heat.
So far, only mild bleaching has been reported, Lesneski said. But scientists remain wary.
When water temps begin to rise about 3.6 degrees Fahrenheit above normal, coral can begin bleaching — and expelling their life-sustaining algae — in just days or weeks.
In Florida, water above about 88 degrees puts coral at risk. On Monday afternoon, only water at about 30 feet deep at five locations feel above the threshold.
For now, Lesneski agreed scientists remain wary.
“ So we have quite a few warm months ahead of us,” she said. “Back in 2023, we saw bleaching occurring through October and, in some cases, corals were not able to recover for months after that. So it's quite a long period of time.”
Surprisingly, the heat hasn’t dampened romance among the coral, which happily spawned all around the Keys. Teams, including students from Florida International University, rushed to reefs off Key Largo to collect the spawn for nursery breeding.
Ken Nedimyer, a pioneering coral breeder and technical director of Reef Renewal USA, said his nursery in Tavernier had a rare elkhorn spawning. Normally, reproductive elkhorn are too bulky for the Christmas tree-shaped framework used to grow coral in nurseries.
“Like a basketball with arms,” he said.
In the past, Nedimyer has sent those elkhorn to labs at the Florida Aquarium and the University of Miami for breeding. Last weekend was the first year the elkhorn spawned in Tavernier.
”We've had to build completely different hardware and do a lot of different things to try to get them to grow that size,” he said. “But years ago I thought, this is what we need to be striving for is growing sexually mature corals so we can start reproducing them because that's the future.”
Coral are unbelievably fixed in their habits and time their spawning precisely to sun and moon patterns, especially if conditions are just right.
“Year after year after year,” Nedimyer said. “The last time I was out watching it personally was 2017, and I told everybody on the boat, I said, they're not going to spawn until 10:20, but at 10:20 they're going to all start spawning. And sure enough they did.”
That has allowed scientists to collect the coral babies and grow nursery collections across Florida, targeting those that have survived bleaching. UM scientists have also collected elkhorn in Honduras, which thrive in hotter Caribbean waters, to begin cross-breeding them with Florida coral to create a newer, more heat-tolerant elkhorn.
“Elkhorn creates these massive reef structures. It's a really, really important reef building coral in the Caribbean and Florida, and it's the only coral that could conceivably keep up with sea level rise," Nedimyer said.
When the 2023 heat wave hit the Keys, he said, the only elkhorn to survive were those bred in the Florida Aquarium and planted on reefs.
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