Extreme ocean heat off Florida has ebbed. But for marine life, the danger remains

In late July, a fierce ocean heat wave ratcheted up temperatures in Florida’s coastal waters to unprecedented highs. One buoy bobbing in shallow, turbid Manatee Bay logged a measurement of 38.3˚ Celsius (101˚ Fahrenheit). That may be the highest temperature ever recorded in the ocean. A week later, that surge in ocean heat had ebbed. But South Florida’s denizens are still in hot water.

The concern is not just that the Manatee Bay buoy recorded shockingly high, hot tub–level temperatures — actually, “close to the limit of hot tub temperatures” — for several days in a row, says Benjamin Kirtman, a climate scientist at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science.

And it’s not just that June and July’s brutally hot water temperatures in the North Atlantic Ocean are linked to shockingly hot temperatures on land. This summer, Miami’s heat index, a measure of air temperature and humidity, soared to a record-breaking streak of nearly two months, reaching a daily heat index of 38° C (100° F).

It’s not even that such ocean heat waves are becoming the new normal, as swells of heat more and more frequently crest atop the baseline warming of the global ocean due to climate change (SN: 2/1/22). Florida’s waters may have hit a record high, but July saw widespread ocean heat waves around the world, from the North Atlantic Ocean to the eastern equatorial Pacific to the Southern Indian Ocean.

“The global oceans have warmed up so much … we’re seeing a ratcheting up that’s unprecedented in the modern instrument record, and maybe in the last 125,000 years,” Kirtman says. “It’s really quite remarkable.”

‘Way outside of the bounds of anything these corals have experienced’

In Florida, the temperatures of the coastal waters have returned to a normal summertime range for now. But the danger remains acute for many ocean dwellers, from corals to fish, says Andrew Baker, a coral biologist also at the University of Miami’s Rosenstiel School.

Murky Manatee Bay, swirling with sediment, isn’t home to corals — but the water temperatures in the reefs around the Florida Keys were still “incredibly hot,” perhaps reaching up to 36° C (96° F), Baker says.

As the sweltering sea temperatures peaked in July, the Coral Restoration Foundation, a nonprofit marine conservation organization based in Key Largo, Fla., found 100 percent coral mortality at one site, Sombrero Reef off Key West. There, heat had caused the corals to bleach.

Bleaching occurs when corals’ symbiotic algae, the main source of their food, flee, leaving the corals colorless and essentially starving. Corals can recover from bleaching, but if the events are too severe or too frequent, they can kill entire reefs. U.S. National Oceanic and Atmospheric Administration records show that the heat burden on corals globally has been rising since the 1980s (SN: 1/4/18).

Even with the return to typical summertime water temperatures off Florida’s coasts, the impacts of July’s heat wave on the region’s corals will linger. That’s because corals have a limit to how much accumulated heat they can tolerate before bleaching. And with this heat wave, the corals have already received far too much heat far too early in the summer, researchers say.

NOAA records from sites across the Florida Keys each tell the same worrisome story — that what’s happened in 2023 so far is “way outside of the bounds of anything these corals have experienced,” Baker says. And the corals still must contend with two more months of expected, but still very hot, water in August and September.

Meanwhile, scientists are racing to save corals growing at nurseries in the Keys, bringing them to onshore laboratories away from the overheated coastal waters. The cultivated corals are part of a decade-long effort to protect the two most important reef species in the region, staghorn and elkhorn coral, from the ever-looming threat of bleaching.

The fledgling finger- to hand-sized corals are cultivated in coastal waters atop bits of PVC tubing, and are ultimately destined to be planted in reefs. As the water temperatures rose, researchers hurried to collect the cultivated corals ahead of their expected spawning in early August. Scientists feared the “heat stress is just too much for these baby corals,” and that they might not spawn at all, Baker says. Happily, some of the rescued staghorn corals, now ensconced in the laboratory, did manage to spawn on August 3, releasing clouds of eggs and sperm into the water. Whether the sperm will fertilize the eggs remains uncertain, but Baker and colleagues are cautiously optimistic.

It’s not just corals in trouble

The overheated water is also bad news for everything from sponges to sea grasses to fish. “There are a lot of studies that show that species experiencing ocean heat waves are migrating [to cooler waters],” says Regina Rodrigues, a physical oceanographer at the Federal University of Santa Catarina in Brazil (SN: 8/10/20). But in tropical regions like the Caribbean and the Gulf of Mexico, where cooler waters are prohibitively far away, “that community doesn’t have anywhere to go.”

That lack of access to an escape route to cooler waters is why the region’s cold-blooded ocean species, including fish, may be even more vulnerable to warming than their counterparts on land. On average, ocean ectotherms spend more time near the upper limits for body temperature than land ectotherms, as marine ecologist Malin Pinsky of Rutgers University in New Brunswick, N.J., and colleagues reported in 2019.

Then there’s the anoxia. As water heats up, it releases oxygen, like bubbles escaping a pot boiling on the stove, leaving less oxygen available for sea life. Such heat-amped anoxic waters have been linked to increased sea grass die-offs as well as fish kills. In June, for example, thousands of fish killed by a low-oxygen event washed up on the Texas Gulf Coast just south of Houston.

Florida’s sea grasses have been in free fall for years, with thousands of hectares of marine sea grass beds wiped out by anoxia as well as nutrient pollution, which can lead to harmful algal blooms that block out the light for underwater plants. The loss of those sea grass ecosystems has been deadly for manatees and other creatures that rely on the grasses for food.

‘It’s just bonkers hot’

What’s driving the brutal ocean temperatures is still uncertain — but human-caused climate change is undeniably at its core, researchers say. “Ninety-three percent of the excess heat in the atmosphere is being absorbed by the ocean,” Rodrigues says. That’s raised the average temperature of ocean waters, “and once the mean temperature is raised, the extremes are easier to achieve.”

Other factors are also likely playing a role, including this year’s onset of the global climate pattern known as the El Niño-Southern Oscillation (SN: 7/13/23). The El Niño phase of that climate pattern tends to increase the global average temperature, and this year’s El Niño is bidding to be “a strong one,” Kirtman says.

“Certainly, one of the questions that’s come up is how much [of the heat] is internal natural variability, and how much a ratcheting up of climate change,” he says.

Local extremes — such as the temporary hot tub in Manatee Bay — may also be influenced by factors such as the shallowness of the water and murkier, less-reflective waters absorbing more heat. 

But, Kirtman says, the global oceans have warmed up so much that El Niño or sediment-laden waters alone can’t possibly explain what’s going on. “This is so crazy, so bonkers. It’s just bonkers hot.”