Extreme ocean temperatures threaten to kill Caribbean corals

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The last global event of this type occurred between 2014 and 2017, a period that, like this year, also featured the return of El Niño conditions, a natural climate cycle that can aggravate background warming due to climate change, increasing to often average air and sea temperatures.

Even with El Niño, the intensity of marine heat waves, particularly off Floridaand its longevity was a surprise, said Ian Enochs, a research ecologist at NOAA’s Atlantic Oceanographic and Meteorological Laboratory.

Heat-induced bleaching occurs as a stress response to abnormal conditions, causing corals to expel small photosynthetic algae that live in their tissues. That makes the colorful corals take on a disturbingly “bleached” shade of white.

Bleaching does not necessarily cause coral death, but the process weakens reefs and makes marine invertebrates more susceptible to disease.

Sea surface temperatures around the world have broke records in recent months, and some of the largest and most persistent temperature spikes were recorded in the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean basin. During the summer, sea surface temperatures off Florida peaked at more than 90 degrees Fahrenheit and remained elevated for weeks.

Part of Enochs’ research has focused on Cheeca Rocks, a reef within the Florida Keys. For more than a decade, Enochs has been monitoring the site, collecting data to compile 3D models of changes to the reef over time. This year, he said, Cheeca Rocks experienced 100% whiteningwithout a single part of the reef remaining intact.

“I’ve never seen anything this size at Cheeca Rocks,” he said. “We were experiencing heat stress levels that were double what we had experienced before at Cheeca Rocks. “If that’s not alarming in terms of the magnitude of this, I don’t know what is.”

Phanor Montoya-Maya, a marine biologist and restoration program director at the Coral Restoration Foundation, an ocean conservation nonprofit, said marine heat waves were so intense this year that many corals didn’t have a chance to adapt.

“On those occasions when the temperature rose so quickly, they didn’t even have time to blanch. They were burned alive,” Montoya-Maya said.

The Coral Restoration Foundation works to raise genetically diverse corals in nurseries and then plant them on reefs around the world. The goal, Montoya-Maya said, is to increase live coral cover on reefs to rebuild populations and increase their resilience.

The situation in the Caribbean is not yet as serious as it is off Florida. But, Manzello said, the bleaching event is still ongoing, and the full extent of this year’s record heat may not be clear in the coming months. What is evident, however, is that long-term trends are cause for alarm.

“It’s concerning because every time we’ve had a global bleaching event, it just keeps getting worse and worse,” he said.

Still, amid such ecological devastation, Enochs said, there are reasons for optimism.

“There is nothing worse than the death of these important species, but at the same time, I have been very surprised that we have not seen more destruction so far,” he said. “There has been a lot of mortality and death, but we have seen some recovery as the water temperature has dropped. And that, to me, means that, in the face of all this, there is still hope.”

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