“When it gets really hot, they often die before they even fully bleach.”
When an intense marine heat wave sent ocean temperatures soaring in 2023 and 2024, coral reefs around the world bleached. New research on the Great Barrier Reef’s One Tree Island shows that more than 50% of surveyed coral colonies that bleached died of heat stress and starvation. And even heat-resistant corals weren’t immune.
When corals are stressed by warm water, they can lose the algae that live in their tissues. This process turns the coral white, earning it the name “coral bleaching.” Sometimes corals can recover, but if the stress is too intense, they die and eventually crumble into rubble and sand.
“What we noticed in more recent times, when it gets really hot, they often die before they even fully bleach,” said marine biologist Maria Byrne at the University of Sydney.
A Global Marine Heat Wave
Record hot temperatures in 2023 were worsened by a strong El Niño. This heat triggered a global coral bleaching event that began in the Caribbean. By early 2024, it had reached the Great Barrier Reef. Australian aerial surveys showed that in some areas of the reef, more than 90% of corals were bleached.
Starting in February 2024, Byrne and her colleagues went to the One Tree Island Reef, located roughly 100 kilometers off the coast of Queensland, Australia, to document how the heat wave affected the reef in the months afterward. The island is protected from mainland coastal pollution and tourism, so the effects of the heat wave could be surveyed independently from those stressors.
Over 161 days, the researchers tracked 462 coral colonies from the peak of the Southern Hemisphere heat wave in February through to the winter of July 2024. On four occasions, they dived at two lagoon sites—a shallow channel and a bay connected to the ocean.
Wearing thin stinger suits to protect them from jellyfish stings, the researchers swam in pairs along marked transects, one researcher capturing videos and the other taking both wide-angle and close-up photos.
The scientists monitored individual corals across surveys using GPS markers and numbered tags. The tags helped them match photos and videos from different dives, allowing them to compare changes over time—whether the corals remained bleached, recovered, or succumbed to disease and algae.
“We match the corals a bit like a jigsaw puzzle,” Byrne said. “It did take a lot of time to match all the photos, thousands of them, to individual corals.”
When bleaching and algae made corals unrecognizable, Byrne’s team had to use natural landmarks—like a giant clam or a patch of soft coral—to track the reef’s changing story.
Widespread White
Reef sensors recorded a peak temperature of 30.55°C (86.99°F). That’s higher than satellite readings. This kind of on-site data should help the scientists identify coral heat stress and bleaching risks better.
In February, almost two thirds of the corals were ghostly white. Even the heat-resistant Porites, usually a refuge for turtles, was affected, with seven out of 10 colonies observed showing signs of bleaching.
“The corals that live on One Tree Island Reef are very used to having extreme summers, and to a degree, it’s surprising they were pushed over the limit so easily.”
By April, bleaching had intensified, affecting 80% of the 462 coral colonies under study. Though bleaching can contribute to slow starvation over weeks or months, extreme heat can lead to sudden die-offs due to heat stress before the coral even turns fully white. By July, more than half of the observed bleached colonies had died, and only 16% showed signs of recovery.
Some species fared worse: Acropora, a fast-growing branching coral, faced near-total collapse after bleaching, with 95% of colonies dead. Goniopora, known for its flowerlike polyps, was severely affected by black band disease, a fatal disorder in which a dark stripe separates the animal’s healthy tissue from an encroaching microbial mat that leaves a dead skeleton in its wake.
The results were published in Limnology and Oceanography Letters.
“The corals that live on One Tree Island Reef are very used to having extreme summers, and to a degree, it’s surprising they were pushed over the limit so easily,” said Stuart Kininmonth, a coral reef ecologist who managed the University of Queensland’s Heron Island Research Station, located 20 kilometers west of One Tree Island. Kininmonth was not associated with the study.
The scale of the die-offs and the low recovery rate at One Tree Island Reef show how hard it can be for reefs to recover amid harsh bleaching events. Although some species (the stony corals Goniastrea and Pavona) experienced lower mortalities, surveys also revealed how quickly the reef turned to rubble after coral died, transforming the underwater landscape and ecosystem.
“We’ve entered a new phase where heat intensities are so frequent—every 2 years instead of every 10—that corals may not have a chance to recover,” Byrne said. “Species that didn’t bleach may be the ones that survive, leading to a different barrier reef with altered species, ecosystem services, and predator-prey dynamics. We’re entering a brave new world.”
—Anupama Chandrasekaran (@indiantimbre), Science Writer
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