Bleached Corals “Sickest” Scientists Have Ever Seen

When corals are stressed, they lose the symbiotic algae that live in their tissues, leading to a loss of color called bleaching. But not all the algae have to leave for the color to be lost; corals can bleach to different degrees, even if they look completely white. The GBR corals that have bleached but are still alive may either recover their algae and survive, or slowly degrade until they ultimately die.

“When you do a visual survey, you can see corals that are white, but it’s difficult to guess which will survive and which won’t,” explained Leggat. Scientists can get a better idea of how many will make it by measuring key indicators of health while the corals are recovering. One of those measures is the density of algae remaining in the coral tissues. “A normal, healthy coral will have about one million to two million algae per square centimeter,” Leggat said.

On average, the scientists are finding 4,000 algae per square centimeter in the bleached corals—500 times less than healthy corals, and 50 times less than corals sampled during other bleaching events. “The scary thing is: we’ve never seen anything this bad,” Leggat told The Scientist. “Portions of the corals we’ve been looking at have absolutely none.” Even laboratory studies have not tested such low algal densities, he added. “We didn’t push corals this hard because we didn’t think we’d see it.”

Leggat said their results have “dire implications” with regard to recovery. “It’s the algae that provide the corals with energy,” he said. “If they haven’t got those algae, they’re probably going to die.” But just how dire remains to be seen. “All we can say is that they’re going to do a lot worse than anything we would have thought.”

The bleaching event is especially devastating because of its severity in the northern 700 km of the GBR—an area that, until now, was considered the nearly pristine. While the southern and central regions have declined over the last few decades, losing 50 percent of their corals in the last 30 years, the northern reefs have remained robust. But now, in just one season, the north may have caught up with the south in terms of coral degradation.

Scientists are uncertain how long it would take for the GBR to recover from this bleaching event, but they fear it’s longer than the corals will have. “We’re seeing 50- and 100-year-old corals that are dying,” said Hughes. “You can’t replace a 50-year-old coral in a decade.”