One-third of Great Barrier Reef corals killed in marine heatwave

The study, published in the journal Nature, showed 29 percent of the almost 4,000 reefs that make up the Great Barrier Reef lost at least two-thirds of their corals over a nine-month period, affecting their ability to support diverse ecosystems.

"When corals bleach from a heatwave, they can either survive and regain their color slowly as the temperature drops, or they can die," Terry Hughes, lead study author and director of the Australian Research Council Centre of Excellence for Coral Reef Studies, said in a statement.
Corals, or coral polyps, are soft-bodied organisms that have a symbiotic relationship with algae living in their tissues. Algae provide food and color to the corals.

Coral bleaching occurs when high water temperatures cause corals to expel the algae, making the coral pale or white.
The study found corals in the northern third of the reef, where water temperatures were most extreme, were the most severely affected, with half of the coral cover in the most shallow areas dying in an eight-month period.

Hughes told the Australian Broadcasting Corp. that heat stress -- not starvation caused by bleaching -- was the direct cause of coral death.
"They cooked because the temperatures were so extreme," he said, referring to temperature-sensitive species of corals which began to die almost immediately when water temperatures rose.

Researchers used aerial surveys to measure the extent and severity of bleaching along the 2,300-kilometer-long reef, while the geographical pattern of heat exposure was mapped from satellites, Kyodo reports.