As Oceans Warm, Coral Bleaching Seen at Greater Depths

Bleached corals some 300 feet below the surface of the Indian Ocean.

Bleached corals some 300 feet below the surface of the Indian Ocean. Diaz et al.

Researchers have discovered coral bleaching hundreds of feet underwater, at a depth where corals were once well insulated from surface warming.

When ocean waters grow too warm, corals eject the colorful algae that inhabit their tissues, turning white. If waters cool, algae can regain their color, but stubbornly high temperatures may prove deadly. With climate change, coral bleaching has become routine in shallow reefs, from Australia to the eastern Pacific.

It was long assumed that deeper reefs would remain safe from warming, but in 2019 researchers recorded coral bleaching some 300 feet underwater along the Egmont Atoll in the western Indian Ocean. Amid a hot spell, bleaching affected 80 percent of corals in some areas, scientists reported in Nature Communications.


“There are no two ways about it, this is a huge surprise,” said study coauthor Philip Hosegood, an oceanographer at the University of Plymouth in the UK. “Deeper corals had always been thought of as being resilient to ocean warming, because the waters they inhabit are cooler than at the surface and were believed to remain relatively stable.”

The unusually warm waters around the Egmont Atoll were due partly to an El Niño-like phenomenon in which the west and east sides of the Indian Ocean are alternately hotter and colder. Climate change nudged already-warm waters higher still, researchers said.

Returning to the site in 2020 and 2022, scientists found that much of the Egmont reef had recovered, though they remain cautious about the future of coral. Said Hosegood, “There are likely to be reefs at similar depths all over the world that are at threat from similar climatic changes.”


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