Coral bleaching destroys Indian Ocean reefs, reports show

A huge majority of corals in the Chagos Marine Reserve in the British Indian Ocean Territory have been killed or damaged by the global bleaching event that is also thought to have affected more than 90 percent of Australia’s Great Barrier Reef.

The problem is caused by the demise of zooxanthellae – tiny, colorful marine algae that creates much of the beautiful color in coral reefs, as well as providing them with energy. Increasingly high sea surface temperatures lead to the death of zooxanthellae, without which coral tissue becomes transparent, revealing the white coral skeleton underneath. Zooxanthellae can return if conditions revert back to something more habitable, but if temperatures continue to increase then the corals can die.

Climate change is a contributor to the phenomenon, as are El Nino events. With the latter, the passing of the event should hopefully lead to some recovery for corals.

The Bleaching is’Very depressing’

The last major bleaching event was in 1998, after which recovery levels were good. However, scientists say the current occurrence is worse, the BBC reports.

“In 1998, the temperature that killed all the corals was probably about 29.5C (81.1F). Last year, in April, at the beginning of the latest bleaching event, it was 30.5C (86.9F) and down to 25m. And this year scientists have been out and it’s the same again,” said Charles Sheppard, the chair of the Chagos Conservation Trust.

Heather Koldewey, from the Zoological Society of London and a CCT Trustee, who led the expedition, said: “I was there two years ago and it’s always an absolute joy to go diving in Chagos because you really get to see what a reef should look like – rich, living corals with abundant fish and other marine life at densities you just don’t get in other places. This was very depressing.”

At 250,000 square miles, the Chagos Marine Reserve is home to superb biodiversity that includes the world’s biggest land crab, the meter-spanning coconut crab.

It has been relatively population free since the late 1960s, with this having the effect of limiting the stress factors that can weaken corals.

“This is what makes Chagos such an important reference site for corals worldwide,” explained Koldewey. “This is not an oil spill, this is not coastal pollution, sewage, or overfishing or siltation. If anywhere can bounce back, it is the Chagos Archipelago, and I hold on to that positive point of view.”

She hopes the reef will be re-invaded by juveniles in deeper, cooler water who will establish new communities.

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Image credit: Dan Bayley

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