What is bleached coral

A.� Although the resultant colour leaves the coral looking devoid of colour, no bleach is actually involved. Nor is it, directly, the result of the suns rays. Bleached coral occurs when the algae that live within coral polyps are forced out.

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Q.� What is the relationship between coral and algae

A.� They have a symbiotic relationship: the coral provides a home for the algae and the algae, called zooxanthellae, provide the coral with most of its oxygen and nutrients.

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However sometimes, under extreme stress the coral expels the algae.

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Q.� So, why the colour change

A.� The algae also provides the coral with their amazing colours, due to a pigment. Once the algae has gone the coral becomes white, hence the term coral bleaching.

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Q.� Is bleached coral in fact dead then

A.� Coral reefs are living organisms. They’re made from the skeletons of millions of marine animals, called coral polyps. As polyps die, new polyps grow on top of and next to the dead ones. Many layers of skeletons from the dead polyps form the limestone structure of a coral reef.

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Normally, bleached coral should be fine after the next cool season, but if all the algae are expelled, the coral will die.

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Q.� How much above average does the sea temperature have to get before coral bleaching occurs

A.� As little as a 1-degree rise in temperature can affect the relationship between coral and algae.

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Q.� So sensitive, it must be an increasing problem then

A.� Yes, up until recently coral bleaching was mainly small scale and localised. However over the last few decades a new phenomenon has appeared, called mass coral bleaching.

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Q.� What causes mass coral bleaching

A.� Probably a combination of general global warming and El Nino events, which create unusual warm water currents.

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by Lisa Cardy