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Bleached boulder coral

Coral Bleaching

During the early months of 1998, 2002, 2004 & 2006, many corals on the Great Barrier Reef, Australia and many other reefs in the world changed their colouration from brown to a brilliant white, purple or green. This ‘whitening’ of the coral is called coral bleaching and is due to a loss of the symbiotic dinoflagellates (algae) living within the tissue of the coral. In a healthy reef, the algae supply the coral with energy (sugars and amino acids) and in turn receive products essential to their survival (ammonia and phosphate).
Pink bleached coralBlue bleached coralYellow bleached coralRed bleached coralWhite bleached coral
Without the brown zooxanthellae present in healthy coral, bleached coral will display the bright white calcium carbonate skeleton and fluorescing pigments.


During a bleaching event, the brownish algae disappear from the tissue of the corals, leading to the observed colour change. This loss of the ‘nutrient factory’ in corals may lead to death of the coral, or the coral may slowly recover. The mass coral bleaching event of 1998 is considered to be the most severe on record - about one-sixth of the world’s coral colonies died!

Many stressful environmental conditions can lead to bleaching, however, elevated water temperatures due to global warming have been found to be the major cause of the massive bleaching events observed in recent years. As the sea temperatures cool during winter, corals that have not starved may overcome a bleaching event and recover their symbionts.

However, even if they survive, their reproductive capacity is reduced, leading to long-term damage to reef systems. Sea temperatures are predicted to continue to rise, and thus, bleaching is expected to occur more and more frequently, leading to the death of large areas of coral reefs worldwide within a few decades!

The Bleaching Process

In recovery

The single greatest determining factor as to whether a coral survives a bleaching event is the amount of time that it is exposed to elevated temperatures under high light conditions. The longer the coral is exposed, the greater the chances of mortality, with many succumbing to disease and being overgrown by algae.

Corals can recover quite quickly from bleaching events once the stresses are relieved, in some cases regaining their colour in a matter of days. However, each bleaching event weakens the overall health of the coral over time.

 

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Other major threats to corals reefs are ocean acidification, overfishing, coral disease, invasive species, and both natural and human disturbances. Read more about this in “Coral Reefs and Climate Change: the guide for education and awaress”.