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Great Barrier Reef Marine Park Authority :: What is Coral Bleaching

What is Coral Bleaching

The primary cause of coral bleaching is high water temperature. Temperature increases of only 1.5–2°C lasting for six to eight weeks are enough to trigger bleaching. When high temperatures persist for more than eight weeks, corals begin to die. Many other stressors can also cause bleaching including sedimentation, pollutants and changes in salinity. These stressors usually operate at local scales. Elevated water temperature is of greater concern as it can affect reefs at regional to global scales. When bleaching occurs at these large spatial scales, it is a mass bleaching event. 

Why do corals bleach?

Many types of coral have evolved to form a special symbiotic relationship with a type of tiny marine algae (called zooxanthellae) in order to survive.  These zooxanthallae avoid predation by living inside the tissue of the corals. In return, the zooxanthellae (which are very efficient food producers) give their excess food to the coral, supplying up to 90 per cent of the energy that a coral needs in order to survive, grow and reproduce.

Coral Polyp

Coral polyp showing its tiny zooxanthellae, seen as small brown dots. Source Kirsten Michalek-Wagner


Coral bleaching occurs when the coral host expels its zooxanthellae. Photosynthetic pigments of the zooxanthellae give corals much of their colour. Therefore without the zooxanthellae, the tissue of the coral animal appears transparent and the coral’s bright white skeleton is revealed. 

Corals begin to starve once they bleach. While some corals are able to feed themselves, most corals struggle to survive without their zooxanthellae. If conditions return to normal, corals can regain their zooxanthellae, return to normal colour and survive. This stress, however, is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease.

Bleached corals often die if the stress persists. Coral reefs suffering severe mortality following bleaching can take many years or decades to recover.

This diagram illustrates the three stages of the death of a coral colony due to bleaching. Part One shows a healthy coral branch covered in coral tissue, known as coral polyps. Each of these polyps contain many tiny zooxanthellae (microscopic algae). The zooxanthellae give the coral its colour. Part Two shows a coral branch which is bleaching. The coral polyps on the branch are expelling their zooxanthellae out into the water column. Without the zooxanthallae, the coral polyps lose their colour and become pale and white. Part Three shows the coral branch which has died as a result of prolonged bleaching. Without the zooxanthellae, all the coral polyps have died, leaving a bare white coral skeleton without any tissue. Without the coral polyps, the coral skeleton has become covered in fine filamentous algae.

Where has coral bleaching occurred?

Image of bleached Acropora - close up
Bleached staghorn coral

Mass bleaching has now affected every reef region in the world. The spatial extent and severity of impacts of coral bleaching have been increasing throughout the world over the last few decades. A particularly severe, worldwide bleaching event occurred in 1998, effectively destroying 16 per cent of the world’s reefs. The Great Barrier Reef was affected by this global bleaching event and by another event in 2002. More localised bleaching occurred in the southern Great Barrier Reef in 2006.

Projected increases in global temperatures suggest this trend will continue over coming decades, placing greater stress on reefs. The Great Barrier Reef Marine Park Authority has developed a Coral Bleaching Response Plan to provide a comprehensive strategy for detecting and responding to widespread coral bleaching during summer.

For more information on coral bleaching see A Reef Manager’s Guide to Coral Bleaching.

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