What is Coral Bleaching

Zooxanthellae (microscopic algae) live in the tissue of many corals in a symbiotic relationship. Up to 90 per cent of the coral’s energy requirements comes from the zooxanthellae so corals are highly dependant on this symbiotic relationship.

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.

What causes 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 disease, 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. 

Where has coral bleaching occurred?

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.

Related links

• Coral bleaching on the Great Barrier Reef
• Current bleaching status on the Great Barrier Reef
• BleachWatch