Coral bleaching

As the climate changes, coral bleaching is predicted to become more frequent and severe. Sea temperature increases and coral stress from other impacts may increase corals' vulnerability to bleaching.

As part of its Reef Health Incident Response System, the Great Barrier Reef Marine Park Authority has a Coral Bleaching Response Plan for detecting and responding to coral bleaching on the Great Barrier Reef.

What is coral bleaching

Many types of coral have a special symbiotic relationship with a tiny marine algae (zooxanthallae) that live inside corals' tissue and are very efficient food producers that provide up to 90 per cent of the energy corals require to grow and reproduce.

Coral bleaching occurs when the relationship between the coral host and zooxanthallae, which give coral much of their colour, breaks down. Without the zooxanthallae, 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 zooxanthallae.

If conditions return to normal, corals can regain their zooxanthallae, return to their normal colour and survive. However, this stress is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease.

Bleached corals often die if the stress persists. Coral reefs that have high rates of coral death following bleaching can take many years or decades to recover.

What causes coral bleaching

The main cause of coral bleaching is heat stress resulting from high sea temperatures.Temperature increases of only one degree celsius for only four weeks can trigger bleaching events.

If these temperatures persist for longer periods (eight weeks or more) corals begin to die. High water temperatures can affect reefs at regional and global scale.

Other stressors can also cause bleaching, including freshwater inundation (low salinity) and poor water quality from sediment or pollutant run-off.

FIGURE 1 SHOWS HEALTHY ACOPORA CORAL, FULL OF COLOUR AND WITH LIVE POLYPS AND ZOOXANTHALLAE LIVING IN ITS TISSUE. FIGURE 2 SHOWS BLEACHED ACOPORA CORAL WITH THE ZOOXANTHALLAE LEAVING THEIR POSITION IN THE CORAL TISSUE. THE CORAL IS WHITE IN COLOUR, DUE TO THE ZOOXANTHALLAE LEAVING. FIGURE 3 SHOWS DEAD ACOPORA CORAL WHICH IS COVERED WITH GREEN TURFING ALGAE. THE CORAL HAS DIED BECAUSE THE WATER CONDITIONS HAVE REMAINED POOR FOR TOO LONG AND THE CORAL HASN'T HAD A CHANCE TO RECOVER FROM THE BLEACHING. PHOTO 1 IS OF A HEALTHY ACOPORA CORAL GARDEN, THE CORAL IS LIVE WITH VIBRANT COLOURS OF PURPLE, GREEN AND BROWN. PHOTO 2 IS OF BLEACHED CORAL. THE CORAL IS COMPLETELY WHITE AND ONLY ITS SKELETON IS LEFT. PHOTO 3 IS OF A DEAD CORAL GARDEN, COVERED IN GREEN ALGAE. THE CORALS' SKELETONS STILL REMAIN, BUT THEY ARE NOT FUNCTIONING.

Coral bleaching on the Great Barrier Reef

The Great Barrier Reef has experienced mass coral bleaching events in the past.

In 1998, there was a global mass bleaching event where 50 per cent of the reefs on the Great Barrier Reef suffered bleaching. During this time, sea temperatures on the Great Barrier Reef were the highest ever recorded.

Mass bleaching also occurred in 2002, with 60 per cent of reefs were affected. This was the largest coral bleaching event on record. Two periods of hot weather resulted in sea surface temperatures a few degrees centigrade higher than long-term summer maxima.

In both events, about five per cent of the Great Barrier Reef's coral reefs were severely damaged.

 

THIS FIGURE SHOWS THAT IN THE 1998 CORAL BLEACHING EVENT, 45 PER CENT OF CORALS ON THE GREAT BARRIER REEF WERE UNAFFECTED, 50 PER CENT WERE BLEACHED AND FIVE PER CENT WERE PERMANENTLY DAMAGED. IT ALSO SHOWS THAT IN THE 2002 BLEACHING EVENT, 35 PER CENT OF CORALS ON THE GREAT BARRIER REEF WERE UNAFFECTED, 60 PER CENT WERE BLEACHED AND FIVE PER CENT WERE PERMANENTLY DAMAGED.