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What is coral bleaching?

Coral bleaching is a stress condition in reef corals that involves a breakdown of the symbiotic relationship between corals and unicellular algae (zooxanthellae). These microscopic plants live within the coral tissue and provide the coral with food for growth and their normal healthy colour.

The symptoms of bleaching include a gradual loss of colour as zooxanthellae are expelled from the coral tissue, sometimes leaving corals bone white.

Bleaching stress is also exhibited by other reef animals that have a symbiotic relationship with zooxanthellae such as soft corals, giant clams, some sponges, etc.

The coloration of a healthy coral (left) is due to microscopic algae called zooxanthellae (center) living within its tissue. When bleaching occurs, the coral loses its zooxanthellae, leaving the white skeleton starkly visible through the transparent tissue (right). Photos courtesy of Prof. Ove Hoegh Guldberg, Centre for Marine Studies, University of Queensland.

Why do corals bleach?

Inshore reefs of the GBR were most severely affected during the 1998 bleaching event. Photo courtesy of Paul Marshall.

The stress factor most commonly associated with bleaching is elevated sea temperature, but additional stresses such as high light intensity, low salinity and pollutants are known to exacerbate coral bleaching. If the causal stress is too great or for too long, corals can die.

Reef corals are very sensitive to sea temperatures outside their normal range. Elevated temperatures of 1 deg C above the long term monthly summer average are enough to cause coral bleaching in many dominant coral species.

When temperatures exceed threshold levels for long enough, the symbiotic relationship between the zooxanthellae and the corals breaks down and bleaching results. Because the zooxanthellae are responsible for the green and brown color that characterises most corals, the loss of zooxanthellae can leave the coral tissue colorless, rendering the bright white of the coral skeleton starkly visible. However, many corals also have other pigments in their tissue, giving them the yellow, pink or blue tinge often seen in shallow water. Coral bleaching sometimes enhances these colors, resulting in bright pastel colors mixed amongst the bright white and pale brown of most corals.

If stressful conditions prevail long enough, bleached corals will die. However, if stressful conditions abate, then the bleached corals can recover their symbiotic algae and return to their normal, healthy colour. The severity of bleaching can vary substantially according to water depth, location and species of corals.

Mass bleaching events

If stressful conditions, such as high water temperatures, prevail at a regional level, large areas of coral reef can be affected in what is known as a mass bleaching event. The mass bleaching events reported on the Great Barrier Reef and elsewhere around the world over the last 5-10 years have been triggered primarily by anomalously high water temperatures.

The Great Barrier Reef (GBR) has suffered two mass coral bleaching events in the last five years: in the summers of 1998 and 2002.

The mass bleaching event that occurred in the summer of 2002 affected between 60% and 95% of reefs in the Marine Park. This was the worst bleaching event ever recorded for the GBR. While most reefs that were surveyed survived with relatively low levels of coral death, some locations suffered severe damage with up to 90% of corals killed. Up to 5% of reefs on the GBR have been severely damaged during each of the last two major bleaching events, including the inshore reefs around Bowen and Mackay, and some reefs in the Coral Sea. Full recovery of these badly damaged reefs will take many years to decades.

Bleaching occurred in almost all tropical oceans in 1997/98, with more isolated bleaching events around the world in succeeding years. Many reef provinces have suffered extensive coral mortality at a regional scale in past bleaching events (up to 90% of coral cover lost in the Maldives, Sri Lanka, Kenya, Tanzania and Seychelles). More information about coral bleaching events around the world can be found on ReefBase.

Bleaching of some GBR inshore reefs was evident from the air during the mass bleaching event of 1998.

Further reading

  • Berkelmans, R. and Oliver, JK. (1999). Large-scale bleaching of corals on the Great Barrier Reef. Coral Reefs 18: 55-60.
  • Hoegh-Guldberg O (1999) Climate change, coral bleaching and the future of the world's coral reefs. Marine and Freshwater Ecology 50: 839-66
  • Marshall, P.A., Baird A. H. (2000). Bleaching of corals in the Central Great Barrier Reef: Variation in assemblage response and taxa susceptibilities. Coral Reefs 19 (2): 155-163.
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