Coral bleaching

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Image:GBReef TempChlorophyll 200602.jpg The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its particular coloration, depending on the clade living within the coral. Under stress, corals may expel their zooxantheallae, which leads to a lighter or completely white appearance, hence the term "bleached".

Coral bleaching is a vivid sign of corals responding to stress, primarily increased water temperatures. Higher rates of bleaching are being linked to global warming, but bleaching may occur from other stress factors, such as solar irradiance (photosyntheticaly active radiation and ultraviolet band light), changes in water chemistry, and silt runoff. Some of these factors are anthropogenic, while others occur naturally.

Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. Corals may then reuptake the same clade of zooxanthellae, or may reuptake a new clade that is better suited to the new environment. Reuptake times can range from a few days to several weeks, depending on the species affected. Ability to withstand stress and bleaching and ability to recover from a bleaching event varies greatly across coral species. Large massive corals, such as Porites lobata is able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to dying following a bleaching event. Recent research has also shown that corals consistently exposed to low levels of stress may in fact be more resistant to bleaching.

Other reef creatures have symbiotic zooxanthellae, which they may also expel under stressful conditions. Bleaching stress is also exhibited by soft corals, giant Tridacna clams and some sponges.

The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed.

Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles and Congo.