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by Lucy Gallagher
What Is Coral Bleaching?
Although coral is often mistaken for a rock or a plant, it is actually composed of tiny, fragile animals called coral polyps. A coral is in fact a colony of many of these coral polyps. Coral polyps have a symbiotic relationship with a microscopic plant which lives within the coral tissue called the zooxanthellae. The zooxanthellae provide the coral with food for growth and their normal healthy color.
A coral polyp
Coral bleaching is the whitening of coral colonies due to the loss of the symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. If the coral is subjected to stressful conditions, the symbiotic relationship between the zooxanthellae and the corals breaks down; for reasons unknown, the zooxanthellae leave the coral polyp (or are rejected) and bleaching results. As mentioned previously, the zooxanthellae provide the color to coral colonies, the loss of zooxanthellae can leave the coral tissue colorless. As a result of bleaching, the corals will often turn bright white but may also lighten to bright pastel colors.
Why do corals bleach?
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.
Corals are very sensitive to sea temperatures outside their normal range—between 25°C (77°F) and 29°C (84°F) depending on location. Elevated temperatures of just 1°C above the long-term monthly summer average are enough to cause coral bleaching in many dominant coral species. 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 color. The severity of bleaching can vary substantially according to water depth, location and species of corals.
How serious is the coral bleaching problem?
Incidences of localized coral bleaching have been reported in all major reef provinces since the 1870s. Since 1983, however, a new phenomenon has appeared, called mass coral bleaching. In such instances, elevated water temperature and increased amounts of UV light cause coral reefs in unrelated areas around the world to bleach during the warm season. Many mass bleachings are correlated with El Niño/La Niña events, which create unusual warm water currents. Many scientists are also concerned that global warming is leading to elevated water temperatures and consequently causing mass bleaching events.
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