When corals undergo certain kinds of stress, much of the zooxanthellae – the symbiotic algae that provide coral polyps with nutrients – are expelled from the coral tissue. Ultimately, weakened corals may die. Bleaching is a frequent symptom of pollution-induced stress, as well as a response to natural factors such as changes in water temperature, salinity levels, and possibly ultraviolet light.[1]
During the El Niņo of 1982-83, large areas of coral reef around the world were severely damaged by high water temperatures, which resulted in coral bleaching. Scientific studies have linked bleaching events to temporary “hot spots,” local areas of unusually high temperatures caused by changes in atmospheric circulation during the El Niņo oceanographic events. (Coral Bleaching Has Been Observed Worldwide provides a sense of the wide incidence of coral reef bleaching.) The effects of the severe El Niņo that started in 1997 have yet to be documented.[2][3]
| Coral bleaching has been observed worldwide |
 
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Notes
1. Brown, “Disturbances to Reefs,” 365.
2. C. M. Eaken, “Where Have all the Carbonates Gone? A Model Comparison of Calcium Carbonate Budgets Before and After the 1982-1983 El Niņo,” Coral Reefs 15, no. 2 (1 Aqaba Environmental Action Plan (Washington D.C., World Bank, 1996), 996), 109-19.
3. P. W. Glynn, “Coral Reef Bleaching: Facts, Hypothesis and Implications,” Global Change Biology 2, no. 6 (1996), 495-509.
