Coral bleaching is the whitening of coral colonies due to the loss of the symbiotic algae, zooxanthellae, from the tissues of coral polyps. It is mostly caused by stress. The host coral polyp provides the algae with a protected environment and a supply of carbon dioxide for its photosynthetic processes. The golden-brown algae serve as a major source of nutrition and color for the coral. The loss of the algae exposes the translucent calcium carbonate skeletons of the coral colony, and the corals look "bleached." Corals may recover from short-term bleaching (less than a month), but prolonged bleaching causes irreversible damage and mortality , for without the algae, the corals starve and die. However, even a sublethal stress may result in increased susceptibility of corals to infections, with resulting significant mortality. Populations of sea urchins, parrot fish, and worms erode and weaken dead reef skeletons, and the reef can be destroyed by storm surges.
The means by which corals expel the zooxanthellae are not yet known. In laboratory experiments the zooxanthellae are released into the gut of the polyp and then expelled through the mouth; however this method has not been observed in the environment. Another hypothesis is that the stressed corals provide the algae with fewer nutrients, which results in the algae leaving the corals. Another possibility is that the algae may produce oxides under stress, which adversely affect the algae.
Bleaching may be caused by a number of stresses or environmental changes, including disease, excess shade, increased levels of ultraviolet radiation , sedimentation , pollution , salinity changes, exposure to air by low tides or low sea level, and increased temperatures. Coral bleaching is most often associated with increased sea surface temperatures, as corals tolerate only a narrow temperature range of between about 77–84°F (25–29°C). Historically bleaching was observed on a small scale, such as in overheated tide pools. However, in the early to mid 1980s, coral reefs around the world began to experience large scale bleaching, with a bleaching event occurring somewhere in the world almost every year. In 1998, coral reefs around the world suffered the most extensive and severe bleaching and subsequent mortality in recorded history, in the same year that tropical sea surface temperatures were the highest in recorded history. Coral bleaching was reported in at least 60 countries and island nations in the Pacific Ocean, Indian Ocean, Red Sea, Persian Gulf, and the Caribbean. Only the Central Pacific region was spared. About 16% of the world's reefs were lost in a period of only nine months. Previous bleaching events had only affected reefs to a depth of less than 49 ft (15 m), but in 1998, the bleaching extended as deep as 164 ft (50 m). The reason sea temperatures throughout the world were so warm in 1998 remains controversial and uncertain. Three theories have been developed—natural climate variability, El Ninño and other climatic variations, and global warming. By the end of 2000, 27% of the world's reefs had been lost, with the largest single cause being the coral bleaching event of 1998. Only half of the reefs lost during 1998 will probably recover, which will add to the 11% already lost to human impacts such as sediment and nutrient pollution and overexploitation.
Many of the bleached coral reef ecosystems may require decades to recover. Human populations dependent on the reefs will lose fisheries, shoreline protection, and tourism opportunities. Trends of the past century indicate that coral bleaching events may become more frequent and severe if the climate continues to warm.
[Judith L. Sims ]
Wilkinson, Clive. ed.Status of Coral Reefs of the World: 2000. Queensland, Australia: Australian Institute of Marine Science, 2000.
Pomerance, Rafe. "Coral Bleaching, Coral Mortality, and Global Climate Change".Report to U.S. Coral Reef Task Force, Bureau of Oceans and International Environmental and Scientific Affairs, U.S. Department of State. March 5, 1999. [cited May 31, 2002]. <http://www.state.gov/www/global/global_issues/coral_reefs/990305_coralreef_rpt.html>.