Coral Reefs

coral reefs, limestone formations produced by living organisms, found in shallow, tropical marine waters. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). The accumulation of skeletal material, broken and piled up by wave action, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator; the reef-forming corals do not grow at depths of over 100 ft (30 m) or where the water temperature falls below 72°F (22°C). Corals are not the only, and in some cases not even the major, reef-forming organisms. Calcium carbonate is also deposited by coralline algae, the protozoan foraminiferans, some mollusks, echinoderms, and tube-building annelid worms. However, any reef formed by a biological community is usually called a coral reef.

Geologically, coral reefs are classified into three main types. Fringing reefs are coral platforms that are more or less continuous with the shore and exposed at low tide. Barrier reefs are separated from the shore by a wide, deep lagoon or surround a lagoon that has a central island. An atoll is a reef surrounding a lagoon that has no central island, with passages through the reef to the sea. It is generally believed that fringing reefs formed as a result of upward and outward growth of corals that became established on rocks near shore; there is disagreement about the nature of barrier reef and atoll formation. Charles Darwin postulated a progression from fringing reef to barrier reef to atoll, as a result of a slow, steady sinking of the seafloor that creates a lagoon and a simultaneous upward and outward growth of coral. Where entire volcanic islands sink, only the reef remains above water, forming an atoll. Not all scientists accept Darwin's proposal, but most current theories involve subsidence of the seafloor, although changes of the ocean level may also be involved.

Sediments accumulate on the lagoon side of atolls and support vegetation; in time the entire lagoon may fill, creating an island. Many such atolls and islands, common in the Pacific and Indian oceans, are inhabited. The Great Barrier Reef of NE Australia is the largest known complex of coral reefs. It is 10 to 90 mi (16–145 km) wide and about 1250 mi (2010 km) long, and is separated from the shore by a lagoon 10 to 150 mi (16–240 km) wide.

Reefs are under numerous environmental pressures, including damage from increased coastal development, water pollution, tourism, runoff containing agricultural chemicals, abrasion by ships' hulls and anchors, and smothering by upstream sedimentation. Coral reefs are sometimes destroyed in fishing when poison or dynamite are used to catch fish and by the harvesting of coral for use in jewelry. During the 1990s, many previously unknown diseases began attacking coral reefs worldwide, causing rapidly spreading damage. Also since the 1990s increasingly warm ocean temperatures have led to recurring episodes of bleaching; caused by heat stress, bleaching results when coral polyps expel the colorful algae they host and depend on. If the water fails to cool in time for the algae to become reestablished, the coral polyps die.

See A. Emery, The Coral Reef (1981); J. A. Fagerstrom, The Evolution of Reef Communities (1987).

The Columbia Encyclopedia, 6th ed. Copyright© 2018, The Columbia University Press.

Coral Reefs: Selected full-text books and articles

Coral Reefs under Threat By McKeown, Alice World Watch, Vol. 23, No. 1, January-February 2010
Impacts of Climate Change on Coral Reefs and the Marine Environment By Creary, Marcia UN Chronicle, Vol. 50, No. 1, April 2013
Saving the Rainforests of the Sea: An Analysis of International Efforts to Conserve Coral Reefs By Mulhall, Marjorie Duke Environmental Law & Policy Forum, Vol. 19, No. 2, Spring 2009
Adapting to a Changing Environment: Confronting the Consequences of Climate Change By Tim R. McClanahan; Joshua E. Cinner Oxford University Press, 2012
Librarian's tip: Chap. 4 "Climate Change and the Resilience of Coral Reefs"
In Hot Water: Global Warming Takes a Toll on Coral Reefs By Schmidt, Charles W Environmental Health Perspectives, Vol. 116, No. 7, July 2008
Peer-reviewed publications on Questia are publications containing articles which were subject to evaluation for accuracy and substance by professional peers of the article's author(s).
The Reel Downfall of Reefs: Controlling Fishing to Save the Coral Reefs By Pala, Christopher E Magazine, Vol. 19, No. 4, July-August 2008
Environment and Tourism By Andrew Holden Routledge, 2003
Librarian's tip: Discussion of coral reefs begins on p. 78
The Geography of Tourism and Recreation: Environment, Place, and Space By C. Michael Hall; Stephen J. Page Routledge, 2002 (2nd edition)
Librarian's tip: "Tourism and Coral Reefs" begins on p. 157
State of Coral Reefs Management: Case Study of Okinawa Island, Japan By Mahichi, Faezeh; Arii, Ken; Sanga-Ngoie, Kazadi; Kobayashi, Shoko Journal of International Business Research, Vol. 11, No. S3, December 2012
Peer-reviewed publications on Questia are publications containing articles which were subject to evaluation for accuracy and substance by professional peers of the article's author(s).
Encyclopedia of Earth and Physical Sciences By Joyce Tavolacci Marshall Cavendish, vol.3, 2005 (2nd edition)
Librarian's tip: "Coral Reefs" begins on p. 338
The Princeton Guide to Ecology By Simon A. Levin Princeton University Press, 2012
Librarian's tip: Part IV Chap. 8 "Seascape Patterns and Dynamics of Coral Reefs"
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