A coral reef is a biologically constructed marine structure formed primarily through the accumulation and cementation of calcium carbonate produced by reef building organisms, especially reef building corals. Corals are animals belonging to the phylum Cnidaria, not plants, and reef building corals generally live as colonies of tiny polyps.
Many of these corals have a symbiotic relationship with microscopic dinoflagellate algae commonly called zooxanthellae, which live within coral tissues and provide photosynthetic products to the coral while receiving nutrients and shelter. The coral polyps secrete calcium carbonate skeletons, and generations of coral growth can build large three dimensional reef structures. A mature reef also contains many other organisms, including algae, sponges, molluscs, worms, crustaceans, fish and microorganisms. Coral reefs are therefore ecosystems as well as geological structures. Their physical framework is produced by biological activity, while erosion, cementation, sediment production and biological interactions continually modify it.
Reef building corals generally require environmental conditions that allow both coral calcification and the productivity of their symbiotic algae. Tropical shallow water reefs are commonly associated with warm, clear, sunlit marine environments because their photosynthetic symbionts need light. Clear water is also important because excessive sediment can smother corals or reduce light penetration. Many reef building corals thrive within a relatively limited temperature range and are sensitive to prolonged extreme temperatures.
Salinity, water chemistry, nutrient availability, circulation and substrate stability also affect reef development. These requirements explain why extensive shallow coral reefs are concentrated in tropical and subtropical regions. Deep water corals form important reef like structures in darker environments and do not depend on photosynthesis in the same way, demonstrating that the term coral reef encompasses systems with different ecological characteristics.
The physical framework of a coral reef develops through the balance between carbonate production and destruction. Living corals build skeletons by depositing calcium carbonate, while calcareous algae and other organisms also contribute carbonate material. When corals die, their skeletons can remain as part of the reef framework and may be colonised by new organisms. Waves and currents break skeletal material into sand and rubble, while boring organisms and chemical dissolution remove carbonate.
Microbial activity can promote cementation that binds loose material together. Over long periods, these processes create complex structures containing cavities, channels and layers of biological and sedimentary material. Reef growth is therefore not equivalent to simple vertical accumulation of living coral. It is a dynamic construction and destruction system in which biological production, physical erosion, chemical dissolution and sediment redistribution occur simultaneously.
Coral reefs support exceptionally diverse biological communities because their three dimensional structures create many habitats. Crevices, caves, reef surfaces and sediment patches provide shelter and feeding areas for different species. Corals themselves can form associations with algae, bacteria and other microorganisms. Herbivorous fish and invertebrates consume algae, predators feed on smaller animals, and decomposers recycle organic material. Competition for space can be intense, with corals, algae, sponges and other organisms interacting through growth, grazing, chemical compounds and physical occupation.
Reef ecosystems therefore involve complex food webs rather than a simple coral and fish relationship. Their biodiversity has ecological importance because species interactions contribute to nutrient cycling, habitat formation and ecosystem stability. Reef structures also provide nursery areas and feeding grounds for species that may later occupy surrounding habitats.
Coral reefs have several major morphological forms. Fringing reefs develop close to shore and may border islands or continental coastlines. Barrier reefs are separated from the land by a lagoon or other relatively deep water area and can extend for very long distances. Atolls are ring shaped reef structures surrounding a lagoon and are commonly associated with volcanic islands that have subsided or become submerged while reef growth continued near sea level.
Patch reefs are smaller isolated reef structures within lagoons or other shallow marine settings. These categories describe broad physical arrangements rather than completely separate biological entities. Geological processes such as subsidence, sea level change, island erosion and reef growth interact to produce reef morphology. The classic model of atoll development proposed by Charles Darwin remains historically important, although modern research recognises that multiple geological and ecological mechanisms can contribute to atoll formation.
Coral reefs have major ecological and human significance. They provide habitat for marine organisms, contribute to fisheries, support tourism and recreation, and can reduce some wave energy reaching coastlines. Their complex structures can dissipate wave energy and influence sediment transport, although the degree of coastal protection depends on reef morphology, water depth, wave conditions and reef condition. Reefs also participate in the global carbon cycle because carbonate production and dissolution move carbon among seawater, organisms and sediments.
Reef associated fisheries can support coastal communities, while reef tourism can have substantial economic importance. However, reef ecosystems are vulnerable to warming, ocean acidification, pollution, sedimentation, destructive fishing practices, disease and physical damage. Their ecological and economic importance makes reef conservation a significant area of marine science.
Coral reefs are vulnerable to bleaching, a phenomenon in which corals lose or expel many of their symbiotic algae and become visibly pale. Bleaching can occur in response to several stresses, with unusually warm seawater being a major trigger for large scale events. Because symbiotic algae provide important energy products, prolonged or severe bleaching can reduce coral growth and survival.
Some corals and symbiont combinations show greater tolerance to heat stress than others, and recovery is possible when stressful conditions are reduced quickly enough. Ocean acidification creates another challenge because increasing carbon dioxide in seawater alters carbonate chemistry and can make calcification more difficult for some marine organisms. Local pressures such as nutrient pollution, sediment runoff and overfishing can interact with global climate stresses. Reef health therefore depends on multiple environmental variables rather than one isolated factor.
A coral reef should be distinguished from coral itself. Coral refers to the animals and their colonies, while a reef is the larger geological and ecological structure created through the accumulation of skeletal material and associated organisms. Not every coral forms a reef, and many coral species live in environments where they contribute little to reef construction. Similarly, reefs can contain organisms other than corals that contribute substantially to carbonate production and framework stability.
Reef scientists therefore consider coral cover, species composition, structural complexity, carbonate production, erosion and associated communities when assessing reef condition. Modern understanding also emphasises that reefs can change between coral dominated and algae dominated states depending on grazing, nutrient levels, disturbance and climate. Coral reefs are consequently living ecosystems embedded within continuously changing carbonate landscapes.
