Hoegh-Guldberg and Dove, 2008:

"The last variable that is a major determinant of where coral reefs are found is light. Because of the dependence of primary production and calcification on light, coral reefs are limited to clear tropical and subtropical waters where depths are less than 100 m. Both light quantity and quality (wavelength) are important, driving the primary step of photosynthesis of the symbionts within corals and many other photosynthetic organisms. Coral reefs only form where the average irradiance is at least 250 μmol M^-2 S^-1 (roughly 10 per cent of surface irradiances in tropical and sub-tropical regions).

Several variables affect the light available for coral reefs. Light enters the outer atmosphere of the Earth (Fig. 7.1) and is selectively filtered such that some wavelengths (ultraviolet, infrared) are largely removed by the ozone layer and water vapour (e.g. clouds). The penetration of Photosynthetically Active Radiation (PAR, 400–700nm) is also reduced by dust and clouds. Then, at the surface of the ocean, more light is reflected, with the amount reflected decreasing as the height of the waves increases. The light that enters the ocean is absorbed by water molecules, or is scattered and absorbed by dissolved compounds, plankton, and suspended sediments. These interactions are wavelength dependent such that its spectral breadth and intensity decrease with depth. In a uniform water column, the intensity of light decreases exponentially as described by Beer’s Law (Fig. 7.1)."

AND

"One of the hallmarks of coral reefs is the high number of mutualistic symbiotic relationships across a large range of organisms. These relationships number in the thousands and involve all sorts of interactions, from those between gobies and burrowing shrimp to the cellular symbioses between sponges and bacteria. One of the central hypotheses surrounding coral reefs is that the large proportion of mutualistic symbioses have arisen due to the low nutrient conditions that dictate the advantages of a close association of primary producer and consumer. The ultimate outcome of these close associations is that the inorganic nutrients required by the primary producer are obtained directly from the animal consumer. This avoids the dilution that would otherwise happen if the nutrients and organic matter were to enter the water column. There is no better example of the ultimate close association than that of reef-building corals.

Reef-building corals form a mutualistic symbiosis with single-celled dinoflagellate protists (genus Symbiodinium) that live inside the gastrodermal cells of corals where they photosynthesise, passing large amounts of captured energy to the coral host (see Fig. 7.7 and Box 7.1). In return for the energy contributed to the coral host, the symbiotic dinoflagellates receive access to inorganic nutrients arising from animal metabolism. The advantages of the close coupling of coral and Symbiodinium spp. are enormous, resulting in large photosynthetic rates that power the metabolically expensive process of calcification. Significantly, only animals that are symbiotic with Symbiodinium calcify at rates that are significant enough to contribute significant amounts of energy to reef accretion. The close relationship between corals and symbiotic dinoflagellates has been in existence for at least 220 million years and is largely responsible for the huge reserves of limestone found in the upper layers of the Earth’s crust. As pointed out elsewhere, the limestone structures generated by corals and other organisms generate the habitat for over a million species of plant, animal, fungi and bacteria worldwide."

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Citation and/or URL

Hoegh_Guldberg, O. and Dove, S., 2008, 'Primary Production, Nutrient Recycling and Energy Flow through Coral Reef Ecosystems' In, The Great Barrier Reef: biology, environment and management, eds P.A., Hutchings, M.J., Kingsford and Hoegh-Guldberg, O., CSIRO Publishing, pp. 59-73.  © ACRS 2008. http://www.publish.csiro.au/pid/5921.htm

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