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Article by: Hubbard, Dennis Geology Department, Oberlin College, Oberlin, Ohio. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.576800 (http://dx.doi.org/10.1036/1097-8542.576800)

Content

Coral reefs Reefs under threat Additional Readings Reefs through time Bibliography

A rigid and wave-resistant marine structure that stands above its surroundings. Biologists and geologists specify that reefs are constructed by organisms that secrete calcium carbonate skeletons. To navigators, a reef is any rocky structure that poses a threat to navigation.

Coral reefs

Modern reef-builders include algae, mollusks, bryozoans, worms, and sponges, but the most important are corals.

Distribution

Coral reefs are most common in warm water (typically ≥20°C) in which calcium carbonate precipitates more easily. Most shallow-water corals are hermatypic, and they host symbiotic zooxanthellae (algal-like dinoflagellates) that provide metabolites through photosynthesis. This provides energy for the coral while removing carbon dioxide that inhibits calcification.

While coral reefs are more common in the shallow tropics, they can be found in both deeper and colder water, where they do not have zooxanthellae. According to the National Oceanic and Atmospheric Administration, reefs dominated by the coral Lophelia pertusa occur off the coast of Florida at depths of 500–850 m; similar features built by Oculina varicosa are found at water depths of 70–100 m in the Gulf of Mexico and the Straits of Florida. Off the coast of Norway, Lophelia reefs occur at depths of 75–155 m, at temperatures ranging from 4 to 13°C . Coral diversity is much lower in these deep-water reefs, but they can harbor a variety of organisms. Between 350 and 400 species of mollusks, crustaceans, and other fauna have been reportedly found within the branches of Oculina off the eastern coast of Florida. See also: Scleractinia (/content/scleractinia /607500)

Living corals and reefs

Most corals are colonial, with each individual polyp measuring from several millimeters to a few centimeters across (Fig. 1a). Each polyp secretes an external calcium carbonate skeleton and sits within a cuplike calyx (Fig. 1b) that is part of the larger colony (Fig. 1c). At night, the coral polyp extends its tentacles to feed. During the day, the coral retreats into its “skeletal cup” and relies on zooxanthellae for nutrition.

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Fig. 1 Photographs illustrating the relationship between (a) individual coral polyps, (b) the skeletal structures that they form, and (c) the colony in which they live. The hemispherical colony of Montastraea faveolata (c; foreground) sits on a sloping Caribbean forereef in 10 m of water. Branching colonies of Acropora palmata (c; background) occupy shallower water near the reef crest. (d) The branching/table-like corals near the crest are responding to similar conditions and give way to more massive forms in deeper water along the Great Barrier Reef of Australia.

Reef types

Generally, coral reefs can be divided into three categories: fringing, barrier, and atoll. This classification is based on Darwin's original observations of mid-Pacific reefs.

Volcanoes form as a moving oceanic plate passes over a rising plume of hot mantle (Fig. 2). As the seafloor moves away from this hot spot, the volcanic lava (basalt) cools and subsides. Coral reefs initially form along the steep slopes of a new volcano in shallow waters close to shore. As the volcano subsides (moves away and sinks), reefs build vertically to stay close to sea level and the light needed for photosynthesis; the lagoon behind the reef progressively widens, and the influence of land decreases. Fringing reefs, which are closest to shore, occur around newly formed and thus high volcanic islands. As the volcanic island slowly subsides, reefs transition to barriers, and eventually to atolls, once the volcano has sunk from sight beneath the surface waters. See also: Atoll (/content/atoll/059700); Oceanic islands (/content/oceanic-islands/464200); Volcano (/content/volcano/735200)

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Fig. 2 Diagram of the evolution of fringing reefs into barrier reefs and atolls as subsiding volcanoes move away from a hot spot. The inset shows the trend of volcanic islands (dark color) and seamounts (sunken volcanoes, light color) associated with the hot spot near Hawaii. The volcanoes get older away from Hawaii.

Coral zonation

The species and shapes of corals on a reef are controlled primarily by light availability, wave energy, and sedimentation, resulting in predictable reef zones (Fig. 3). Branching corals, such as Acropora palmata in the Caribbean Sea (Fig. 1c; background), occur along the reef crest, where abundant light drives the photosynthesis needed to support rapid growth. Vigorous wave energy clears sediment from the branching species that have no biological means of sediment removal. Although the species of branching and table-like corals vary by ocean, these types of corals usually occur near the reef crest, in response to similar processes (Fig. 1d). Further down the reef front, slower-growing hemispherical corals (Fig. 1c, foreground; Mixed/Massive Zone in Fig. 3) are better suited for removing sediment coming down the slope. In even deeper water, platelike colonies (Fig. 3) respond to continually decreasing light levels. This shape places all the polyps on upward- facing surfaces, optimizing the colony's ability to gather light—much like solar panels.

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Fig. 3 Zonation on a typical Caribbean reef. The shift from branching corals to mixed/massive and platy corals down the reef front is a response to decreasing light and wave energy, accompanied by increasing sedimentation with depth.

Reef building

Reefs have traditionally been thought of as structures built by intact and interlocking corals growing atop one another over the course of hundreds to thousands of years (6000–12,000 years for modern reefs). However, recent coring studies have recognized that much of the reef interior consists of toppled and degraded corals, sediment, and open cavities. Most of the sediment is produced by (1) fish and urchins grazing on algae that cover dead coral surfaces and (2) sponges, mollusks, and worms that excavate into the carbonate substrate seeking shelter (a process known as bioerosion). This nearly equal mix of intact and broken corals plus loose sediment is subsequently bound together by encrusting organisms (for example, coralline algae) and carbonate cement. Thus, the edifice that is the coral reef owes its final structure as much to bioerosion, cementation, and encrustation as it does to skeletal production by corals. See also: Algae (/content/algae/022000); Corallinales (/content/corallinales/161700)

Reefs through time

Reefs have not always been built by corals. Microbial stromatolites formed nonskeletal mounds approximately 3 billion years ago (Fig. 4). Photosynthesis by these widespread reef-builders provided the oxygen needed for the evolution of complex organisms. Over the ensuing 550 million years, the evolution of reef-building organisms was controlled by changes in ocean chemistry (which affected calcification), changes in sea level (which increased or decreased space for colonization), increases in habitat variability, and increases in competition and predation. See also: Paleoecology (/content/paleoecology /483700)

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Fig. 4 Major reef-building episodes (REEFS column) through geologic time. The major organisms responsible are listed. The origin of other important organisms that were associated with these reefs are also indicated (ACCESSORIES column).

In the mid-Cambrian period (543–490 million years ago), archaeocyathids, followed by early corals (rugose and tabulate) and stromatoporoids (calcifying sponges), may have evolved due to increased grazing pressure (Fig. 4). Early coral evolution paralleled the rapid evolution of herbivorous fish, particularly those with jaws in the Devonian (417–354 million years ago). The diversification of photosynthesizing dinoflagellates may have coincided with the radiation of both grazing and boring species from Triassic through Cretaceous time (approximately 250–65 million years ago). In this regard, increases in grazing/boring versus calcification resembled an “arms race” between organisms that built reefs and those that bioeroded them for food or shelter. See also: Archaeocyatha (/content/archaeocyatha/047400); Rugosa (/content/rugosa/595600); Stromatoporoidea (/content/stromatoporoidea/661100); Tabulata (/content/tabulata/676100)

Reefs under threat

Dramatic changes have occurred in reefs over the past few decades. In the mid-1970s, coral cover commonly exceeded 50–75%. Since then, coral abundance has plummeted on many reefs, especially those closer to population centers. The low nutrient levels characteristic of healthy reefs rose in response to increased septic waste, agricultural fertilizers, and sedimentation. Macroalgae (seaweeds) that were previously limited by low nutrients suddenly took over. Sedimentation from dredging and deforestation not only blocked sunlight needed for photosynthesis, but it also eliminated the hard, stable substrate needed for coral recruitment. The ensuing shift from coral-dominated reefs to dense macroalgal communities was exacerbated by overfishing (especially of grazers that kept algae populations balance) and the loss of other key species, such as the loss of the long-spined sea urchin (Diadema altillarum) in 1983 from disease. Even the deep reefs described earlier are under threat, mostly by breakage related to trawling for deep-sea fishes.

Recently, global factors have added to the more direct assaults on reef habitat and the organisms that keep the benthos in balance. Rising temperatures have caused corals to expel their zooxanthellae, exposing the bare skeleton through the

5 of 7 9/12/2016 6:38 AM Reef - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/reef/576800 remaining clear coral tissue (bleaching). If temperatures remain elevated for more than 2–3 weeks, the corals die. Starting with white band disease in the late 1970s, corals have been assaulted by episodes of tissue degeneration. Arguments abound over the specific causes of recent declines, but the scientific community is unanimous in its assessment of likely anthropogenic ties for the global degradation of coral reefs.

Dennis Hubbard

Bibliography

D. K. Hubbard, Dynamic processes of coral-reef development, in C. Birkeland (ed.), Life and Death of Coral Reefs, Chapman and Hall, 1997

T. P. Hughes, Catastrophes, phase shifts and large-scale degradation of a Caribbean coral reef, Science, 265:1547–1551, 1994 DOI: 10.1126/science.265.5178.1547 (http://dx.doi.org/10.1126/science.265.5178.1547)

J. Pandolfi et al., Are U.S. coral reefs on the slippery slope to slime?, Science, 307:1725–1726, 2005 DOI: 10.1126/science.1104258 (http://dx.doi.org/10.1126/science.1104258)

L. L. Richardson, Coral diseases: What is really known?, Trends Ecol. Evolut., 13:438–443, 1998 DOI: 10.1016/S0169-5347(98)01460-8 (http://dx.doi.org/10.1016/S0169-5347(98)01460-8)

G. D. Stanley, The History and Sedimentology of Ancient Reef Systems, Kluwer Academic/Plenum Press, New York, 2001

Additional Readings

P. Hutchings, M. Kingsford, and O. Hoegh-Guldberg, The Great Barrier Reef: Biology, Environment and Management, CSIRO Publishing, Collingwood, Victoria, Australia, 2008

A.Mascarelli, Climate-change adaptation: Designer reefs, Nature, 508(7497):444–446, 2014 DOI: 10.1038/508444a (http://dx.doi.org/10.1038/508444a)

J. M. Pandolfi et al., Projecting coral reef futures under global warming and ocean acidification, Science, 333(6041):418–422, 2011 DOI: 10.1126/science.1204794 (http://dx.doi.org/10.1126/science.1204794)

National Geographic: Great Barrier Reef (http://ngm.nationalgeographic.com/2011/05/great-barrier-reef/holland-text)

National Oceanic and Atmospheric Administration: Coral Reef Watch (http://coralreefwatch.noaa.gov/satellite/index.php)

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