Reef
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[[Image:PamalicanAfterLiftOff.jpg|thumb|[[Pamalican]] island with surrounding reef, [[Sulu Sea]], [[Philippines]].]]
[[Image:Reef.jpg|thumb|A reef surrounding an [[islet]].]]
In nautical terminology, a '''reef''' is a [[Rock (geology)|rock]], [[bar (landform)|sandbar]], or other feature lying beneath the surface of the water yet shallow enough to be a hazard to [[ships]]. Many reefs result from [[abiotic]] processes—deposition of sand, wave erosion planning down rock outcrops, and other natural processes—but the best-known reefs are the [[coral reefs]] of [[tropical]] waters developed through [[biotic]] processes dominated by [[coral]]s and [[calcareous]] [[algae]].
Reefs can be created artificially either by special construction or through deliberately sinking ships, but one can argue that these "reefs" are not real ones, as it is seldom the case that an artificial obstruction would be created that is a hazard to shipping. These structures are usually created to enhance physical complexity on generally featureless sand bottoms in order to attract a diverse assemblage of organisms, especially [[fish]]. Thus, "[[artificial reef]]" is a misnomer, though firmly established as the term used for man-made underwater habitat structures.
== Biotic reef types ==
There are a number of biotic reef types, including [[oyster]] reefs, but the most massive and widely distributed are tropical [[coral reef]]s. Although corals are major contributors to the framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault from ocean waves are calcarous algae, especially, although not entirely, species of [[coralline algae]].
== Geologic reef definition ==
Geologists define reefs and related terms (for example, bioherm, biostrome, carbonate mound) using the factors of depositional relief, internal structure, and biotic composition. There is no consensus on one universally applicable definition. A useful definition distinguishes reefs from mounds as follows. Both are considered to be varieties of organosedimentary buildups: sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework. Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework. Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria. Excellent examples of biostromes formed by cyanobacteria occur in the Great Salt Lake of Utah (USA), and in Shark Bay, Western Australia.
Cyanobacteria do not have skeletons and individuals are microscopic. Cyanobacteria encourage the precipitation or accumulation of calcium carbonate and can produce compositionally distinct sediment bodies that have relief on the seafloor. Cyanobacterial mounds were most abundant before the evolution of shelly macroscopic organisms, but they still exist today (stromatolites are microbial mounds with a laminated internal structure). Bryozoans and crinoids, common contributors to marine sediments during the Mississippian (for example), produced a very different kind of mound. Bryozoans are small and the skeletons of crinoids disintegrate. However, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief.
== Geologic reef structures ==
Ancient reefs buried within [[Stratigraphy|stratigraphic]] sections are of considerable interest to [[geologist]]s because they provide paleo-environmental information about the location in [[History of Earth|Earth's history]]. In addition, reef structures within a sequence of [[sedimentary rock]]s provide a discontinuity which may serve as a trap or conduit for [[fossil fuel]]s or mineralizing fluids to form [[petroleum]] or [[ore]] deposits. Corals, including some major extinct groups [[Rugosa]] and [[Tabulata]], have been important reef builders through much of the [[Phanerozoic]] since the [[Ordovician]] period. However, other organism groups, such as calcifying algae, especially members of the red algae [[Rhodophyta]], and mollusks (especially the [[rudist]] bivalves during the [[Cretaceous]] period) have created massive structures at various times. During the [[Cambrian]] period, the conical or tubular skeletons of [[Archaeocyatha]],an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the [[Bryozoa]] have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the [[Scleractinia]], arose after the [[Permian]]-[[Triassic]] extinction that wiped out the earlier rugose corals (as well as many other groups), and became increasingly important reef builders throughout the [[Mesozoic]] Era. They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of [[calcite]] and have a different symmetry from that of the scleractinian corals, whose skeletons are [[aragonite]]. However, there are some unusual examples of well preserved aragonitic rugose corals in the late [[Permian]]. In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) may have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian).
== See also ==
*[[Benjamin Kahn]]
*Shears N.T. (2007) Biogeography, community structure and biological habitat types of subtidal reefs on the South Island West Coast, New Zealand. ''Science for Conservation 281''. p 53. Department of Conservation, New Zealand. [http://www.doc.govt.nz/upload/documents/science-and-technical/sfc281.pdf]
== External links ==
*[http://www.nature.org/joinanddonate/rescuereef/ Coral Reefs of the Tropics]: facts, photos and movies from [[The Nature Conservancy]]
*[http://www.photolib.noaa.gov/reef/ NOAA Photo Library]
*[http://www.reef.org/ Reef Environmental Education Foundation]
*[http://nosdataexplorer.noaa.gov/nosdataexplorer/ NOS Data Explorer] - A portal to obtain NOAA National Ocean Service data
[[Category:Reefs| ]]
[[Category:Stratigraphy]]
[[Category:Coastal geography]]
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