
A reef is a ridge or
shoal of rock,
coral or similar relatively stable material, lying beneath the surface of a natural body of water.
Many reefs result from natural,
abiotic processes—
deposition of sand, wave erosion planing down rock outcrops, etc.—but there are also reefs such as the
coral reefs of tropical waters formed by
biotic processes dominated by corals and
coralline algae, and
artificial reefs such as shipwrecks and other anthropogenic underwater structures may occur intentionally or as the result of an accident, and sometimes have a designed role in enhancing the physical complexity of featureless sand bottoms, to attract a more diverse assemblage of organisms. Reefs are often quite near to the surface, but not all definitions require this.
Earth's largest coral reef system is the
Great Barrier Reef in Australia, at a length of over .
Biotic reef
There is a variety of biotic reef types, including
oyster reefs and
sponge reefs, but the most massive and widely distributed are tropical
coral reefs.
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
calcareous algae, especially, although not entirely, coralline algae.
The preferred substrate for oyster larvae is the shells of oysters so they tend to settle on adult oysters and thereby develop layers building upwards, eventually forming a fairly massive hard stony calcium carbonate structure on which other reef organisms like sponges and seaweeds can grow, and provide a habitat for mobile benthic organisms.
These biotic reef types take on additional names depending upon how the reef lies in relation to the land, if any. Reef types include
fringing reefs,
barrier reefs, and
atolls. A fringing reef is a reef that is attached to an island. A barrier reef forms a calcareous barrier around an island resulting in a lagoon between the shore and the reef. An atoll is a ring reef with no land present. The reef front (ocean side) is a high energy locale whereas the internal
lagoon will be at a lower energy with fine grained sediments.
Biotic mound structures
One 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 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. Examples of
biostromes formed by cyanobacteria occur in the
Great Salt Lake in
Utah, and in
Shark Bay on the coast of
Western Australia.
Cyanobacteria do not have skeletons, and individuals are microscopic. Cyanobacteria can encourage the precipitation or accumulation of calcium carbonate to produce distinct sediment bodies in composition 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 instance, 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.
The
Proterozoic Belt Supergroup contains evidence of possible
microbial mat and dome structures similar to stromatolite and chiggen reef complexes.
Geologic reef
Rocky reefs are underwater outcrops of rock projecting above the adjacent unconsolidated surface with varying relief. They can be found in depth ranges from intertidal to deep water, and provide a substrate for a large range of sessile benthic organisms, and shelter for a large range of mobile organisms.
Geologic reef structures
Ancient reefs buried within
stratigraphic sections are of considerable interest to
geologists because they provide paleo-environmental information about the location in
Earth's history. In addition, reef structures within a sequence of
sedimentary rocks provide a discontinuity which may serve as a trap or conduit for
fossil fuels 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 molluscs (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 event 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).
Artificial reef
An artificial reef is a human-created underwater structure, typically built to promote
marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of
trawling nets,
or improve
surfing.
Many reefs are built using objects that were built for other purposes, for example by sinking oil rigs (through the
Rigs-to-Reefs program),
scuttling ships, or by deploying
rubble or
construction debris. Other artificial reefs are purpose built (e.g. the
reef balls) from
PVC or concrete. Shipwrecks become artificial reefs on the seafloor. Regardless of construction method, artificial reefs generally provide stable hard surfaces where
algae and invertebrates such as
barnacles,
corals, and
oysters attach; the accumulation of attached marine life in turn provides intricate structure and food for
assemblages of fish.
See also
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References
Sources
* 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
External links
''Reef Rescue'' - Smithsonian Ocean PortalCoral Reefs of the Tropics facts, photos and movies from
The Nature Conservancy
NOAA Photo LibraryReef Environmental Education Foundation NOS Data Explorer- A portal to obtain NOAA National Ocean Service data
Atolls – Distribution, Development and Architecture
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Category:Stratigraphy
Category:Coastal geography
Category:Coastal and oceanic landforms