Areef is a ridge orshoal of rock,coral, or similar relatively stable material lying beneath the surface of a natural body of water.[1] Many reefs result from natural,abiotic (non-living) processes such asdeposition of sand orwave erosion planning down rock outcrops. However, reefs such as thecoral reefs of tropical waters are formed bybiotic (living) processes, dominated by corals andcoralline algae.Artificial reefs, such as shipwrecks and other man-made underwater structures, may occur intentionally or as the result of an accident. These are sometimes designed to increase the physical complexity of featureless sand bottoms to attract a more diverse range oforganisms. They provide shelter to various aquatic animals which help prevent extinction.[2] Another reason reefs are put in place is for aquaculture, and fish farmers who are looking to improve their businesses sometimes invest in them.[3] Reefs are often quite near to the surface, but not all definitions require this.[1]
Earth's largest coral reef system is theGreat Barrier Reef in Australia, at a length of over 2,300 kilometres (1,400 miles).
Reefs may be classified in terms of their origin, geographical location, depth, andtopography. For example a tropical coral fringing reef, or a temperate rocky intertidal reef.
A variety of biotic reef types exists, includingoyster reefs andsponge reefs, but the most massive and widely distributed are tropicalcoral reefs.[1] 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 arecalcareous algae, especially, although not entirely,coralline algae.
Oyster larvae prefer to settle on adult oysters and thereby develop layers building upwards. These eventually form a fairly massive hard stony calcium carbonate structure on which other reef organisms likesponges andseaweeds can grow, and provide a habitat for mobile benthic organisms.[1]
These biotic reef types take on additional names depending upon how the reef lies in relation to the land, if any. Reef types includefringing reefs,barrier reefs, andatolls. A fringing reef is a reef that is attached to an island. Whereas, a barrier reef forms a calcareous barrier around an island, resulting in alagoon between the shore and the reef. Conversely, an atoll is a ring reef with no land present.
The reef front, facing the ocean, is a high energy locale. Whereas, the internal lagoon will be at a lower energy with fine grained sediments.
Both mounds and reefs are considered to be varieties of organosedimentary buildups, which are sedimentary features, built by the interaction of organisms and their environment. These interactions have a synoptic relief and whose biotic composition differs from that found on and beneath the surroundingsea floor. However, reefs are held up by a macroscopic skeletal framework, as what is seen on coral reefs.Corals and calcareous algae grow on top of one another, forming a three-dimensional framework that is modified in various ways by other organisms and inorganic processes.[5]
Conversely, mounds lack a macroscopic skeletal framework. Instead, they are built by microorganisms or by organisms that also lack a skeletal framework. A microbial mound might be built exclusively or primarily bycyanobacteria. Examples ofbiostromes formed by cyanobacteria occur in theGreat Salt Lake inUtah, United States, and inShark Bay on the coast ofWestern Australia.[5][6]
Cyanobacteria do not have skeletons, and individual organisms are microscopic. However, they 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, for instance, are microbial mounds with a laminated internal structure. Whereas,bryozoans andcrinoids, common contributors to marine sediments during theMississippian period, produce a different kind of mound. Although bryozoans are small and crinoid skeletons disintegrate, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief.[5][7]
Rocky reefs are underwater outcrops of rock projecting above the adjacent unconsolidated surface with varying relief. They can be found in depth ranges fromintertidal to deep water and provide a substrate for a large range of sessile benthic organisms, and shelter for a large range of mobile organisms.[9] They are often located in sub-tropical, temperate, and sub-polar latitudes.
This sectionneeds expansion with: relative abundance of rocky vs biotic reef and global distribution, ecological importance. You can help byadding to it.(February 2021)
Ancient reefs buried withinstratigraphic sections are of considerable interest togeologists because they provide paleo-environmental information about the location inEarth's history. In addition, reef structures within a sequence ofsedimentary rocks provide a discontinuity which may serve as a trap or conduit forfossil fuels or mineralizing fluids to formpetroleum orore deposits.[10]
Corals, including some major extinct groupsRugosa andTabulata, have been important reef builders through much of thePhanerozoic since theOrdovician Period. However, other organism groups, such as calcifying algae, especially members of the red algae (Rhodophyta), and molluscs (especially therudist bivalves during theCretaceous Period) have created massive structures at various times.
During theCambrian Period, the conical or tubular skeletons ofArchaeocyatha, an extinct group of uncertain affinities (possibly sponges), built reefs.[11] Other groups, such as the Bryozoa, have been important interstitial organisms, living between the framework builders. The corals which build reefs today, theScleractinia, arose after thePermian–Triassic extinction event that wiped out the earlier rugose corals (as well as many other groups). They became increasingly important reef builders throughout theMesozoic Era.[12] They may have arisen from a rugose coral ancestor.
Rugose corals built their skeletons ofcalcite and have a different symmetry from that of the scleractinian corals, whose skeletons arearagonite.[13] However, there are some unusual examples of well-preserved aragonitic rugose corals in theLate 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).[5]
An artificial reef is a human-created underwater structure, typically built to promotemarine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use oftrawling nets,[14] or improvesurfing.[15]
Many reefs are built using objects that were built for other purposes, for example by sinking oil rigs (through theRigs-to-Reefs program),scuttling ships, or by deployingrubble orconstruction debris. Other artificial reefs are purpose built (e.g. thereef balls) fromPVC or concrete. Shipwrecks become artificial reefs on the seafloor. Regardless of construction method, artificial reefs generally provide stable hard surfaces wherealgae and invertebrates such asbarnacles, corals, andoysters attach; the accumulation of attached marine life in turn provides intricate structure and food forassemblages of fish.
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.[1]
“General Information on Reefs.” General Information on Reefs – Reef & Ocean Ecology Lab. Accessed February 1, 2024. https://www.reefoceanlab.org.au/resources/general-information-on-reefs/#:~:text=Rocky%20reefs%20are%20more%20typical,many%20parts%20of%20New%20Zealand.
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