Reef

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.^[1] Many reefs result from natural, abiotic (non-living) processes such as deposition of sand or wave erosion planing down rock outcrops. However, reefs such as the coral reefs of tropical waters are formed by biotic (living) processes, dominated by corals and coralline 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 of organisms. 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 the Great Barrier Reef in Australia, at a length of over 2,300 kilometres (1,400 miles).^[4]

When corals are exposed to high-stress events such as temperature change, air exposure, sun exposure, and nutrient availability, the zooxanthellae algae that live within the coral's tissues are expelled, leaving the coral colorless- or bleached. The most common coral stressor is exposure to heat.^[5][6]

The loss of energy-producing algae leaves the coral exposed, weak, and nutrient deficient. This affects the coral's ability to provide for marine ecosystems.^[7] The algae are unable to return back to the coral's tissues so long as the stress event that caused the initial bleaching still remains. If the stress event is not mitigated, the coral will eventually die. Corals are important species in marine ecosystems. Corals act as resources for a third of marine populations, using them for habitat, protection, and spawning grounds.^[8]

A global coral bleaching event is a significant period of mass coral bleaching in reefs across the globe. Coral reefs and the monitoring of bleaching activity in them is primarily overseen by the Global Coral Reef Monitoring Network (GCRMN)​, International Coral Reef Initiative (ICRI)^[9], and the National Oceanic and Atmospheric Administration (NOAA) (for the United States).^[10]

In February 2023, significant coral bleaching was observed in reefs, which continued until April 2024, with bleaching observed across at least 83 different countries. This major event was classified by the ICRI and the NOAA as our 4th major global coral bleaching event.^[11][12] The first 3 bleaching events occurred across 18 years, the first event observed in 1998, and the 3rd event occurring in late 2015 to early 2016.^[13] 84% of the world's reefs have been affected by bleaching during the recent 4th global bleaching event. During the 1st global bleaching event in 1998, only 21% of the world's reefs saw bleaching impacts.

Coral reefs act as a barrier for coastlines, blocking forceful waves and storms from causing extreme damage to shores and the buildings and people that live on them. With the depletion of coral reefs, these coastlines are significantly more vulnerable to damages from storms, potentially costing lives and millions of dollars in property damages.^[14]

The main conservation and mitigation efforts for coral reefs are identifying high-risk areas and observing for emergency events, so that emergency response is possible and quick. Repopulating coral, mitigating stress environments, and improving existing water quality are all main ways of reviving and protecting our world's coral reefs.^[15][16]

The word "reef" traces its origins back to the Old Norse word rif, meaning "rib" or "reef". Rif comes from the Proto-Germanic term ribją meaning "rib".^[17]

Reefs may be classified in terms of their origin, geographical location, depth, and topography. For example a tropical coral fringing reef, or a temperate rocky intertidal reef.

A variety of biotic reef types exists, including oyster reefs and sponge reefs, but the most massive and widely distributed are tropical coral 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 are calcareous 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 like sponges and seaweeds 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 include fringing reefs, barrier reefs, and atolls. A fringing reef is a reef that is attached to a landmass. Whereas, a barrier reef forms a calcareous barrier around a landmass, resulting in a lagoon 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 surrounding sea 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.

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 by cyanobacteria. Examples of biostromes formed by cyanobacteria occur in the Great Salt Lake in Utah, United States, and in Shark Bay on the coast of Western Australia.^[18]

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 and crinoids, common contributors to marine sediments during the Mississippian 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. ^[19]

The Proterozoic Belt Supergroup contains evidence of possible microbial mat and dome structures similar to stromatolite reef complexes.^[20]

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.^[21] They are often located in sub-tropical, temperate, and sub-polar latitudes.

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.^[22]

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.^[23] 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). They became increasingly important reef builders throughout the Mesozoic Era.^[24] 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.^[25] 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).

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,^[26] or improve surfing.^[27]

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 and Electrified Reefs). 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.

• Coral reef – Outcrop of rock in the sea formed by the growth and deposit of stony coral skeletons
• Placer (geography) – Submerged bank or reef
• Pseudo-atoll – Island that encircles a lagoon
• Reef Hobbyist Magazine – US magazine focused on reef life

• 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 1 February 2024. [2]
• "Coral Reefs ~ Marinebio Conservation Society." MarineBio Conservation Society, 10 November 2023. [3]

• Reef Rescue – Smithsonian Ocean Portal
• Coral Reefs of the Tropics Archived 19 February 2011 at the Wayback Machine: facts, photos and movies from The Nature Conservancy
• NOAA Photo Library
• Reef Environmental Education Foundation
• NOS Data Explorer – A portal to obtain NOAA National Ocean Service data
• Reef formation
• Atolls – Distribution, Development and Architecture