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Coral reef

Submitted by Mousumi Sepai, Last Modified on 2021-03-15

Coral reefs are tropical,under water, swallow water, calcareous structures supporting a diverse association of marine plants and animals. A reef mainly consist of reef building cnidarians for example, colonies of coral polyps and of calcareous red algae held together by calcium carbonate.Most of the time coral reefs are formed by stony corals, bunch of polyps groups. These structures form a enter locking frame work upon which are deposited sediments that derived from the break down of skeleton materials and of other organisms secreting caco3.All these organisms contribute to the reef formation. Scleractinian corals plays the most important role for reef building. Coral reefs play very important role in ecosystem services for tourism, fisheries and shoreline protection.

Distribution of coral reef:

  • Coral animals thrive based in warm, swallow,well-illuminated and well-oxygeneted water.
  • The condition are found in tropical and subtropical seas.
  • Coral reefs are confined between 30 north and 30 south latitudes.
  • Coral reefs are normally found in Caribbean sea including Florida, Bahamas and west indies, in the east coast of Africa including Malagasy,Maldives, the Malayan Archipelago, coast of Australia and newmarous Pacific islands.
  • In India coral reefs are found in both east and west coast including lakshadweep and Andaman and Nicobar islands.
  • Coral reefs extend over an area of some 68 million square miles in tropical and subtropical seas.

Conditions for growth of coral reefs:

Coral reefs are fragile, partly because they are sensitive to water conditions. They are under threat from excess nutrients (nitrogen and phosphorus), temperatures up and down, acid level, excess fishing (e.g., from blast fishing, cyanide fishing, spearfishing on scuba), chemical products, and harmful land-use practices, including runoff and seeps.

  • Physical Condition:
    1. Light:Reef building corals(hermatypic corals) grow only in shallow waters ranging to a depth of 60 meter where they ridge sufficient light to support photosynthesis.
    2. Temperature:Temperature limits distribution of corals .Corals are restricted to waters where the minimum temperature does not fall below 18 to 20 c. Temperature above 29 c is lethal.

    3. Water movement:

      Coral reefs are continuously washed by oceanic currents which deliver oxygen and nutrients, and remove waste products. However extremely stronge currents and waved action are detrimental.
  • Chemical conditions:
    1. Salinity:[Average 35 ppt (27 ppt Nacl + 8ppt other salts)] Coral reefs grow in water with fairly stables salinity . Heavy rain falls and runs off from rivers reduce salinity which can be lethal to corals.
    2. Oxygen: Coral reefs occur where the water is well oxygeneted.

Types of coral reefs:

  1. Fringing reef(shore reef):

    These reefs lie much closer to the shore or border and this may be an island or a straight of continental coast line. In either case the intervaning water constitutes only a narrow Chanel or lagoon. These are the most common types of reefs. The fringing reef is extended from the shore up to 400 meters and takes the contour of the shore. It has a reef edge, called the front where the most active coral growth occurs. Fringing reefs are mainly formed on the shore line at the low water level and expand towards the sea as they grow in size and the final width depends on where the sea bed level begins to drop steeply. Between the front and the shore there lies a more or less flat surface which is known as the reef flat. The fringing reef is largely composed of coral, sand, mud, dead coral and other debris. It is partly composed of coral colonies and other animals. Fringing reef is cut by a narrow water channel Lagoon and the depth of the water channel is 37 to 55 meters. The fringing reefs of the Red Sea are "some of the best developed in the world" and occur along all its shores except off sandy bays.

  2. Barrier reef:

    Barrier reefs are separated from the mainland or shore by a deep channel caled lagoon. These are similar to the later stages of a fringing but are separated from the adhasent landmass by a lagoon of 90-110meters deep. The typical example of the barrier reef is the great Barrier reef of the northern coast of Australia known as Great Barrier Reef. It resembles the fringing coral reef, but it follows the contour of the shore less regularly. Other major examples are the Belize Barrier Reef and the New Caledonian Barrier Reef. Barrier reefs are also found on the coasts of Providencia, Mayotte, the Gambier Islands, on the south-east coast of Kalimantan, on parts of the coast of Sulawesi, south-eastern New Guinea and the south coast of the Louisiade Archipelago.

  3. Atoll:

    These are usually oceanic. They are oval or circular or horse-shoe-shaped reef and encloses a circle lagoon without a central island. They are usually formed from the fringing reefs which are mostly around volcanic islands . Over time, the island be gradually worn away by natural agents and sinks below sea level sometimes it is also be formed by the sinking of the seabed or rising of the sea level. The parts of the reef platforms may emerged as islands breaks in the reef provide passage into the central lagoon which are almost from a few k.m to 64-80 k.m across and the depth of the lagoon is 37 to 55 meters. The lagoon usually contains inner islands and reef. Over 300 atolls are present in the Indo-Pacific region, mostly in the South Pacific, where they usually occur in mid-ocean, like in the Caroline Islands, the Cook Islands, French Polynesia, the Marshall Islands and Micronesia.

  4. Platform reef:

    Platform reefs, variously called bank or table reefs, can form on the , offshore rights, middle of the ocean, any suitable place where the seabed rises close enough to the surface of the ocean to where the reef building corals can able to survive . These are flat structures without lagoons. They raised on the shallowed part of the continental shell and may appear between the coast and the barrier reefs and they may associated with atolls .They are also regarded as patch reefs. They found in various size, from a few hundred metres to many kilometres across. They are usually is oval to elongated. Sometimes the parts of these types of reefs can attached to the the surface and form sandbanks which leads to small islands around which are basic structure of the fringing reefs. A lagoon may form In the middle of a platform reef. platform reefs of the northern Mascarenes are several thousand kilometres from the mainland.

Other reef types or variants

  • Apron reef - This type reefs are and resembling a fringing reef, but more sloped, growing out and downward from a point or peninsular shore. The initial stage of a fringing reef.
  • Bank reef – isolated, flat-topped reef larger than a patch reef and usually on mid-shelf regions and linear or semi-circular in shape; a type of platform reef.
  • Patch reef – common, isolated, comparatively small reef outcrop, usually within a lagoon or embayment, often circular and surrounded by sand or seagrass. Type of platform reef.
  • Ribbon reef – long, narrow, possibly winding reef, usually associated with an atoll lagoon. Also called a shelf-edge reef or sill reef.
  • Habili – reef specific to the Red Sea; does not reach near enough to the surface to cause visible surf; may be a hazard to ships (from the Arabic for "unborn"),
  • Microatoll – community of species of corals; vertical growth limited by average tidal height; growth morphologies offer a low-resolution record of patterns of sea level change; fossilized remains can be dated using radioactive carbon dating and have been used to reconstruct Holocene sea levels.
  • Cays –

    small, low-elevation, sandy islands formed on the surface of coral reefs from eroded material that piles up, forming an area above sea level; can be stabilized by plants to become habitable; occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including the Caribbean and on the Great Barrier Reef and Belize Barrier Reef), where they provide habitable and agricultural land

  • Seamount or guyot –

    formed when a coral reef on a volcanic island subsides; tops of seamounts are rounded and guyots are flat; flat tops of guyots, or tablemounts, are due to erosion by waves, winds, and atmospheric processes

Profile of a reef:

  • All the 3 reef types show certain similarities I profile.
  • The seaward side of the reef, called the reef front.
  • Rises from lower depth to a level just below the water surface.
  • The inclination of the reef front varies from gentle to steep slope . In some reefs the lower part of the front in vertical.
  • The reef front does not present a flat wall like barrier to the incoming wave energy.
  • Instead, there are finger like sea ward projections, that alternate with deeper sand filled pockets . These formation disperses wave energy.
  • Behind the reef front is a reef flat or reef platform.
  • It may be short or may extend back several hundred meters. The reef platforms may be dissected by channels several meters depth.
  • The reef flat may be paved with rocks of sand or corallined boulders.
  • The reef platforms ends at the shore in fringing reef or descents into the lagoon of atolls and barrier reefs.
  • The profile clearly indicates different environment that support different kinds of coral and other animals.

Reef formation:

The building of the reef involves constructive and destructive phases. Skleractinian corals and larger skeleton of other animals from the frame-work materials or the bricks of the reef platform while the finer skeleton materials from the mortar.

The destructive phase may begin to any exposed surface of the coral which is attached by the boring sponges and bivalves . Living corals are attached from the underside which sometime leads to the topping of the upperdom.

When the living colony dies, the underline skeleton becomes riddled with boring organisms. Eventually a large po that support a wide range of other organisms rtion is fragmented. Due to aquamolition and deposition and final excaved materials the borars are kills and the destructive phase comes to an end.

The debries produced by boring as well as fine skeletons and shells of different animals settled into cravices and holes slowly filling the places between the larger frame-work pieces. The growth of the coral then proceeds.

The diversity of a species in coral reef

Coral reefs form some of the world's most productive ecosystems,a variety of animals, including fish, seabirds, sponges, cnidarians (e.g. corals and jellyfish), worms, crustaceans (e.g. shrimp, spiny lobsters and crabs), mollusks , cephalopods, echinoderms (e.g. starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes. Mammals are rare on coral reefs, but humans are visiting cetaceans such as dolphins the main exception.

Besides Cnidarians, many other attached animals lives on reefs including clams, tunicates, and bryozoans. Many coral reef animals, for example, variety of fishes and crustaceans feed in the sea grass and use the reefs for protection, breeding or hideouts for e.g. cardinalfish and squirrelfish hide during the day, while damselfish, surgeonfish, triggerfish, wrasses and parrotfish hide from eels and sharks.There are also certain sponges and worms. That bore into coral surface. The reefs protect the mangroves and seagrass from strong currents and waves that would damage them or wash away the sediments in which they are rooted.

Likenes(1973) estimated that rate of primary production of coral reefs exceeds that of tropical rain forest. Reef building corals of western, pacific and eastern Indian ocean are highly diverse. More than 600 coral species are over 2000 species of fish are found.

Sea snakes feed mainly on fish and their eggs. Marine birds, such as herons, gannets, feed on reef fish. Some land reptiles, such as monitor lizards, the marine crocodile and semiaquatic snakes, such as Laticauda colubrina associate with reefs. Sea turtles, particularly hawksbill sea turtles, feed on sponges.

Enemies of coral reef:

Coral reef phases intense and sometimes complex biological disturbance periodic out-breaks of a predator star-fish, called crown of thorn(Acanthaster planci) which eat corals, have devastated large areas of coral reefs in Indo-Pacific region.

Corals also complicit vigorously among themselves for light and space. They actively attack and killed neighbouring corals of other clones the differ genetical. Coral reefs are dying around the world. In particular, runoff, pollution, overfishing, blast fishing, disease, invasive species, overuse by humans and coral mining and the digging of canals and access into islands and bays are localized threats to coral ecosystems. Broader threats are sea temperature rise, sea level rise and ocean acidification, all associated with greenhouse gas emissions. Other threats include the ocean's role as a carbon dioxide sink, atmospheric changes, ultraviolet light, ocean acidification, viruses, impacts of dust stormscarrying agents to far-flung reefs, and algal blooms.

Air pollution can stunt the growth of coral reefs; including coal-burning and volcanic eruptions. Pollutants, such as Tributyltin, a biocidereleased into water from anti-fouling paint can be toxic to corals.

Rock coral on seamounts are threatened by bottom trawling. Reportedly up to 50% of the catch is rock coral, and the practice smashes coral structures to rubble. These ecosystems take years to regrow, destroying coral communities faster than they can rebuild.

Another cause for the death of coral reefs is bioerosion. Various fishes graze corals and change the morphology of coral reefs making them more susceptible to other threats. Only the algae growing on dead corals is eaten and the live ones are not. However, this act still destroys the top layer of coral substrate and makes it harder for the reefs to sustain.

Human-influance of coral reef:

Corals are intensively harvested and bleached for decoration. Some coral reefs have been heavily fished both for food and aquarium trades. In the Philippines over 60% of the area once covered by corals has been destroyed by dynamite in the recent years.

Physiology of coral combination:

  • The reef building corals contain a green algae (zooxanthella sp.) in th egastrodermal cells.
  • The contribution of this algae to the calcification process involves fixing of CO2 and bi-carbonate
  • Acoording to Goreau's in interpretation calcium ion are taken up from the sea water and pass through the tissues to be absorbed on an extracellular organic matrix.
  • There it is incorporated with bi-carbonate to form CaCO3 Ca+2HCO3 -----> Ca(HCO3)2
  • The Ca(HCO3)2 then breaks into CaCO3 and Carbonic Acid.
  • Ca(HCO3) ----> CaCO3 + H2CO3
  • CaCO3 is deposited as corallum while carbonic acid is taken up by the shells.
  • The Carbonic acid is then broken into CO2 and H2O and the reaction is catalized by the enzyme Carbonic anhydraze
    H2CO3 --------------------------->CO2 +H2O
  • The excess CO2 produced as a result of calcification in then utilized by the symbiotic green algae for photosynthesis.

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