Coral reefs are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals. Coral reefs are built by colonies of tiny animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps belong to a group of animals known as Cnidaria, which also includes sea anemones and jellyfish. Unlike sea anemones, coral polyps secrete hard carbonate exoskeletons which support and protect their bodies. Reefs grow best in warm, shallow, clear, sunny and agitated waters.
Coral Reefs are often referred as “rainforests of the sea”. Coral Reefs occupy less than 0.1% of the world's ocean surface, yet they provide a home for 25% of all marine species.
Conditions for the growth of Coral Polyps:
(i) Corals are found mainly in the tropical oceans and seas because they require high mean annual temperature ranging b/w 68°F and 70°F (20°C – 21°C) for their survival
(ii) Corals cannot survive in the waters having wither very low temperature or very high temperature
(iii) Corals do not live in deeper waters because they die in waters deeper due to lack of sufficient amount of sunlight and oxygen which are very much required for the growth of Coral Polyps.
(iv) There should be clean sediment-free water because muddy water or turbid water clogs the mouths of coral polyps resulting into their death
(v) It may be pointed out that though coral polyps require sediment-free water but fresh water is also injurious for the growth of corals. This is why corals avoid coastal land and live away from the areas of river mouths.
(vi) Very high proportion of oceanic salinity is injurious for the growth of coral polyps because such water contain little amount of calcium carbonates whereas lime is important food for coral polyps.
(vii) Ocean currents and waves are favourable for corals because they bring necessary food supply for the polyps. Current and waves also determine the shapes of coral reefs.
(viii) Corals grow in open seas and oceans but they die in lagoons and small enclosed seas because of lack of supply of food.
(ix) Coral Reefs are generally long but narrow in width
(x) The continuity of coral reefs is broken wherever rivers drain into the seas and oceans.
Coral Reef Distribution:
Types of Coral Reefs:
è Major Types:
(01) Fringing Reef:
· Coral Reefs developed along the continental margins or along the islands are called as Fringing Reefs
· Though Fringing Reefs are usually attached to the coastal land but sometimes there is a gap between them and land. Thus lagoon is formed between the fringing reef and the land. Such lagoon is called as “boat channel”.
(02) Barrier Reef:
· The largest coral reefs off the coastal platforms but parallel to them are called as Barrier Reefs.
· Barrier Reefs are the largest, most extensive, highest and widest reefs of all types of coral reefs.
· Great Barrier Reef, located parallel to the east coast of Australia is the largest of all the barrier reefs in the world.
· Barrier Reef are seldom found as continuous chains rather they are broken at many places and thus the lagoons have contact with the open seas and oceans through tidal inlets.
(03) Atolls:
· A ring of narrow growing corals of horseshoe shape and crowned with palm trees is called atoll.
· It is generally found around in an island or in elliptical form on a submarine platform. There is a lagoon in the middle of coral ring.
· Atolls are found in Antilles Sea, Red Sea, Chian Sea, Australian Sea, Indian Sea.
Atolls are divided into three types:
(i) Island Atoll: having an island in the central part of the lagoon enclosed by circular reef
(ii) True Atoll: characterized by circular reef enclosing a shallow lagoon but without island
(iii) Coral Island / Atoll Island: does not have island in the beginning but later on island is formed due to erosion and deposition by marine waves.
è Minor types:
There are other types of Coral Reefs, but they are less important and not widespread in distribution. Anyway look into it:
Patch Reef: refers to a small patch of colonies, smaller in size than a fringing reef.
Table Reef: looks like a table of coral colonies placed on the sea floor.
Ridge Reef: as the name implies, is a long ridge of corals.
Micro- Atoll: is an atoll-like reef structure within a major atoll.
Theories for the origin of coral reefs:
(i) Subsidence Theory of Darwin
(ii) Stand Still Theory of Murray
(iii) Glacial Control Theory of Daly
(iv) W M Davis Theory
Coral Reef types in India:
Ø In India, only two types of Coral reefs are found: Fringing Reefs and the Atolls
Ø Fringing reefs are common [Gulf of Mannar, Gulf of Kachchh, Andaman and Nicobar Islands]
Ø Atolls [Lakshadweep group of islands]
Ø Barrier reefs are not known form Indian seas though there was a record of such a reef off Andamans in the 1920s. This has never been reconfirmed.
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Why there is absence of coral reefs on the central east and west coasts of India?
The conditions in the central east and west coasts of India, especially salinity and high sediment load, are not ideal for coral growth. Most major rivers of India, like the Ganges, flow into the sea on the east coast, bringing in lots of sediments that would not allow the corals to grow. On the west coast, the monsoon is intense from June to August. The fresh water flow into the sea at this time reduces salinity to less than half of the normal and the sea water becomes murky brownish with the sediments.
Coral Bleaching
· The increase in temperature causes bleaching in the corals wherein the corals lose their algae and become white in colour. This process is called as Coral Bleaching. The Coral Bleaching cause death to the corals.
· Global Warming has been reported as the major factor of coral bleaching.
· Different types of Coral Bleaching
(01) Catastrophic Bleaching: Adversely affecting 95% of shallow water corals in Bahrain, Maldives, Sri Lanka, Singapore and Tanzania
(02) Severe Bleaching: Accounting for 50%-70% death of corals in Kenya, Seychelles, Japan, Thailand and Vietnam
(03) Moderate Bleaching: Resulting into 20%-50% coal mortality but with quick recovery
(04) Insignificant Bleaching: No Bleaching
Coral Bleaching in India
ü The large scale coral bleaching have been reported in the Andaman and Nicobar Islands.
ü According to the National Institute of Oceanography (at Goa), the coral reefs in Lakshadweep have suffered great damage from coral bleaching due to bacterial diseases and warmer sea temperature
ü The corals in the Gulf of Kutch have been bleached due to siltation
Threats to Coral Reef:
§ Deforestation, industrialization etc causing global warming adversely affect coral reefs in their habitats
§ Scientists claim that about 10% of the corals have died and become skeletons due to global warming caused by industrialization
§ Besides global warming, human activities at local to regional levels such as pollution of oceanic water through excess flux of sediments and nutrients, industrial effluents, urban wastes, sewage, over fishing, clearance of maritime forests etc cause damage to coral reefs
§ Recent studies have shown that 58% of the world’s coral reefs are threatened by human activities.
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