Study Material | Test Series
Drishti IAS
call1800-121-6260 / 011-47532596
Drishti The Vision Foundation
(A unit of VDK Eduventures Pvt. Ltd.)
Coral Reefs
Jul 21, 2014

Coral reefs are marine ridges or mounds, which have formed as a result of the deposition of calcium carbonate by living organisms, predominantly corals, but also a rich diversity of other organisms such as coralline algae and shellfish. 

An individual coral is known as a polyp, a very small and simple organism consisting mostly of a stomach topped by a tentacle-bearing mouth. The polyps extend their tentacles at night to sting and ingest tiny organisms called plankton and other small creatures. 

Thousands of identical polyps live together and form a coral colony. Each polyp excretes a calcium carbonate exoskeleton beneath it and, over long periods of time, the skeletons of many coral colonies add up to build the structure of a coral reef. Many other species – fish, invertebrates, algae and microorganisms – make their homes on and around this reef. Reefs only occur in shallow areas that are reachable by sunlight because of the relationship between coral and algae. 

Coral lives a symbiotic life. Inside the sac of each coral polyp lives one-celled algae called zooxanthellae. The algae gives off oxygen and other nutrients that the coral polyp needs to live and in return the polyp gives the algae carbon dioxide and other substances the algae needs. That is why coral reefs grow so near the surface of the water where it is the sunniest--the algae need sunshine for photosynthesis.

In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and beautiful colors of many stony corals. Sometimes when corals become physically stressed, the polyps expel their algal cells and the colony takes on a stark white appearance. This is commonly described as “coral bleaching”

Coral Reef Formation :

Coral reefs begin to form when free-swimming coral larvae attach to submerged rocks or other hard surfaces along the edges of islands or continents. As the corals grow and expand, reefs take on one of three major characteristic structures —fringing, barrier or atoll. 

Fringing reefs:  They are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. 

Barrier reefs: Grow at border shorelines, but at a greater distance. They are separated from their adjacent land mass by a lagoon of open, often deep water. 

Atoll: If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms. Atolls are usually circular or oval, with a central lagoon. 

Condition for Coral Growth :

There are certain special conditions which are necessary prerequisites for the growth and development of corals. That is why the reef building corals and their associates are not uniformly deposited throughout the tropical warm ocean waters.

For the growth of corals in such large numbers, as is necessary to give rise to coral deposits, there must be a reasonable balance of favourable conditions. The most important of these conditions are the following:

  • For the growth and development of corals the surface temperature of the ocean must be above   20°C. Corals cannot live if the temperature of the sea water falls below this.

  • The water must be shallow, with a depth not exceeding 25-30 fathoms. With increasing depth, the amount of calcium as well as the temperature of water goes on decreasing, so that the coral polyps and other sea creatures which live on calcium carbonate do not thrive. That is why the reef building corals live in shallow waters and on the surface of the seas.

  • The water must be normally saline, and, therefore, along the ocean margins where the water becomes fresh by the inflow of rivers, reef building corals do not grow and develop.

  • Corals need clear water that is free from abundant sediments. That is why coral reefs cannot develop where rivers enter the sea or where wave-erosion causes muddy coastal water.

  • There must be adequate food supply to nourish the abundant life of the coral reef. The most favourable condition for this purpose is the presence of continuously flowing ocean currents which provide to the stationary reef building organisms the much needed food supply.
    For example, extensive coral reefs are found on the east coasts of Australia, Central America, and Africa, which are washed by warm ocean currents flowing along them. On the contrary, corals are found only in scattered patches on the west coasts of these continents.

  • If the salinity of the ocean water is very high, the lime content is bound to be low, which is not favourable for the growth of corals. Therefore the average salinity ranging from 27% to 40% is ideal for the proper growth of corals and other reef forming creatures.

  • The most essential prerequisite for the formation of coral reefs is the presence of sub-marine platforms which must lie near the sea shore, or should be attached to some islands. The depth of water on such platforms should not exceed 50 fathoms. Remember that the corals build their permanent colonies only on these platforms.

  • Corals cannot live for long out of water, and are therefore, rarely found above the low-tide level. On the other hand, their growth is retarded at depths much exceeding 25 or 30 fathoms.

  • Corals need clear oxygenated water with sufficient supplies of microscopic life as food.

  • Since food supplies are plentiful on the seaside of a growing reef, the corals tend to grow 

Distribution :

  1. Various species of corals are found in all oceans of the world, from the tropics to the Polar Regions.

  2. Coral reefs are mainly concentrated towards western margins of ocean.

  3. Reef-building corals are scattered throughout the tropical and subtropical Western Atlantic and Indo-Pacific oceans, generally within 30°N and 30°S latitudes.
    • Western Atlantic reefs include these areas: Bermuda, the Bahamas, the Caribbean Islands, Belize, Florida, and the Gulf of Mexico.
    • The Indo-Pacific ocean region extends from the Red Sea and the Persian Gulf through the Indian and Pacific oceans to the western coast of Panama. Corals grow on rocky outcrops in some areas of the Gulf of California.
    • Great Barrier Reef along east coast of Australia is largest coral reef.

Main threats to coral reefs :

Coral reefs have survived tens of thousands of years of natural change, but many of them may not be able to survive the havoc wrought by humankind. Roughly one-quarter of coral reefs worldwide are already considered damaged beyond repair, with another two-thirds under serious threat. Major threats to coral reefs and their habitats include:

  • Destructive fishing practices: These include cyanide fishing, blast or dynamite fishing, bottom trawling, and muro-ami (banging on the reef with sticks). Bottom-trawling is one of the greatest threats to cold-water coral reefs.

  • Overfishing: This affects the ecological balance of coral reef communities, warping the food chain and causing effects far beyond the directly overfished population.

  • Careless tourism: Careless boating, diving, snorkeling, and fishing happens around the world, with people touching reefs, stirring up sediment, collecting coral, and dropping anchors on reefs. Some tourist resorts and infrastructure have been built directly on top of reefs, and some resorts empty their sewage or other wastes directly into water surrounding coral reefs. 

  • Pollution: Urban and industrial waste, sewage, agrochemicals, and oil pollution are poisoning reefs. These toxins are dumped directly into the ocean or carried by river systems from sources upstream. Some pollutants, such as sewage and runoff from farming, increase the level of nitrogen in seawater, causing an overgrowth of algae, which 'smothers' reefs by cutting off their sunlight.

  • Sedimentation: Erosion caused by construction (both along coasts and inland), mining, logging, and farming is leading to increased sediment in rivers. This ends up in the ocean, where it can 'smother' corals by depriving them of the light needed to survive. The destruction of mangrove forests, which normally trap large amounts of sediment, is exacerbating the problem.

  • Coral mining: Live coral is removed from reefs for use as bricks, road-fill, or cement for new buildings. Corals are also sold as souvenirs to tourists and to exporters who don't know or don't care about the longer term damage done, and harvested for the live rock trade.

  • Climate change: Corals cannot survive if the water temperature is too high. Global warming has already led to increased levels of coral bleaching, and this is predicted to increase in frequency and severity in the coming decades. Such bleaching events may be the final nail in the coffin for already stressed coral reefs and reef ecosystems. Likely impacts of climate change to coral reefs:

    • Coral Bleaching : Corals are extremely sensitive to temperature changes. Increased water temperatures, which may be linked to global warming, can cause mass coral bleaching. Bleaching occurs when coral polyps, stressed by heat or ultraviolet radiation, expel the symbiotic algae that live within coral tissues. When the algae are expelled, the coral appears white or “bleached.” These algae provide corals with most of their food and oxygen. Corals can recover after short periods of bleaching, but as the length and severity of the stress increase so does coral mortality. Coral bleaching events and subsequent reef mortality are expected to become more frequent as sea temperature increases. Other causes of Coral bleaching are: 
        • oxygen starvation caused by an increase in zooplankton levels as a result of overfishing
        • increased sedimentation
        • Pollution from urban or agricultural run-off
        • Sedimentation from undersea activities like dredging
       • Disease
    • Slower Coral Growth: Sea level is expected to increase in the range of 6 to 37.5 inches (15 to 95 cm) over the next century (IPCC, 2001). The vertical growth rate of coral is likely to be slower than this increase. As a result, corals will be deeper, receive less sunlight and grow more slowly. The combined effect of deeper reefs and slower growth will cause two problems for coastal areas: 1) corals will not be able to protect the shore as effectively and wave energy could increase in strength; and 2) smaller reefs will produce smaller amounts of reef sediment which builds and supports island land-bases.
    • Physical Damage to Coral Reefs: Increased coral mortality is expected as storm events and cyclones become more frequent and intense. Coral reef growth may not be able to keep pace with these destructive events.
     Coral Mortality : Rising sea temperatures and sea levels and increasing frequency of storms will increase coral mortality and seriously endanger coral reefs, especially those already under stress. These climatic changes could become the proverbial straw that breaks the camels back for reefs facing stresses such as poor water quality, destructive fishing and tourism impacts.

    Steps that can be taken to protect coral reefs from climate change:

    • Create Effective Marine Protected Areas (MPAs): Create MPAs in areas that are less prone to bleaching events   because of local cold-water currents or upwelling.
    • Lessen Other Pressures on Coral Reefs: Reefs with fewer stresses will be more likely to recover from coral bleaching and adapt to increased temperatures. Countries and communities need to enforce laws against coral destruction, as well as control pollutants, and promote sources of construction material other than coral. Controlling coastal development through an Integrated Coastal Zone Management (ICZM) strategy can help protect reefs from long-term stresses.
    • Identify Ways To Adapt : Governments, especially those of island nations, need to assess ways to adapt to these   changes in coral reefs and develop a national strategy to deal with these impacts in consultation with local communities and the private sector.
    • Adopt Policies and Treaties To Reduce Greenhouse Gas Emissions and Climate Change. The reduction of greenhouse gases will decrease the severity of global climate change. All countries are encouraged to support, ratify and implement the Kyoto Climate Change Convention. All countries are also encouraged to participate in the work of the UN Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC).
    • Protect and Enhance Ecosystems That Absorb Greenhouse Gases. The loss of some ecosystems, especially forests and wetlands, contributes a significant amount of carbon dioxide and other greenhouse gases to the atmosphere. Governments can mitigate the severity of climate change by protecting and enhancing these ecosystems, a strategy known as natural carbon sequestration. Natural carbon sequestration is the process of  removing carbon dioxide from the atmosphere by enhancing ecosystems, such as forests, that absorb greenhouse.

Distribution of Coral Reef in India :

Palk Bay :

Coral reefs on the Tamil Nadu coast are located in Palk Bay near Rameswaram and in the Gulf of Mannar. Palk Bay is separated from the gulf of Manner by Mandapam peninsula and Rameswaram island. The reef is centered on 9 °17'N and 79° 15'. There is only one fringing reef in the Palk bay, which lies in an east-west direction along the mainland from the Pamban channel at the Pamban end of the bridge to Rameshwaram Island. This reef is 25-30km long, and generally less than 200m wide; maximum depth is around 3 m. Visibility is poor around 1 meter and it is badly affected by the north east monsoon. The reef flat is relatively broad from Pamban channel to the southern end near Ramnad and narrow from Pamban to south of Rameshwaram.

Diversity in the Palk bay reef consists of common sea grasses, sixty five species of coral have been recorded with a large number in the family Acroporidae. Turtles and Dugongs are found in the area. Squid breeding grounds occur near Rameshwaram.

The Gulf of Mannar :

The Gulf of Mannar reefs on the other hand are developed around a chain of 21 islands that lie along the 140 km stretch between Tuticorin and Rameswaram (Krishnamurthy, 1987; Kumaraguru, 1997). These islands are located between latitude 8°47' N and 9° 15' N and longitude 78° 12' E and 79° 14'E. A detailed account of each Island is provided by Krishnamurthy (1987) and Deshmukh and Venkatramani (1995). The islands lie at an average of about 8 km from the main land. They are a part of the Mannar Barrier reef which is about 140 km long and 25 km wide between Pamban and Tuticorin (Venketesan, n.d). Different types of reef forms such as shore platform, patch, coral pinnacles and atoll type are also observed in the Gulf of Mannar. The islands have fringing coral reefs and patch reefs around them. Narrow fringing reefs are located mostly at a distance of 50 to 100 m from the islands. On the other hand patch reefs arise from depths of 2 to 9 mt and extend to 1 to 2 km in length with width as much as 50 meters. Reef flat is extensive in almost all the reefs in the gulf of Mannar. Reef vegetation is richly distributed on these reefs. The total area occupied by reef and its associated features is 94.3 sq km. Reef flat and reef vegetation including algae occupies 64.9 and 13.7 sq km, respectively

Andaman and Nicobar Group of Islands :

The Andaman and Nicobar group of Islands are located in the SE of the Bay of Bengal, between 6°-14° N lat and 91 °-94° E longitude. They are the emerged part of a mountain chain and lie on a ridge which extends southward from the Irrawaddy delta area of Burma, continuing the trend of the Arakan Yoma range.

They consist of 350 islands, of which only 38 are inhabited along with a number of exposed islets and rocks. The principal of these are the North Andaman, Middle Andaman with Ritchies archipelago to the east, South Andaman, little Andaman, Baratang and Rutland island. Barren island lies to the east with Narcondum and other extinct volcanoes to the north. The larger islands are mountainous and several are fully forested. Mangroves form extensive coverage along the shores. Annual rainfall is about 3000 mm and salinity of the waters is around 33 ppt. The coral reefs are of fringing type and except for a few investigation reports the reefs of the area still largely remain unknown. A deep oceanic ridge along 10°N seperates the Andaman Group and the Nicobar group of Islands. The general orientation is north-south.

The Gulf of Kutch :

The Gulf of Kutch located at 22°15'-23°40' N Latitude and 68°20'-70°40' East Longitude, is one of the indentations found on the northern side of the Saurashtra Peninsula. It has and area of approximately 7350 sq km. The Gulf which is aligned approximately E-W is about 170 km long and 75 km wide at the mouth, after narrowing down abruptly at a longitude of 72° 20' it gets divided into three major creek systems at the island of Satsaida bet. The southern shore is fringed by numerous live and some dead coral reefs, islands and extensive mud flats, that dry at low tide. In contrast the northern coast is fringed by tidal flats only.

The coral formations of the Gulf of Kutch represent one of the extreme northern limits of corals in the Indian Ocean. The approach to the corals is difficult due to the existence of vast intertidal mud flats which are difficult to negotiate by foot at low tide. The sudden influx of tidal waters also renders it risky to work on the exposed bank.

These reefs are mostly of fringing type along with offshore platform reefs, patch reefs and coral pinnacles. There are some 40 islands with patchy coral formation of which the largest is Pirotan Island. The coral reefs are in a highly degraded condition. The major source of degradation has been mud deposits on various coral reefs e.g. Bural Chank, Kalubhar, Munde ka bet and Jindra reef. Mud over reef occupies a major portion (117.1 sq km. of the reef). The reef area of the gulf of Kutch is 148 sq km and the total area occupied by the reef is 315 sq km .

West Coast of India :

The west coast of India between Bombay and Goa is reported to have submerged banks with isolated coral formations. Coral patches have been recorded in the intertidal regions of Ratnagiri, Malvan and Redi, south of Bombay and at the Gaveshani Bank, 100 Km west of Mangalore. Ponies, Coscinarares, Turbinaria, some favids and Pseudosiderastrea are reported. All the genera recorded are massive or encrusting without any of representation of ramose forms. Siltation is of high rate and salinity may drop to 20 ppt during monsoon in these habitats which may restrict the growth of ecologically sensitive forms of ramose corals. 

Hermatypic corals along the shore are reported from Quilon in the Kerala coast to Enayem in Tamilnadu. Pocilipora spp is the most common genus in this area.Accropora is found with representation of three species. Pseudosiderastrea and Ponies spp are also found. A recent investigation has shown that 29 species in 17 genera of scleractinians occur in this area 

The Lakshadweep Islands :

The Lakshadweep islands lie scattered in the Arabian sea about 225 to 450 km from the Kerala coast. Geographically, the islands lie between 8°N - 12°3'N lat. And 71 °E- 74°E longitude. The islands consist of coral formations built up on the Laccadive-Chagos submarine ridge rising steeply from a depth of about 1500 m to 4000 m off the "west coast of India. The U.T of Lakshadweep along with the Maldives and the Chagos Archipelagoes form an interrupted chain of coral atolls and reefs on a contiguous submarine bank covering a distance of over 2000 km. This ridge is supposed to be a continuation of the Arravali Mountains, and the islands are believed to be remnants of the submerged mountain.

There are 36 tiny islands, 12 atolls, 3 reefs and 5 submerged banks, covering an area of 32 km2 with lagoons occupying about 4200 km2. Only 11 of the 36 islands are inhabited. They are Andrott, Amini, Agatti, Bangaram, Bitra, Chetlat, Kadmat, Kalpeni, Kiltan, Minicoy and the headquarters at Kavaratti. The Minicoy island is separated from the rest of the islands by a 180 km wide stretch of sea known as the nine degree channel. Kavaratti is the administrative headquarters. Agatti houses the only airport and airstrip. A resort catering to international tourists has been functioning in Bangaram since 1988 and a resort catering to national tourists with a dive school has been set up at Kadmat in 1995. In addition tourist huts have been erected at Kavaratti, Minicoy and Agatti.

The islands are flat and scarcely rise more than two meters. They are vulnerable to storms and sea erosion. They are made up of coral sand and boulders which have been compacted into sandstone. These islands have a warm humid climate (air temperature 17°-38° C, humidity 70%). The surface water temperature varies between 28-31 °C. While the salinity ranges from 34-37 ‰ Ground water is found a couple of meters below the land surface and is replenished by an annual rainfall of about 150 cm during the south west monsoon from June to September.

Helpline Number : 87501 87501
To Subscribe Newsletter and Get Updates.