Coral reefs are some of the most diverse ecosystems in the world. Coral polyps are the animals which are primarily responsible for building reefs. Coral Reefs may vary from large reef-building colonies to graceful flowing fans to even small, solitary organisms. Thousands of species of corals have been discovered. While some of the corals live in warm, shallow, tropical seas and some others in the cold, dark depths of the ocean. Coral Reefs provide shelter to a large number of marine animals and hence these are considered as one of the most diverse ecosystems in the world. These reefs are present in the shallow water hence they are susceptible to climate change and are affected by anthropogenic activities.
Favorable conditions for coral reef formation
Optimum Sea Surface Temperature: Reef-building corals cannot tolerate water temperatures below 18° C. However, some of the corals grow with an optimum temperature range of 23ºC to 29ºC.
Optimum Salinity: Most reef-building corals also require very saline (salty) water ranging from 32 to 42 parts per thousand.
Transparency of water: The water must also be transparent so that a maximum amount of sunlight penetrates through it. This is the reason corals do not grow near the mouths of the large rivers It is because of the turbidity in water produced by the sediments present in the river water.
Symbiotic Association
Sunlight is important as most reef-building corals contain photosynthetic algae, called zooxanthellae. These microscopic algae live in their tissues. The corals and algae share a Symbiotic relationship.
The coral provides a protected environment and the compounds zooxanthellae need for photosynthesis.
In return, the algae produce carbohydrates (the coral uses that for food and oxygen). The algae also helps the coral remove waste. Since both partners benefit from association, this type of symbiosis or biotic interaction is called mutualism.
Deep-sea Corals Deep-sea corals live in much deeper or colder oceanic waters. These corals lack zooxanthellae. These corals rely heavily on photosynthesis to produce food. Deep sea corals take in plankton and organic matter for much of their energy needs. |
Types of Coral Reefs
The coral reefs have been classified on the basis of their locations into different types such as fringing, patch, barrier and atoll. Following are three different types of reefs.
Fringing Reef
Coral reefs which are developed along the continental margins or along the islands are called fringing reefs. The seaward slope is steep and vertical while the landward slope is gentle while the upper surface is uneven and corrugated. These conditions are favourable for the formation of coral reefs.
Fig. Fringing Reefs
Boat channel: Although fringing reefs are usually attached to the coastal land but sometimes there is a gap between the reefs and the land. A lagoon is formed between the fringing reef and the land. Such a lagoon is termed as the boat channel.
Barrier Reef
The largest coral reefs which are formed off the coastal platforms but parallel to them are called barrier reefs. Barrier reefs are the largest, most extensive, highest and widest reefs of all types of coral reefs.
Fig. Barrier Reef
Characteristics of Barrier Reefs
Slope: The average slope of these reefs is about 45° but some barrier reefs are characterized by a 15°-25° slope. There is an extensive but shallow lagoon between the coastal land and the barrier reef.
Continuity: Barrier reefs are found as continuous chains in some places only. At most of the places, these are broken at many places and thus the lagoons have contact with the open seas and oceans through tidal inlets. Sometimes, these tidal inlets are so wide that ships enter the lagoons through them.
Example of Barrier Reef: The Great Barrier Reef
Great Barrier Reef which is located parallel to the east coast of Australia, is the largest of all the barrier reefs of the world. This reef is located between 9°S to 22°S latitudes. It stretches for a length of 1200 miles (1920 km) and thus covers about two-thirds of the coastal length of Queensland province of Australia. The northern and southern parts of this reef are 80 miles (128 km) and 7 miles (11km) away from the coast the reef from the Coast Ranges is between 20 to 30 miles (32 to 48 km).
Atoll
A ring of narrow growing corals of horseshoe shape which is usually crowned with palm trees is called atoll. These are generally found around an island or in elliptical form on a submarine platform. Usually, there is a lagoon in the middle of the coral ring. The depth of the lagoon ranges between 40 to 70 fathoms.
Types of Atolls
Atolls can be divided into 3 types. These are:
True Atoll: True atoll is characterized by a circular reef which encloses a shallow lagoon but without an island.
Island Atoll: Island atolls are those atolls which have an island in the central part of the lagoon enclosed by a circular reef, and
Coral Island: Coral island or Atoll Island are those atolls which do not have an island in the beginning but later on the island are formed due to erosion and deposition by marine waves.
Distribution of Coral Reefs in India
In India, three major reef types are found. These are atolls, fringing and barrier reefs. The mainland coast of India has two widely separated areas which contain coral reefs. These are the Gulf of Kutch in the northwest, which has some of the most northerly reefs in the world, and Palk Bay and The Andaman and Nicobar Islands have fringing reefs around many islands. The Lakshadweep Islands are atolls and have extensive coral reefs.
Fringing Reefs: The fringing reefs are contiguous with the shoreline. These are the most common forms of reef found in the Andaman and Nicobar group of Islands.
Patch Reefs: Patch reefs are isolated and discontinuous patches of reefs. These reefs lie shoreward of offshore reef structures. These are found in the Gulf of Mannar and Kutch.
Barrier Reefs: Barrier reefs are linear offshore reef structures which run parallel to coastlines and arise from submerged shelf platforms. There is a long barrier reef (329 km) on the west coast of the Andaman and Nicobar Islands.
Atolls: Atolls are circular or semi-circular reefs which arise from subsiding sea floor platforms as coral reef buildings keep ahead of subsidence. Lakshadweep group of islands are examples of Atolls. These islands are scattered in the Arabian Sea about 225 – 450 km from the Kerala coast.
Geographical Location of Coral Reefs in India
The Gulf of Mannar
The Gulf of Mannar reefs have been developed around a chain of 21 islands. These are located along the 140 km stretch between Tuticorin and Rameswaram. They are a part of the Mannar Barrier Reef which is located between Pamban Island and Tuticorin. In this group of coral reefs, different types of coral reefs are found such as shore platforms, patch reefs, coral pinnacles and atoll types.
Fig. Gulf of Mannar Coral Reefs
The Gulf of Kutch
The Gulf of Kutch is located at 22°15'-23°40' N Latitude and 68°20'-70°40' East Longitude. The coral formations of the Gulf of Kutch represent one of the extreme northern limits of corals in the Indian Ocean. The reefs which are found in the Gulf of Kutch are mostly of fringing type. They consist of offshore platform reefs, patch reefs and coral pinnacles.
The Lakshadweep Islands
The Lakshadweep Islands are located in the Arabian Sea from the Kerala coast. The islands consist of coral formations which are built up on the Laccadive-Chagos submarine ridge off the west coast of India. Most of the Coral reefs of the islands are atoll type except one coral reef at Androth, which is of platform reef.
Fig. Distribution of Coral Reefs in India
Global Distribution of Reefs
Reef-building corals are found in tropical and subtropical regions around the world. These are found in the Indo-Pacific and Atlantic Oceans, normally between the Tropic of Capricorn and Tropic of Cancer within 30°N and 30°S latitudes. There are two major regions of the reef-building corals. These are
Western Atlantic Reefs: Western Atlantic Reefs include areas such as Bermuda, the Bahamas, the Caribbean Islands, Belize, Florida, and the Gulf of Mexico.
The Indo-Pacific Ocean Region: 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. In the Pacific region, corals grow on rocky outcrops in some areas of the Gulf of California.
Fig: Global Distribution of Coral Reef
Coral Bleaching
Coral bleaching refers to the loss of algae from the corals. It results in the white colouration of the coral polyps which indicates the death of corals. Global warming and ocean acidification have been reported as the major factors of coral bleaching.
Coral Bleaching Event of 1997
The coral bleaching which was recorded during 1997-98 has been cited as the most catastrophic event. It accounted for large-scale deaths of corals in the tropical oceans of 60 countries and island nations. In 1998 when large-scale coral bleaching accounted for 70 per cent of the death of corals off the coasts of Kenya, Maldives, Andaman and Lakshadweep islands in the Indian Oceans and 75 per cent of death in the Seychelles Marine Park System and the Mafia Marine Plant off Tanzania was reported by Clive Wilkinson of the Global Coral Reef Monitoring Network (GCRMN) of Townsville (Australia).
Threats to the Coral Reef: Need for Conservation
There has been unprecedented degradation of the coral reef system across the world. The coral reef system has a high natural, cultural and economic. The anthropogenic activities have led to a major threat to the existence of coral reefs. Climate change, tourism, fishing, coastal development, eutrophication and sedimentation have been linked to severe degradation of coral reefs.
Climate Change and Ocean Acidification: As the reef-building corals cannot thrive the temperatures above 18ºC or sometimes between 25 ºC to 29 ºC, global warming which increases the ocean water temperature leads to bleaching.
Unsustainable Fishing Practices: Fishing practices such as cyanide fishing, blast or dynamite fishing, bottom trawling and Muroami (banging on the reef with sticks) etc. can lead to coral bleaching.
Unsustainable Tourism: The engagement of humans with the coral reefs due to careless boating, diving and fishing, snorkelling with people touching reefs, stirring up sediment, collecting coral, and dropping anchors on reefs can lead to the destruction of Coral Reefs. Sometimes tourist resorts and infrastructure are built directly on top of reefs in the middle of the ocean connecting through pathways to shorelines can also lead to the destruction of the coral reef.
Pollution: The dumping of urban and industrial waste, sewage, agrochemicals, and oil pollution directly into the oceans leads to the poisoning of the reefs. These toxins are carried by river systems from sources to oceans and can also damage the coral reefs to a greater extent. Pollutants from sewage and runoff from farmlands often increase the level of nitrogen in seawater. It leads to the overgrowth of algae, which 'smothers' reefs as the eutrophication leads to blocking of the sunlight.
Sedimentation: Erosion formed by construction along the coasts and inland, mining, logging, and farming increases the volume of sediments in rivers. These sediments are dumped into the oceans by the rivers leading to turbidity. The turbidity again blocks the sun's rays leading to the death of corals.
Use of Coral as Building materials: Coral mining is currently done in many places in Southeast Asia and is used as a cementing agent. This has further led to overexploitation of the coral reefs and their depletion.
Use of Coral in decoration: Corals have been widely used in the beautification of aquariums. It has led to their mining resulting in bleaching.
Conservation of Coral Reefs
There are various methods which have been used for the conservation of the coral reefs. These are:
Coral Farming: The method utilizes coral fragments which can be farmed in underwater nurseries (in situ). It takes advantage of natural conditions to grow. Coral fragments can also be propagated in terrestrial facilities (ex-situ) that consist of arrays of tanks equipped with carefully controlled water quality management systems. After the formation of corals, these are transplanted into their natural habitats.
Aquarium Farming: The other alternative for popular restoration techniques is to grow corals in aquaria and out-plant them onto natural or artificial reefs.
Coral Gardening: Coral gardening is a method of growing coral polyps to help restore reefs around the globe.
Micro-fragmentation: The Micro-Fragmentation technique consists of breaking the corals into smaller pieces of 1 to 5 polyps, using a specialized band-saw. This stimulates the coral tissue to grow, allowing them to grow into clones at 25 to 50 times the normal growth rate.
Note for UPSC Aspirants: For UPSC aspirants interested in exploring further, here are some keywords to guide your research:Coral reefs, diversity, coral polyps, symbiotic relationship, mutualism, favorable conditions, sea surface temperature, salinity, transparency of water, deep-sea corals, types of reefs, fringing reef, barrier reef, atoll, conservation.
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