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CORAL REEF ECOSYSTEM
Coral reefs form the most dynamic ecosystem providing shelter and nourishment to thousands of marine flora and fauna. They are the protectors of the coastlines of the maritime states. A few genera of corals are supposed to be older than prairies. This unique ecosystem is most productive because of its symbiotic association with algae called Zooxanthellae. Though they are the builders of the most massive structures ever created by living beings in the world, they are very fragile and vulnerable to natural disturbances, and human activities. Maritime states and the coastal populations mostly depend upon the coral reef ecosystem for their day to day life.
Coral reefs are tropical, shallow water ecosystems, largely restricted to the area between the latitudes 30o N and 30o S, the exact arial extent of coral reefs in the world is unknown and extremely difficult to estimate. However, Smith (1978) has produced a figure of 600,000 Sq. Km for reefs to a depth 30 m. About 60% of the world’s reefs occur within the area covered by the Indian Ocean region estimated as 73,600 000 Sq. km, according to IUCN.UNEP (1985) about half of which are in the Indian Ocean, Red Sea and Gulf and half in the “Asiatic Mediterranean”, an area bounded by Indonesia to the west northern Australia to the South, the Philippines to the east and mainland Asia to the north (Smith, 1978).
The Indian landmass forms a major physical division between the Arabian Sea and the Bay of Bengal. Oceanographically, the Bay of Bengal differs from the Arabian Sea in maintaining clockwise circulation of major currents during both the northeast and southwest monsoons. The circulation in the Arabian Sea reverses, with surface water masses circulating counter clockwise during the northeast monsoon and clockwise during the southwest monsoon. There is also a major difference in salinity. In the Arabian Sea, evaporation exceeds precipitation and runoff, leading to the formation of highly saline water masses that flow south. The Bay of Bengal has comparatively low salinity due to high runoff and precipitation; during the southwest monsoon, maximum salinity is found at depths of about 500 meters, as highly saline water moves into the Bay from the Indian Ocean.
In India, all the three manor reef types (atoll, fringing and barrier) occur, and the region includes some of the most diverse, extensive and least disturbed reef areas of the Indian Ocean, many of which are among the least scientifically known (Fig. 1). The mainland coast of India has two widely separated areas containing reefs: the Gulf of Kutch in the northwest, which has some of the most northerly reefs in the world, and Palk Bay and Gulf of Mannar in the southeast. There are patches of reef growth on the West Coast, for example coral reefs at Malvan. The Andaman and Nicobars have fringing reefs around many islands, and a long barrier reef (329 km) on the west coast. The reefs are poorly known scientifically but may prove to be the most diverse in India and those in the best condition. The Lakshadweep has extensive reefs but these are equally poorly known.
GEOLOGIC HISTORY OF CORAL REEFS IN GENERAL
In the case of most corals, numerous individuals called polyps unite to form colonies. The formation of coral reefs is a most interesting phenomenon. The first reefs began to form in the World’s Oceans with the advent of filamentary algae about 2 billion years ago. These reefs, many of which are still visible in the fossil record, are quite distinctive in that they consist of laminated limestone flanked by reef debris. The beginning of the Cambrian period some 600 million years ago saw the introduction of the first animal into the reef building community. This was a stony sponge like animal called archaeocyathid (Greek for “ancient cup”). These animals associated themselves with the stromatolite clumps and contributed to the accumulation of reef material. Then, about 540 million years ago, the archaeocyathids simply disappeared without any apparent cause.
In middle Ordovician time, some 480 million years ago, animals rejoined the reef-building process. These animals included the stony sponges called stromatoporoid and the stony coelenterates – the first among the corals. Then around 350 million years ago, the reef community was again devastated by unknown factors or environmental changes.
With the beginning of the Carboniferous period, the reef community again began to flourish. The revitalized community contained the rugged stromatolites, along with bryozoans, brachipods and corals. For 115 million years, this new community flourished by radiating into thousands of new species that accumulated giant reefs still preserved in the fossil record. Then, as before, disaster struck. This time (225 million years ago) the destruction was total for no reefs were found anywhere in the world for the next 10 million years.
About 215 million years ago another reef community began to establish itself slowly, As before, the algae were the most important members of the community, but several new families of coral, the Scleractinians, became prominent. At this point, there was another interval -20 million years in this case – in which reefs were unknown in the world. This period was followed by an impressive reef expansion in which a hither to obscure bivalve mollusk known as a rudist played the most prominent role. For 60 million years, the rudists rivaled both the algae and coral as a reef builder. Then came the great extinction that occurred at the close of the cretaceous period, some 62 million years ago. Nearly a third of all animals known at that time were not to survive. Of the 115 genera of dinosaurs, none survived and in the reef community, the rudists, along with two-thirds of the coral genera perished.
Again, a period of 10 million years was to pass before the reef community could begin to flourish. Since that times the corals have become the most important single members of the community. There have been several periods of decline in reef building since that time, but no great extinction. Oddly enough, the last Ice Age, with its great swings in sea level, had little effect on the reefs. Today reef communities are worldwide but are restricted to a relatively narrow sanctuary on either side of the equator where conditions still favor their growth.
The origin of coral reefs has been debated by oceanographers for over a century. Since coral do not grow below about 65 ft and can survive only brief periods above water, the question arises as to how coral reef formations several hundred feet in thickness could have formed. Charles Darwin, as a result of his voyage aboard the Beagle in 1831, suggested that such reef growth was made possible by the gradual subsistence of the pedestal upon which the reef first began to grow. In other words, the reef organisms grew upward to compensate for the gradual submergence of their platform. More recently, at the end of the last Ice Age, gradual rising of the sea level because of the melting of glaciers has been added to the subsistence mechanism, is a possible explanation.
Reefs are home to more species than any other ecosystem in the sea. The total number of reef species in the world is still unknown, but up to 3,000 species can be found together on a single reef in Southeast Asia and over 1,000 on a single Caribbean reef. Only tropical rainforests estimated by some to be home to a staggering 30 million insects, have a greater number of species, although due to the vast number of fish that inhabit them, reefs contain a larger number of vertebrates than rainforests. Reefs also contain many more major animal groups (Phyla) than any other ecosystem on land or in the sea.
The richest reefs, with the greatest diversity of plants and animals are in the region bounded by Indonesia, Malaysia, the Philippines and southern Japan. Of the 793 or so reef corals that are known in the world, 600 are found in this region; over 400 are found in the Philippines and Japan, and about 350 in Indonesia, although there are probably many more to be discovered here. Up to 200 corals may occur on a single reef in South East Asia and it is estimated recently that there may be up to 400 species of corals may recorded in India (GOI & UNDP GEF report 2001). This high diversity of coral species in India extends equally to other reef associates and is partly because of the greatest area of reefs found here and partly because of its geological history. The variety of species on a reef decreases eastwards across the Pacific.
TYPES OF REEFS
These grow along the edges of continents and around islands, close to shore but sometimes separated from it by a shallow lagoon. Fringing reefs are common in the Gulf of Mannar, Andaman, and Nicobar Islands. Where there is murky water caused by soil run-off, fringing reefs rarely grow to any substantial depth.
These develop along the edges of continental shelves or around islands that have become partially submerged, and are separated from the mainland or island by a wide, deep lagoon. More corals that are fragile grow on the lagoon side of the barrier than on the open side where they would have to withstand the force of larger and more violent waves. This type of reefs can be found in Andaman & Nicobar Islands however the best known example is the Great Barrier Reef which extends for nearly 2000 km along the east coast of Australia and represents about three percent of the total of the world’s reefs.
These generally begin as fringing reefs around volcanic islands. As the island subsides, because of the sea floor sinking or the sea level rising, the fringing reef forms a circular barrier reef separated from the island by a lagoon. When the island finally disappears, the circle of reefs is left, sometimes capped with small coral islands, enclosing lagoon. The whole structure is called an atoll. Atoll varies in size from tiny Bitra, the smallest of just 0.10 sq. km, to the largest, Andrott, 4.84 sq. km in Lakshadweep.
Apart from these, there are a few minor reef types as follows. Platform reef, Patch reef, Coral pinnacle, Reef flat, Coralline shelf, Coral heads, and Live coral platform.
CORAL REEFS IN INDIA
Indian subcontinent with its coastline extending over 8,000 km and subtropical climatic condition has very few coral reef areas when compared to other regions of the world. In India, the reefs are distributed along the east and west coasts at restricted places. However all the major reef types are represented. Fringing reefs are found in Gulf of Mannar and Palk Bay. Platform reefs are seen along the Gulf of Kutchh. Patchy reefs are present near Ratnagiri and Malvan coasts. Fringing and barrier reefs are found in Andaman and Nicobar Islands. Atoll reefs are found in Lakshadweep. The absence of reef in Bay of Bengal (North East Coast) is attributed to the immense quantity of freshwater and silt brought by the rivers such as Ganga, Krishna and Godavari. Satellite imagery (SAC, Ahmedabad) shows scattered patches of corals in the intertidal areas and occasionally at subtidal depths along the West Coast of India notably at Ratnagiri, Malwan and Rede Port.
The mainland coast of India has the Gulf of Kutch in the Northwest (Gujarat State) and Palk Bay and the Gulf of Mannar in the southeast (Tamil Nadu State). Other than these important off shore island groups of India, the Andaman and Nicobar in the Bay of Bengal and Lakshadweep in the Arabian Sea also have extensive reef growth. The total area of coral reefs in India is estimated to be 2,374.9 Sq. km.
Table 1: Area Estimates of Coral Reefs in India (Km2)