CORAL REEFS

In tropical seas many kinds of coral animals and marine organisms such as coral polyps, calcareous algae, shell-forming creatures and lime-secreting plants live in large colonies. Though they are very tiny creatures, their ability to secrete calcium carbonate within their tiny cells has given rise to a peculiar type of marine landform. They exist in numerous species of many forms, colours and shapes. Under favourable conditions, they grow in great profusion just below the water-level. Taking coral animals as a whole, the polyps are the most abundant and also the most important. Each polyp resides in a tiny cup of coral and helps to form coral reefs. When they die, their limy skeletons are cemented into coralline limestone.

There are also non-reef-building species such as the ‘precious corals’ of the Pacific Ocean and the ‘red coral’ of the Mediterranean which may survive in the colder and even the deeper waters. As a rule they thrive well only in the warmer tropical seas.

The reef-building corals survive best under the following conditions.

1) The water temperature must not fall below 20°C (68°F.). This virtually limits the area distribution of corals to the tropical and subtropical zones. Again, they will not flourish where there are cold streams because of the upwelling of the cold water from the depths that cools the warm surface water. This explains why coral reefs are generally absent on the western coasts of continents. On the other hand the warming effect of the warm currents, e.g. the Gulf Stream, means that corals are found far to the north of the West Indies in the Atlantic Ocean. The pacific and the Indian Oceans, however, have the most numerous coral reefs.

 

2) The depth of the water should not exceed 30 fathoms or 55 m (180 feet), because beyond this depth sunlight is too faint for photosynthesis to take place. This is essential for the survival of the microscopic algae, on which the coral polyps depend. Shallow water of less than 30 m (100 feet) is ideal. But there should always be plenty of water as polyps cannot survive for too long out of water.

 

3) The water should be saline and free from sediment. Corals therefore survive best in the moving ocean water well away from the silty coasts or muddy mouths of streams. The corals are best developed on the seaward side of the reef, where constantly moving waves, tides and currents maintain an abundant supply of clear, oxygenated water. They also bring an adequate supply of food in the form of microscopic organism.

 

Types of Coral Reefs

There are three main types of coral reefs.

1) Fringing reefs: A fringing reef is a coralline platform lying close to the shore extending outwards from the mainland.

It is sometimes separated from the shore by a shallow lagoon. It is widest when fringing a protruding headland but completely absent when facing the mouth of a stream. The outer edge grows rapidly because of the splashing waves that continuously renew the supply of fresh food. The reefs may be about a kilometre wide, lying just above the level of low water and sloping steeply downwards on the seaward side to a depth of about 30 m (100 feet).

 

2) Barrier reefs: A barrier reef is separated from the coast by a much wider and deeper channel or lagoon (Fig. 11.7).

The reef is partially submerged. If it lies above the water-level and sand can accumulate on it, a little vegetation is possible. The barrier reefs have narrow gaps at several places to allow the water from the enclosed lagoon to return to the open ocean. Such gaps are very useful for shipping and provide the only entrances for ships to enter or leave the lagoon. The best known barrier reef is the Great Barrier Reef off the coast of Queensland, Australia. It is 1930 km (1200 miles) long, separated from the coast by a channel 160 km (100 miles) wide in places and over 60 m (200 feet) deep.

 

3) Atolls: Atolls are similar to barrier reefs except that they are circular in shape, enclosing a shallow lagoon without any land in the centre. The encircling ring is usually broken in a few places to allow the free flow of water.

On the inside of the reefs, sand and limestone debris collect and palm trees like coconuts may grow. Such palm trees thrive well in the brackish water of the lagoon. The nuts fall into the water and are distributed widely by floating from one coral island to another. The calm waters are useful for fishing and canoeing. Some of the large atolls, e.g. Suvadiva in the Maldives, west of Sri Lanka, have a lagoon over 64 km (40 miles) across. A number of them provide essential air bases for trans-Pacific aircraft.

 

The Probable Origin of Coral Reefs

The subject of the origin of coral reefs has been studied and debated for over one and a half centuries. Several theories have been suggested but none is universally accepted.

The most widely accepted theory is that put forward by the great scientist Charles Darwin, after his voyage to the Pacific islands in 1842. It is known as the subsidence theory. Darwin assumed that all coral reefs began as fringing reefs around an island or the topmost portions of extinct volcanoes that stood above the ocean bed.

Due to a general downwarping of the earth’s crust, the islands gradually subsided. The corals continued to grow upwards to keep pace with the subsidence. The growth was more vigorous at the outward edge than the landward edge because of the more favourable living conditions for corals, so the encircling reef widened. It then formed a barrier reef, with a lagoon between the island and the reef. Eventually, when the land completely submerged, only the outer rims of the reefs were seen, forming an atoll. The submerged island was covered by a layer of sediment so that the characteristic circular lagoon is generally shallow. Thus atolls mark the position of the former islands (Fig. 11.10).

More recent researches by oceanographers have revealed that the ocean floor has, in fact, been subjected to subsidence especially in the Pacific. Darwin’s explanation was therefore generally correct.

Amongst the other theories, perhaps that of the American geographer R.A. Daly on glacial control theory put forward in 1910 is worth consideration. During his visit to Mauna Kea in Hawaii, he noticed the close relationship between glaciation and the development of coral reefs. He believed that during the height of the Ice Age, the water was too cold for any coral growth to take place. With the absence of a coral barrier, marine erosion was able to attack and lower the islands. With the return of the warmer climate, the water that was locked up in the ice sheets melted. Consequently, there was a rise in the sea-level which in some cases submerged these lower islands.

On these wave-planed platforms, corals began to grow upwards at the rate of a third of a metre in a decade to keep pace with the rising water-level (Fig. 11.10b). Where islands still project above sea-level, coral reefs and atolls were thus formed. Recent evidence of boring through coral formations seems to favour Daly’s explanation of a change in sea-level and consequent erosion of the islands. However, the deepest borings reveal basaltic rocks. These correspond to the subsided islands envisaged by Darwin. Thus a combination of the two theories accounts for all the important features of coral reefs and atolls.