2. What is a coral reef?

Surprisingly, the mammoth reef systems which occur in many parts of the world result from the combined activities of the small, humble coral polyp. Reefs are made up of layers of coral skeletons cemented together by coralline (encrusting) algae to form an extremely hard limestone structure. The world's largest reef system, the Great Barrier Reef, has developed in this manner. It is not one reef but a complex of almost 2,900 individual reefs stretching for a distance of about 2,300 kilometres. These individual reefs have resulted from the growth of hard corals on eroded formations (eg. valleys, hills and ridges) which millions of years ago were out of the water when sea-level was much lower.

The Great Barrier Reef comprises many different types' of reef formations. Those that have developed around continental islands (eg. those in the Whitsunday Islands) are termed fringing reefs. Those that form long thin formations along the outside edge of the northern part of the Great Barrier Reef (much like a string of sausages) are termed barrier reefs. These can be up to 30 kilometres long and are divided by narrow passages. There are also a variety of other formations including lagoonal, crescentic and planar reefs.

The formation of these different reef types depends largely on the surface on which they are being formed, the recent geological history of the area (ie. changes in sea level and land formations), and water temperature and exposure to wave action. It has little to do with the types of corals that were important in their formation. Some reefs (eg. Heron Island off Gladstone and Green Island off Cairns) have sandy islands on them. The combined action of waves and water currents causes the deposition of finely eroded coral material which over many years may lead to the development of these islands.

Most reefs have a solid front which receives much of the force of the ocean swell. This is called the windward side of the reef. It forms a wall which slopes (sometimes quite steeply) to the bottom of the seabed (generally 40-50 m in the Great Barrier Reef). The top of this structure, termed the reef flat, may become exposed at low tide. It is normally dotted by small, squat corals which are able to withstand the force of the sea. The substrate between the corals is covered by a hard crust of coralline algae. Often behind the reef front is an area of deeper water (sometimes 15-20 metres in depth) which has a sandy bottom and small isolated islands of coral, called reef patches. This is the lagoon. Some reefs also have another barrier along the back (the leeward side) which encloses the lagoon. It may be continuous or consist of a series of broken reef patches.

The diversity in the structure and size of reefs in the Great Barrier Reef is all the more amazing when it is considered that they have been formed as a result of a group of animals which often measure no more than a couple of millimetres in diameter.

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