A reef is a rock, sandbar, or any other feature lying beneath the surface of the water. There are many types of reefs namely barrier reef, fringing reef, platform reef and ribbon reef. Some of the reefs are separated from land by deep lagoon while some lack lagoons.
The Great Barrier Reef is not a single reef as the name suggests but a combination of all the four types of reef. The northern and the southern section of the Great Barrier Reef are quite different from each other. As the northern section is more close to the equator, they are having the most diversity. Here, the ribbon reefs dominate. Their windward sides deflect strong waves and currents, providing a calm inner section with scattered fringing reefs and patch reefs.
Satellite image of part of the Great Barrier Reef adjacent to the Queensland coastal areas of Airlie Beach and Mackay.
Around the central section, we can find that the reefs are farther from the mainland. The broad, shallow area hosts patch reefs and small coral islands. This section is the most accessible and gets the most tourist traffic. Scattered among the 3,000-plus reefs are low-lying coral or sand islands called cays. The coral reef looks like a garden made up of millions of tiny animals. The cays support many different types of plant communities including mangrove, rainforest and grassy, depending on the rainfall they receive. The islands form from debris such as coral sand, shells and hard algae piling up against the tops and edges of the reefs.
In the southern section the reefs are seen more closely to land. Many submerged platform and patch reefs form the underwater landscape. Heading south further, the reefs gradually disappear as the shelf narrows and temperatures dip to levels unsuitable for coral growth.
A variety of colourful corals on Flynn Reef near Cairns
Along with the 1,500 species of fish, the Great Barrier Reef also hosts more than 200 species of birds, 4,000 types of mollusks, 30 species of whales and dolphins, and six species of sea turtles. There are fish smaller than our fingernail and bigger than a car. There are around 20 species of sea snakes, some of which possess venom 20 times stronger than the most poisonous land snake.