||Coral Sea, Australia
||Approx. 348,000 sq. km
||Great Barrier Reef Marine Park
|World Heritage Site
Origins of the Reef
The great barrier reef is the largest reef system in the world. It is composed of over 2,900 individual reefs and around 900 islands that stretch for over 2,600 kms (1,616 miles) along the coast of Queensland, north east Australia.
The great barrier reef first began to grow 18 million years ago but the present phase of reef growth is 6 - 8,000 years old. The reef reaches from the Torres Strait in the north to the passage between Lady Elliot Island and Fraser Island in the south. In the north and south the reefs lie close together, but in the central section where they are more widely scattered.
Between the reefs and the shore is the main great barrier reef lagoon. This shallow water area, rarely more than 100 m (328 ft) deep, lies over a silt covered plain and is sheltered by the off shore reefs. In contrast, the seaward side of the reef, known as the edge or reef slope, drops away steeply to the sea bed thousands of metres below. It is exposed to the full fury of storms and waves and it is this area that has the most active coral growth.
How the Reef is Formed
The reef is formed by billions of tiny organisms called coral polyps and essentially reefs are living rocks. The polyps embed themselves in protective cups of limestone, secreted by their outer skin cells. Each year the reef expands upwards and outwards as the polyps reproduce and new polyps grow. To tie the reef together each polyp is connected to is neighbours by strands of living tissue.
Seaweeds called calcareous algae are also very important reef builders, they make limestone rock which adds to the structure. Red algae, that lives mainly at the reef edge, produce a substance that acts like a mortar that cements areas of sediment together.
This huge structure contains only a thin layer of living coral on the surface that covers layers and layers of empty limestone skeletons - the new polyps simply grow over the old polyps beneath. Each species of coral has its own growth pattern so there is a great variety of shapes and colours which make up a beautiful reef.
The islands of the great barrier reef support 2,195 known plant species, three of which are endemic to the islands. The northern islands have 300 - 350 species which tend to be woody, whereas the southern islands have 200 which are herbaceous. The Whitsunday region is the most diverse; it supports 1,141 different plant species. All of the species of plant life rely on birds for propagation.
There are 500 species of marine algae or seaweed that live on the reef including 13 species that belong to the genus Halimeda. These deposit calcareous mounds up to 100 m (328 ft) wide, creating mini ecosystems on their surface which have been compared to rainforest cover. Fifteen species of seagrasses can be found on the reef, the most common of which belong to the Halophila and Halodule genera.
The Great Barrier Reef supports a wide diversity of life, including species that are endemic to the reef system.
Thirty species of cetaceans have been recorded on the reef including humpback whales, dwarf minke whales and the indo-pacific humpback dolphin. Large populations of dugong feed on the seagrass beds as do six species of sea turtle including the green sea turtle, leatherback sea turtle, hawksbill turtle, loggerhead sea turtle, flatback turtle and the olive ridley.
Approximately 125 species of shark, stingray and skates live on the reef and over 1,500 species of fish including clownfish, red bass, red-throat emperor, several species of snapper and coral trout. 49 species of pipefish and 9 species of seahorse have been recorded on the reef.
Saltwater crocodiles live in mangrove and salt marshes on the coast near the reef and although nesting hasn't been reported the population is wide ranging but low density. At least 7 species of frog inhabit the islands of the reef and 17 species of sea snake live in warm waters up to 50 m (164 ft) deep.
There are 215 species of birds that visit the reef, or nest or roost on the islands including the white-bellied sea eagle and the roseate tern. The nesting sites are mainly on islands in the northern and southern regions of the reef with 1.4 - 1.7 millions birds using the sites to breed.
Around 5,000 species of mollusc have been recorded on the reef including the giant clam, and various nudibranchs and cone snails. There are at least 330 species of ascidians on the reef system and between 300 and 500 species of bryozoans.
Around 400 species of hard and soft corals inhabit the reef which breed in mass spawning events that are triggered by rising sea temperatures, the lunar cycle and the diurnal cycle.
Climate change is the biggest threat to the great barrier reef, rising ocean temperatures increases coral bleaching and it is expected to become an annual occurrence. As well as an impact on coral, climate change has implications for other reef life too; if the water becomes too warm some species of fish may seek new habitats, which in turn could lead to chick mortality in some species of predatory sea bird. Also it will have an affect on the habitat of sea turtles.
Pollution and declining water quality is another threat to the great barrier reef. The rivers of north eastern Australia pollute the reef during tropical flood events, with over 90% of this pollution coming from farm runoff.
Fishing is another threat faced by the reef. Over fishing of keystone species, such as the Giant Triton, can disrupt food chains that are vital to marine life. It also impacts the reef through increased water pollution, by-catch of unwanted species and habitat destruction from nets, trawling and anchors.
Several commercial shipping routes pass through the great barrier reef and shipping accidents are a pressing concern. There have been over 1,600 known shipwrecks in the reef region and each one carries the threat of pollution.
The Crown-of-Thorns starfish preys on coral polyps and large outbreaks of these starfish can be devastating for a coral reef. They can consume 1 sq m of coral each day and outbreaks of this species is believed to occur in natural cycles, worsened by poor quality water and over fishing of their natural predators.
The Great Barrier Reef Marine Park and the Great Barrier Reef World Heritage Area differ slightly in terms of boundaries. The world heritage area is approximately 2,600 sq kms larger because it takes in a few small parts of the Queensland Coast too, such as inlets and coves, whereas the marine park just covers the area of sea. The approximate figures are:
GBR Marine Park - 345,400 sq kms
GBR World Heritage Area - 348,000 sq kms.
The great barrier reef can be seen from space and it is the world's biggest single structure made by living organisms.
The Queensland National Trust named the great barrier reef a state icon of Queensland.
The reef is a very popular tourist destination, especially in the Whitsunday Islands and Cairns regions.
Tourism is an important economic activity for the region and it generates around AU$1 billion each year.
The great barrier reef is considered to be one of the seven natural wonders of the world.