After Indonesia, Australia has the largest area of coral reefs of any nation, nearly 50,000 square kilometers, or some seventeen percent of the world's total area of reefs. Conditions for reef development vary considerably along the coastline. In the far west the climate is dry and there is little terrestrial runoff. Reef development is not continuous, though away from loose coastal sediments there are important areas, including Australia's best developed fringing reefs. The southward flowing Leeuwin Current is also important on this coastline, bringing warm waters to relatively high latitudes and enabling the development of some unique reef communities. Further north there are several reefs on the outer edges of the continental shelf. These include remnants of what may have been a substantial barrier reef structure drowned as a result of rising sea levels over geological time scales. The northern coastline is less known, however this is an area of high terrestrial runoff, and the waters are shallow and turbid, greatly restricting reef development. The eastern boundary of the Arafura Sea is marked by a narrow constriction, the Torres Strait. East of here, the world's largest coral reef complex commences, extending out to the margins of the continental shelf and continuing southwards as the Great Barrier Reef. The warm, southward flowing East Australia Current also supports the development of high latitude reefs along Australia's eastern shores  to the south of the Great Barrier Reef. Other reefs are found in Australia's offshore waters. Most notable among these are the extensive reef structures of the Coral Sea, east of the Great Barrier Reef.

Australia also administers the Cocos (Keeling Islands and Christmas Island in the Indian Ocean, both of which have significant coral reefs.

Australia's original human population, the Australian Aborigines, are thought to have inhabited the country for more than 40,000 years. These people, and the Torres Strait Islanders who occupy parts of the far northeast of the country, have traditionally made considerable use of reef resources. It seems likely, however, that their overall impacts remained minimal. Population densities were low, and a large area of offshore reef remained inaccessible to them.

The dominant human impacts on coral reefs can now be related to fisheries and terrestrial runoff from deforestation, overgrazing and certain agricultural practices. Compared with most countries, however, these impacts remain few. Population densities are low in all coral reef areas, while the location of many reefs at some distance from shore further protects them from human impacts.

Considerable resources have been put into coral reef research in Australia, and, despite the vast area of reefs in the country, there is a good deal of information describing their distribution and biodiversity. Equally importantly, the great majority of Australia's reefs fall within protected areas. The Great Barrier Reef Marine Park is the largest protected reef in the world, and is well managed with a detailed zoning plan, providing areas of strict protection alongside much larger areas of multiple use.

The reefs in the west of Australia encompass a variety of types in a very broad range of oceanographic conditions. For the most part this is a very dry coastline with very little terrestrial runoff. It is also, from a human perspective, very sparsely populated, and poorly documented. One critical oceanographic feature is the Leeuwin Current which flows south from Indonesia, carrying warm waters to relatively high latitudes, particularly along the continental shelf edge.

Along the mainland coast, reefs are discontinuous but very well developed in places. In the north the continental shelf is very wide and dominated by turbid waters with strong currents. Reef development is little known off the Eighty Mile Beach, through further west there are scattered reefs among the Dampier Archipelago and the Monte Bello Islands. Here, as the continental shelf narrows, there is a great range of oceanographic conditions associated with the gradient between nearshore turbid waters and clear offshore waters, mixed by the complex current regime.

Australia's longest continuous fringing reef system is the Ningaloo Reef which follows some two hundred and thirty kilometers of the coastline running southwards from North West Cape. The reef flats are well developed, lying between 0.5 and 7 kilometers offshore. The reefs receive the full impact of oceanic waves, so corals tend to be quite low and compact. Biodiversity is very high, with some three hundred species of coral, nearly five hundred species of fish and over six hundred mollusks. The area is also noted for the appearance of whale sharks. These giant plankton-feeders occur in considerable numbers between mid-March and mid-May.

The marine areas of the Shark Bay World Heritage Site are of considerable interest, including some of the most extensive seagrass communities in the world and harboring what is probably the largest dugong population in the world (over 10,000). Monkey Mia Bay has become famous for a tame group of bottle nosed dolphins, but the region is also of considerable importance for other cetaceans, including humpback and southern right whales. Hamelin Pool, within the Shark Bay area, is one of the few places in the world where there are actively growing stomatolites. Hypersaline conditions prevent the survival of most organisms, but photosynthetic bacteria and microalgae survive and form microbial mats as they trap and bind sediments. Over the last 4,000 years these mats have developed into relatively large structures -columns or mounds up to 1.5 meters high.

The southernmost true reefs in the Indian Ocean are around the Houtman Abrolhos Islands which lie close to 20 degrees South on the edge of the continental shelf. Lying on the edge of the continental shelf they are directly affected by the Leeuwin Current which moderates the winter temperatures, and may also have a critical role in larval supply. Considering their high latitude, these reefs have a significant diversity, with over one hundred and eighty coral species and over two hundred and thirty fish.

Further south there is no true reef development, though Rottnest Island off the coast near Perth is fringed by shallow platforms where some twenty five species of zooxanthellate corals have been recorded. The importance of the warming effect of the Leeuwin Current is equally strong here. Low human populations generally restrict impacts on the reefs off the west coast of Australia, although there is some fishing in all areas. Around the Dampier and Monte Bello islands there is increasing pearl oyster farming, petroleum exploitation and now some tourism, although these impacts are not very high. Two large protected areas have been declared which provide at least some protection for the reefs in Ningaloo and Shark Bay.

Cocos (Keeling) Islands and Christmas Islands are two territories administered by Australia with important oceanic communities. Located far out in the Indian Ocean, Cocos (Keeling) consists of two atolls on the Cocos Rise, nearly mid-way between Australia and Sri Lanka. They are dominated by the southeast trade winds and swept by the westward flowing equatorial current most of the year, and are occasionally impacted by tropical cyclones. These islands were in fact the only atolls where Charles Darwin ever landed during his voyage on the Beagle in 1836.

Christmas Island is a high, mountainous island some fifteen kilometers across, reaching a height of three hundred and fifty nine meters. Fringing reefs surround much of it, and while the reef faunas clearly contain Indian Ocean elements they show a close affinity to Southeast Asia.  Diversity is somewhat limited by a moderate range of reef habitats. The island has important seabird nesting colonies, including the endemic Christmas Island frigate bird.  Large numbers of crabs are also noted, including thirteen land crabs, the best known of which are red crabs Gecarcoidea natalis, which have a population of some 120,000,000 individuals and undertake an annual mass migration to spawn in the sea. Over sixty percent of the island and much of the fringing reef is protected in a national park.

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North of Port Headland and Eighty Mile Beach the continental shelf of Australia widens considerably while the coastline of Indonesia and East Timor forms a northern boundary enclosing the Timor Sea. To the east of Darwin this continental shelf widens further still and connects Australia to New Guinea across the Arafura Sea and the Gulf of Carpentaria. This is Australia's least known and least populated coastline. Wide areas are dominated by an intricate network of rivers and channels with extensive mangrove communities.  To the east, in the Arafura Sea, the waters are shallow and turbid and there is little reef development. Fringing reefs are reported further west, but are very poorly described. The only reefs in this region which have received attention are those lying in the northwest on the continental shelf edge or just beyond.

The Rowley Shoals, Scott Reef and Seringapatam Reef are shelf edge atolls lying on the continental slope in clear oceanic waters. An extensive live of other reefs, including Lynher, Cartier, Ashmore and Hibernia, lies just on the continental shelf, and it has been suggested that these may in fact be barrier structures.  A number of deeper shoals on the shelf edge indicated that there may have been a more extensive barrier reef along this shelf during recent periods of lower sea level, but that only these structures kept up with rising sea levels. Tidal ranges are very high around these reefs and there is considerable wave energy, so the reef crests are dominated by coralline algae, while only compact coral formations have developed on windward shores. This is an area regularly affected by cyclones.

Coral cover is typically high. Ashmore Reef has the greatest biodiversity in the region: some 255 species of hermatypic corals have been recorded, 747 fish, 433 mollusks and 192 echinoderms. This region also probably has a greater diversity of sea snakes than anywhere else in the world, with twelve species recorded at Ashmore Reef.  Seabird nesting colonies are also extremely important  and seventeen species (with an estimated 50,000 pairs)  have been recorded nesting on the islands of Ashmore.

The more northerly reefs lie relatively close to Indonesia and are regularly fished by Indonesians under a joint agreement. Elsewhere, including near Scott Reef, there is some extraction of natural gas, and further exploratory drilling and the establishment of new oil and gas platforms could bring further human impacts to these otherwise remote reefs. Ashmore Reef and the Rowley Shoals all have some degree of legal protection, and there is ongoing monitoring of Scott Reef and the Rowley Shoals.

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The largest coral reef system in the world runs along the northeastern coastline of Australia, stretching from the Warrior Reefs in the northern Torres Strait for well over 2,000 kilometers to the Capricorn-Bunker group of reefs and islands in the south. Although many of the reefs which make up this system form part of a true barrier reef following the continental shelf on its outermost edge, the Great Barrier Reef is actually a highly complex system including nearly 3,000 separate reefs and coral shoals, as well as high islands with fringing reef systems.

The origins of the Great Barrier Reef can be largely traced back some 2 million years, when continental drift brought the northern coastline of Australia into tropical latitudes and some minor reef development began. Widespread development is thought to be much more recent however, and can be largely traced back within 500,000 years, making it a much younger structure than many oceanic atolls. As with coral reefs the world over, periods of reef building were continually interrupted by changes in climate and shifting sea levels. Typically reef accretion was confined to relatively short periods of higher sea level when reef structures began to build up on the margins of continental shelf. As sea levels fell, the reefs died, became land and were subject to erosion forces which in many places reduced their size again. High sea levels returned and allowed new reef growth, typically most prolific on the remaining structures of the earlier reefs. At the present time in geological history, sea levels are particularly high, such that the base of many of the present reef lies in depths unsuitable for active reef growth. However, active reef building continues on the ancient structures and the reef continues to thrive. The most recent period of growth is probably about 8,000 years.

Although best seen as a continuous reef complex, it is possible to distinguish a number of ecological regions within the Great Barrier Reef. These are the Torres Strait area, Northern Section, Central Section, The Swain and Pompey Complexes and The Capricorn-Bunker Group.

Torres Strait located in the far north of Australia receives considerable freshwater and sediment input from the Papua New Guinea coastline, however there are several very extensive platform reefs across the relatively shallow waters of the Strait. The westernmost areas have the shallowest  and most turbid waters. A large chain of reefs runs between Prince of Wales Island and Moa Island. Like other reefs in the area, these show a very clear east-west alignment associated with the high velocity tidal currents running through the area. The Warrior Reefs further to the north and east run in a chain towards the coastal town of Daru in Papua New Guinea. Sediment loads are high in this area, and much of the shallow surface of these reefs is dominated by soft muds, although they are fringed by coral on their eastern margins. Finally there is a wide area of platform reefs around Darnley Island, stretching out towards the edge of the continental shelf and the near continuous reefs which mark the northern edge of the outer barrier reef.

The Northern Section of the Great Barrier Reef has as its most distinctive feature the well developed ribbon reefs typically less than 500 meters wide but extending up to 25 kilometers in length and separated by relatively narrow passes. They are located on right on the edge of the continental shelf, and depths drop rapidly to over 1,000 meters only a few hundred  meters from the eastern edges of some reefs. For about 80 kilometers, in the northernmost sector of these ribbon reefs, there are spectacular deltaic formations in the channels between the reefs, formed by the powerful currents which flow between the reefs.

Inshore of the ribbon reefs there are well developed mid-shelf and inner shelf reefs, while there are also wide areas of submerged Halimeda-dominated shoals and banks. This is one of the only areas where there are fringing reefs directly adjacent to the mainland coast, although coral cover and diversity are limited. Raine Island just off the continental shelf has the largest nesting populations of green turtles in the world as well as some of the most important seabird rookeries.

The Central Section extends from Mossman in the north to the barrier reef offshore from the Whitsunday and Lindeman Islands. Over this area the continental shelf gradually widens, with reef development largely restricted to its outer third. Closer to the mainland the waters are subject to considerable fluctuations in turbidity and salinity due to the seasonal flooding of rivers. The reefs in this region are younger than those to the north. Many have lower and less extensive reef flats, and coral cays are largely absent, while their outer reef crests are often only clearly developed on the windward southeastern margins. Overall the reefs are less tightly packed and hence do not form such a continuous barrier. In addition to the barrier reef structures there are important fringing reef communities associated with a number of high island groups, notably the Palm Islands and the Whitsunday and Lindeman Islands to the south.

The Swain and Pompey Complexes are in a sector of the Great Barrier Reef where the continental shelf is at its widest and the main reefs are furthest from shore. The Pompey Complex has a number of submerged reefs on the edge of the continental shelf. However, about ten kilometers back from this edge is a vast and complex array of very large reef  platforms separated by countless meandering channels making a nearly solid mass of reefs nearly 200 kilometers in length and up to twenty kilometers wide. To the south the Swain Reefs form a second distinctive complex, dominated by many much smaller and even more closely spaced reefs where large numbers of small coral cays have developed. Inshore of the main Swain and Pompey Complexes reef development is limited, however there are some reefs close to the mainland and associated with island groups such as the Northumberland Islands and Percy Isles.

The Capricorn-Bunker Group lie south of the Swain Reef Complex and are a relatively small complex, well defined with steeply sloping reef edges and deep inter-reefal waters.  There are several well developed coral cays, included One Tree Island and Heron Island, which are among the best known reefs of the Great Barrier Reef.

Levels of biodiversity are generally very high in the Great Barrier Reef, with some 350 coral species, 1,500-2,000 species of fish, and over 4,000 species of mollusks. As might be expected with any reef system traversing such a wide latitudinal range, there is a gradual diminution of species diversity towards higher latitudes. Even more notable are cross-shelf differences. Close to the mainland there are high levels of nutrient inputs, sediments and freshwater, while offshore such inputs diminish and conditions on the outer reefs can be considered near oceanic, with low levels of nutrients and clear waters. These differences have led to considerable variation in the species assemblages depending on their location on the continental  shelf.

One of the most spectacular phenomena in nature was first observed on the Great Barrier Reef - the mass spawning of many corals which takes place every year for a few nights after a particular full moon in the late Australian spring -typically November. At this time, the majority of scleractinian coral species, together with many other reef organisms including sponges, holothurians, polychaetes and giant clams, undergo a mass spawning event. This is highly synchronized, with individuals of  the same species releasing eggs and sperm often minutes of one another over wide areas of the reef. This phenomena was first observed in November 1982, and yet is one of the most spectacular events on any coral reef.

In addition to its considerable diversity in terms of coral reef organisms, the Great Barrier Reef is also an extremely important region for other marine and coastal ecosystems, most notably seagrass and mangrove communities. Mangroves generally lie a considerable distance from coral reef communities -with the exception of a few fringing reef systems. But some 37 mangrove species from 19 families have been recorded at the Great Barrier Reef, with the highest levels of diversity in the "wet tropics" north of Cairns. Seagrass communities are also widespread, with some 3,000 square kilometers of mapped seagrass shallows, and an estimate of at least 2,000 square kilometers of deep seagrasses. Both seagrass and mangroves are extensively used as breeding and nursery grounds by many species, including a number of commercially important species, and some reef species. Seagrass beds are also important for some turtle species as well as large populations of dugongs. Green, hawksbill, loggerhead and flatback turtles all nest in considerable numbers in the region. In addition some twenty-six species of cetacean are resident and visitors to the Great Barrier Reef, including significant numbers of humpback whales which breed in the southern and central waters. There are also important seabird communities on the Great Barrier Reef, with over 55 major nesting islands and some 1.4 to 1.7 million breeding birds from twenty-three species, with a further thirty-two non-breeding species.

The Great Barrier Reef has been one of the regions most extensively impacted by the crown-of-thorns starfish, with the first mass outbreak of this predator observed on Green Island, off Cairns, in 1962. While there is much to learn about these outbreaks, it is clear they have had a significant impact on the ecology of the region, causing apparently periodic massive losses of live coral cover.

In general, the Great Barrier Reef  is not heavily affected by human activities, but there are some concerns that deforestation, poor agricultural practices and high concentrations of agricultural chemicals and nutrients in terrestrial runoff may have some impacts, particularly on those reefs closest to the mainland. The majority of reefs, however, are far offshore and this, combined with the prevailing long-shore currents, reduces the effects of land based sediments and pollutants. The distance from the mainland of most reefs also makes access more difficult, while the coastal population adjacent to the reef is small overall and does not generally exert a very large direct impact on the reefs, except for some commercial fisheries.

Utilization of marine resources by the wider population if far more significant than that by indigenous communities, including the Aboriginal inhabitants of Australia.  Recreational fishing is extremely popular, although it typically targets the reefs closest to the mainland and near the major population centers. The vast majority of the Great Barrier Reef receives protection as the Great Barrier Reef Marine Park, the world's second largest protected area. This covers most of the lagoon and all of the offshore reefs from the Capricorn-Bunker group to the northern tip of Cape York Peninsula. A large proportion of the remaining coastal waters and terrestrial areas of the offshore islands which are not covered by the park fall within other protected areas. The park itself is zoned. About 80 percent of its total area is open for general use including commercial fishing and trawling (with permits). Only about five percent is closed to fishing activities, but this includes over 120 reefs (about twelve percent of the total). The park is managed by a specially designated federal agency, the Great Barrier Marine Park Authority, in collaboration with the Queensland Department of Environmental and Heritage.

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Australian Government Department of Environment and Heritage: Great Barrier Reef
Australian Government Great Barrier Reef Marine Park Authority
Australian Marine Sciences Association
Dugongs: Australian
Great Barrier Reef  
National Geographic website on the Great Barrier Reef
Reef Base: Australia

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Due east of the Great Barrier Reef and the Torres Strait, Australia holds jurisdiction of a large number of reef formations lying some distance off the continental shelf. The majority of these are located in an area known as the Coral Sea. Most are atoll formations, stretching from Osprey Reef in the north to Saumarez Reef in the south.  In general they have relatively low coral cover, and both coralline algae and sponges make up a considerable proportion of the substrate. Total algae cover is often greater than coral cover. There are important bird nesting colonies on some of the coral cays, while the beaches are widely utilized by nesting turtles.

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Reef Base: Australia

South of the Great Barrier Reef there are several reefs and coral communities at high latitudes. The south flowing East Australia Current has an important role to play in maintaining these communities, bringing warm waters ass well as the potential for new larval recruits to settle on the reefs. Biodiversity is low in these areas but they remain of significance since they represent the ecological limits of many species.

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Reef Base: Australia

Extracted and adapted from The World Atlas of Coral Reefs, by Mark D. Spalding, Corinna Ravilious and Edmund P. Green, published by the University of California Press . For more complete and in-depth coverage of the topics presented in this webpage, I recommend highly purchasing a copy of this beautifully illustrated book. Just click on the University of California Press link above to do so.