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Sea country: Marine and transition zone environments

Australia's marine area is one of the largest in the world, extending over about 16 million square kilometres from Antarctica to near-equatorial latitudes—more than double Australia's land area. The length of the coastline of Australia's mainland and islands is about 61,700 km (ABS 2006). At least one-third of this coastline borders the tropical savannas region.

Estuaries | Coral Reefs | References |


North Australian beaches are important nesting sites for a number of species of marine turtles

Australia's marine and coastal regions host a broad variety of habitats ranging from estuaries and mangroves, dunes and beaches, rocky and coral reefs, seagrasses, gulfs and bays, seamounts, and a huge area of continental shelf. Some of Australia’s most spectacular cliff coastlines, mangrove forests, archipelagos, island groups and coral reef systems are found within the tropical savannas region.

These include sections of the world famous Great Barrier Reef and the Bonaparte Archipelago off the Kimberley coast.

These coastal and marine habitats are home to a wealth of fauna and flora species. On a broader scale these regions contribute to Australia having:

  • The highest levels of biodiversity in the world for a number of types of marine invertebrates.
  • The highest mangrove species diversity.
  • The world's largest areas and highest species diversity of tropical and temperate seagrasses, and
  • One of the largest areas of coral reefs (SoE 2001).

Two broad habitat types found within the coastal region of the tropical savannas are estuaries and coral reefs.


Estuaries are common along the tropical savannas coast. Estuaries are semi-enclosed coastal water bodies commonly occurring where freshwater from rivers or creeks meet the sea. Here, a mixing of salt and fresh water occurs, creating many different habitats, essential for the survival of the many plants and animals who have adapted to living here. Estuaries are important as;

  • They naturally filter the water keeping the water quality high.
  • They provide habitats for fish, birds and other wildlife to live, feed and reproduce. Many types of fish, shellfish, crustaceans and other marine animals rely on the sheltered waters as protected places to breed and lay their eggs.
  • They have high nutrient levels and generally sheltered waters which provide ideal environments for fish and other animals to lay eggs and for their young to feed and grow.

Estuaries are important for other reasons too. They are popular places where people live and recreate, and tourists like to visit. Fishing, boating, camping, hunting and bird watching are popular pastimes in tropical savanna estuaries. Estuaries also hold important cultural significance for many Indigenous people .

Commercial fishing and aquaculture often thrive in estuaries, which are also the preferred sites for ports and harbours. Most of Australia's near-pristine estuaries are located away from population centres. The majority of estuaries in the Northern Territory are in near-pristine condition, primarily as a result of low population pressure and minimal development in the estuary as well as upstream in the catchment.

Australia's estuaries face a number of pressures from urban and industrial development in coastal areas, and from disturbance through land use and vegetation clearance in catchments. For example, estuaries are often used for dumping, sand or water extraction, construction of marinas, ports and canal estates, and are susceptible to changes in natural flows caused by the construction of dams and weirs. Such pressures threaten the condition of estuaries by causing excess nutrient concentrations, sedimentation, loss of habitat, weed and pest infestation, and the accumulation of pollutants.

Coral reefs

Coral reefs are made up of dead corals and other organisms that have a limestone skeleton. These are cemented together by some algal species and by physical processes. The reef builds at a rate of a few millimetres a year until it reaches the water’s surface, and then it starts growing horizontally. Reefs build as a result of the growth of corals and other living creatures. The accumulation of sand and rubble formed when organisms are broken down by waves and animals, such as worms and sponges that bore into the coral, also add to reef growth (CRC Reef 2002).

Reefs are among the most complex and diverse ecosystems in the world. The Great Barrier Reef which lies off the eastern coast of Australia’s tropical savannas region is the largest coral reef in the world, consisting of about 3,000 individual reefs covering an area of 345,950 square kilometres, 2300km in length. This huge reef system is teeming with wildlife and supports more than 1500 species of fish, 4000 types of mollusc and more than 200 species of birds. Some of the fauna you can find here include fluorescently coloured nudibranchs, chameleon-like cuttlefish, eels, trigger fish, dugong, green turtles and whales.

Australian coral reefs face a number of pressures and threats. These include:

  • Sediment and nutrient runoff into coastal areas from agriculture and land use practices and increasing industrial and urban development.
  • Increased recreational and commercial fishing.
  • Increasing pressure from tourism developments.
  • Threats from invasive and pest species such as the crown of thorns starfish, and
  • Coral bleaching possibly due to global warming (SoE 2001).

A global assessment of reefs found that about 25% of the world's reefs have effectively been lost. A massive coral bleaching event in 1998 destroyed about 16% of the world's coral reefs in nine months (Wilkinson 2000). It is thought that half of these reefs will never recover.

Coral bleaching occurs when the sea surface temperature goes over a certain level, usually just over 30 0 C. The coral appears ‘bleached’ (white) because the algae which live alongside the coral polyps and provide the coral with nutrients in the coral tissues are expelled, allowing the white calcium carbonate skeleton to show through the clear animal tissue cover. If the temperature remains high for more than two weeks, the coral dies.


Australian Bureau of Statistics (2006) Australian Yearbook 2006

CRC Reef (Cooperative Research Centre for Reef Research) 2002, Australia 's Coral Reefs

Great Barrier Marine Park Authority

SoE (Australian State of the Environment Committee) (2001). Coasts and Oceans , Australia State of the Environment Report 2001, CSIRO Publishing on behalf of the Department of the Environment and Heritage, Canberra.

Zann, L.P. (1995). Our Sea, Our Future: Major findings of the State of the Marine Environment Report for Australia.
Department of the Environment, Sport and Territories, Canberra.