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A report commissioned by Environment Australia October 1997
Commonwealth of Australia
ISBN 0 642 54548 0
Australia's ocean environments are as rich and varied as any on earth. They are linked to three of the world's large ocean basins, the Pacific, Indian and Southern Oceans and encompass all five of the major climate zones, from tropical and subtropical through to southern temperate, subpolar and polar.
The ocean is in constant movement from heating and cooling and the influence of tides and winds which stir waves and currents. The main Australian ocean currents are the East Australian Current, which brings warm equatorial and Coral Sea water down the east coast, and the Leeuwin Current, which transports warm, low salinity water down the west coast. These meet the Antarctic Circumpolar Current in the south. There is also the periodic influence of the wind-driven Southern Oscillation, known as El Niņo which influences ocean surface temperatures with far-reaching effects on the weather.
At any one time conditions in the upper ocean are governed by many factors, such as solar radiation, rainfall, river flow, evaporation, sea-ice formation and tides. Occasional strong winds and storm surges can also have a major effect. Across the shallow continental shelf wind patterns largely determine the speed and direction of water currents and the resulting wind-driven waves are the major source of changing currents in most of Australia's shelf waters. This includes the Great Barrier Reef, the New South Wales Shelf, Bass Strait, the Great Australian Bight and the North West Shelf.
At times these currents are strong enough to bring upwellings of nutrients but, in general, Australian waters are low in nutrients. As a result they have low biological productivity and large ocean areas around Australia are considered `virtual deserts'.
On the other hand some inshore areas are thick with highly productive mangroves, seagrasses and coral communities that are adapted to low-nutrient conditions.
Australian coastal sea life is divided into two main biogeographic regions: the temperate south, and the tropical north, which overlap on the western and eastern coastlines. In the south, which has been geographically isolated for around 40 million years, about 80-90 per cent of species are endemic, which means they are found only in Australia. In the north, which is connected by currents to the Indian and Pacific Ocean tropics, only about 10 per cent are endemic.
The division into these two biogeographic regions, with broad transitional zones, has long been accepted. Recent subdivision of these into 60 more specific biogeographic regions has occurred, providing a useful tool for analysing representations in Marine Protected Areas (MPAs).
Physical ocean features
Significant physical features of Australia's ocean include:
The Australian mainland is surrounded by a continental shelf which is about 200 metres deep and ranges between 15 and 500 kilometres wide. This covers an area of about 2.5 million square kilometres and is connected to Papua New Guinea in the north and Tasmania in the south. Geologically, the shelf is very similar in make up to the continent, in that it is has a foundation of crustal granite, whereas the deep ocean bed has been laid down on basalt.
The shelf extends out from the foreshore and steadily drops away until it reaches the shelf break where the seabed then falls more dramatically into what is known as the continental slope. The shelf break can be found about 10 kilometres offshore from Fraser Island on the east coast and off North West Cape on the west coast. In the north it is about 500 kilometres offshore on the Arafura shelf.
The continental shelf was formed as a result of the movement of huge tectonic plates, which, over many millions of years, saw the moving apart of the larger Gondwanaland supercontinent followed by a series of collisions and uplifts.