Natural history of Rottnest Island
The geographical history of Rottnest Island has been dominated by changes in sea level. These changes occurred either as sea water became trapped and released when ice sheets advanced and retreated, or as the land slowly rose and fell in response to changing stresses in the earth's crust.
It is believed that Rottnest Island was separated from the mainland 7,000 years ago. The sea level rose, cutting the Island off from the land mass, and it is now the largest in a chain of islands (which includes Garden and Carnac Islands) on the continental shelf opposite Perth. These islands all are formed of limestone rocks with a thin covering of sand. The limestone base of Rottnest Island has an effect on all life on the Island, including the types of plants which can grow on it, the species of animals which can feed upon the plants, and the extent to which humans can make use of the Island.
Habitats and Salt Lakes
The Island has six major habitats: coastal, salt lakes, brackish swamps, woodlands, heath and settled areas. Salt lakes occupy ten per cent of the area of Rottnest Island. Many of them - including Lake Baghdad, Lake Vincent, Herschel Lake, Garden Lake, Government House Lake and Serpentine Lake - are permanent and have surrounding beaches. Other lakes such as Pink Lake, Lake Sirius, Lake Negri and the twin Pearse Lakes may dry out in summer.
Rottnest Coral Reefs
The limestone coral reef surrounding Rottnest grew approximately 100,000 years ago when the sea level was thought to be at least three metres higher than the present day. This reef system is fed by the warm Leeuwin Current and provides a home to much of Rottnest's marine life, as well as presenting a significant hazard for shipping.
The Leeuwin Current
The Leeuwin Current is a warm-water current of tropical origin that flows southwards down the Western Australian coast, most strongly in autumn and winter, which maintains much higher sea temperatures off our coast than occur off the west coasts of Southern Africa and South America.
It has two effects on our coastal waters - it transports tropical marine larvae down from more northern latitudes, and it also raises winter water temperatures to enable many of these organisms to survive at Rottnest Island.
Satellite images have revealed some of the complex current patterns associated with the Leeuwin Current. The accompanying images show the Leeuwin Current in red (often enveloping the Island in warm water), with the coolest water in blue.
Average temperatures derived from the satellite data have shown that the coastal waters near the eastern end of the Island vary from about 23°C in summer to 19°C in winter. Thanks to the Leeuwin Current, this is a much smaller temperature range than occurs near the mainland coast, where the temperature falls to about 15°C in winter.
Acknowledgement: Satellite imagery provided by Alan Pearce, CSIRO Marine Research (courtesy of WASTAC).
N9/50190: Satellite image showing the Leeuwin Current (in red) flowing southwards down the Western Australian coast in September 1994. The black line marks the edge of the continental shelf (approximately the 200 m depth contour), and the mottled-white areas towards the top of the image are clouds. Note the large eddy west of Cape Leeuwin carrying the warm water offshore into the much cooler (blue) oceanic waters, as well as the relatively cool water in Geographe Bay at this time of year.
N11/1097: In December, the Leeuwin Current (in red) is weaker and in this image has in fact almost "turned back on itself" to form a very large anticlockwise eddy. The cool Capes Current (blue) is evident flowing northwards from Geographe Bay towards Rottnest Island, and there is a narrow band of warm water along the coast.
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