Thursday, 10 June 2010

Underwater treasure off Rotto

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LOCATED at 32 degrees south, the giant canyon also known as Rottnest Trench or Swan Canyon begins its deep-sea course almost immediately at the 50m-deep water contour about 20km west of Rottnest Island.

From there, it winds its way out to depths of up to four kilometres, emptying into a vast abyssal plain, about 200km off the WA coast.

depth
A three-dimensional cross section image of the Perth Canyon, which ranges in depth from 700m to 400m west of Rottnest. / Image: Courtesy Curtin University

Like most underwater canyons around the continental fringe, Perth Canyon formed after thousands of years of scouring by river outflows into the sea at rates faster than today.

The depth of the continental shelf at the head of the canyon rises rapidly from 200m to 1000m, where blue whales gather to feed and frolic in schools of shrimp brought to the surface by giant upwellings of plankton-rich water, mostly between the months of December and May.

Basking in this rich biomass are blue whales, pygmy blue whales, sperm whales, beaked whales, dolphins and dugongs.

Curtin researcher Susan Rennie, who completed her PhD thesis on the canyon, says the Leeuwin Current had a peculiar impact on the canyon.

“The Leeuwin Current is an unusual current.

“Submarine canyons tend to be areas of high productivity. But the current leaves the coast nutrient poor. In the Perth Canyon, the Leeuwin Current tends to suppress the upwelling of nutrients in the water.

“It tends to cause down-welling, so nutrients cannot reach the layers near the surface.”

These deep ocean eddies, which are usually spin-offs from the southward-flowing Leeuwin Current, can reach sizes visible from space, often migrating away from the WA coast as pairs of nutrient-rich pools of warm water.

Eddy formation in the Leeuwin Current is particularly strong during winter, when the current is stronger.

Scientists from the University of WA, Murdoch, the CSIRO and three US, French and Spanish research institutions have observed ocean eddies dragging fish larvae away from the Perth coast.

One eddy, or vortex, measured 200km wide and 1km deep, spinning at speeds of up to 5kmh just off the Perth Canyon, where eddies are known to form, carrying plankton closer to the surface - the swirling biomass spotted from space by NASA’s Aqua satellite.

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