Rusty stirs up the sea floor off WA

Scientists have for the first time been able to measure properties of the ocean such as turbidity during a tropical cyclone in Australia. The results have taken them by surprise.

As Tropical Cyclone Rusty crossed the Pilbara coast last week, the turbidity levels exceeded the maximum range of the scientists’ instruments.

The high turbidity levels were due to sediment and organic matter being stirred up, or re-suspended, from the sea floor at 30 metres depth.

The measurements were collected from a remote-controlled underwater glider that was on its routine quarterly journey from Broome north towards Scott Reef, about 200 kilometres offshore.

“We’ve deployed gliders on about 150 missions over the past 4–5 years all around Australia and none of them had turbidity levels beyond our instrument range from the surface right down to the sea floor”, says Professor Charitha Pattiaratchi from the University of Western Australia ,who leads the Ocean Glider Facility of Australia’s Integrated Marine Observing System (IMOS).

Turbidity is caused by the action of both waves and currents, and the data collected shows that the speed of the ocean currents almost doubled, he says.  

Nutrients were also stirred up from the sea floor, resulting in a bloom of microscopic plants called phytoplankton, which was evident by an increase in the chlorophyll concentrations.

The bloom is so big—about the size of Tasmania—that it can easily be seen from space, as satellite images show.

“We’ve not been able to collect this type of data until now. Normally we take samples from a ship or a boat but it is not possible to be on board a research vessel in 125km/hour winds. The glider gives us an opportunity to collect data under extreme conditions. We couldn’t have done it otherwise. Satellites provide us information on the ocean surface conditions but gliders tell us what’s happening below the surface.”

IMOS operates a fleet of remote-controlled underwater ocean gliders that take measurements from Australian waters. Designed to withstand extreme weather and equipped with a variety of sensors, they move horizontally through the water while collecting vertical profiles and sending the data back to base via satellite in near-real time. 

Funding for the Australian National Facility for Ocean Gliders comes from the Australian Government.

The ocean gliders and other observing equipment were funded with $6 million over three years by the Western Australian Government as part of an investment to extend IMOS into the Pilbara and Kimberley regions.