Australasian Science Magazine

By Stephen Luntz

The waters of the Great Barrier Reef have been mapped in high resolution, in many cases for the first time.

Shallow waters are hard to map by boat, says Dr Robin Beaman of James Cook University’s School of Earth and Environmental Sciences, but are also generally the most important. Not only are they the places where boats are most likely to come to grief, but shallow reefs have the most biodiversity as the shape of the sea floor has more influence on currents when covered by only a thin layer of water.

North of Cairns, where the coastal shelf is narrow and crossed by many shipping channels, aircraft-borne lasers have been used to map the seafloor but nearly half of the Reef had not been mapped in any detail until a team of German and Australian scientists used satellite data to map 350,000 km2 to a horizontal resolution of 30 metres.

“In order to know the way wave energy spreads across the reef we need to know the shape of the top,” says Beaman. “Scientists badly need data on internal parts, reef flats, knobs and bumps. We need good spatial tools that are consistent and high resolution.”

A wealth of Landsat images is now available using sunlight reflected off the sea floor, providing plenty of opportunity for back-up when clouds interfere with one image. The German EOMAPing tool allows these to be converted to 3D maps.

“Generally we can get good depth resolution to 30 metres, although accuracy becomes limited below 20 metres,” says Beaman. “In very clear water we can go deeper.” Close to the coast, where sediment is more of an issue, depths of more than 5 metres can be a challenge.

The results will be particularly useful for hydrodynamic modelling, Beaman says. “We are trying to understand how flood plumes move through the reef. For modelling we need accurate shapes of the sea floor.” Information about the depths of reefs will also help to determine which are most vulnerable to climate change and human disturbance.

Beaman says the technique will be valuable for Pacific Island nations that are keen to know the health of their own reefs. “It can help us understand how fish larvae disperse,” he says. “There is no end to other examples.”