900,000 truckloads of dirt: Uncovering the Great Barrier Reef’s secret

Normanby River erosion: A team of experts led by Griffith University’s Andrew Brooks has identified the main sources of sediment blighting the Great Barrier Reef. Photo: Griffith UniversityNew techniques to identify and reduce the sources of sediment blanketing parts of the Great Barrier Reef have landed Andrew Brooks and his team at Griffith University this year’s Australian Museum Eureka Prize for environmental research.

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The equivalent of some 900,000 dump-truck loads of dirt ends up on the reef each year. Blocking sunlight and covering corals, the sediment adds to challenges facing reefs that are already threatened by warming waters from climate change.

Associate Professor Brooks and his team studied the Normanby catchment in Cape York and the journey of fine sediment flowing into what was – before the past two summers of widespread coral bleaching – some of the most pristine areas of the reef.

Along with soil loss, the erosion hotspots are sources of nutrients that promote the crown of thorns starfish, another threat to corals. Other important ecosystems, such as sea grasses, are also adversely affected by the run-off.

Land-use changes, such as from grazing, have led to big increases in dirt being washed down the rivers. In the Normanby’s case, sediment loads have more than doubled compared with natural levels, while some other catchments near the reef have seen a five-fold increase.

Contrary to previous thinking, Professor Brooks’ team showed the great bulk of the sediment is not from cleared slopes but rather gullies and stream banks.

Some 40 per cent of the sediment from the Normanby came from gullies, and 40 per cent of that from just one property. This finding prompted the Queensland government to pay $7 million for the 55,000-hectare Springvale property to begin destocking and rehabilitation.

Just 1000 hectares of one property in the Normanby catchment contributed about 16 per cent of total sediment reaching the sea. Photo: Griffith University

“It’s a major thing from our research,” Professor Brooks said. “We can focus rehabilitation on much smaller portions of the landscape.”

The research has implications for other catchments, such as the Burdekin to the south, which drains an area about twice the size of Tasmania.

Of that, the Bowen subcatchment supplies about 70 per cent of the dirt load while accounting for only 10 per cent of the water. Some 30-50 per cent of the sediment comes from as little as 2000 hectares of land, Professor Brooks said.

Apart from helping improve the targeting of some $60 million in government funding for reef water quality, Brooks’ team was recognised for its innovative field and remote sensing techniques that refined sediment modelling.

Gully erosion accounts for much of the sediment ending up in the rivers and ultimately the reef. Photo: Griffith University

The research should also be useful for other states, such as NSW and Victoria. Erosion sources in those areas are more likely to be from stream banks rather than gullies, which can then be the focus of revegetation efforts, Professor Brooks said.

He highlighted the effort to tap specialists from diverse fields, including marine and estuary experts: “This is all one connected system, you can’t compartmentalise.”

While acknowledging the recent devastating impact marine heatwaves have had on the Great Barrier Reef – with as much as a half of corals dying from the heat stress in less than two years -Professor Brooks said governments couldn’t “drop the ball on water-quality issues”.

“If we can improve that one, the reefs have a better chance of bouncing back,” he said.

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