New study shows impact of sediment on Great Barrier Reef worse than first thought

Updated

Sediment being washed into the ocean from rivers is continuing to damage the Great Barrier Reef and is having a more widespread impact than scientists first thought.

New research into the impact of river run-off has led to renewed calls for better land management practices.

Sediment is one of the biggest pressures on the health of inshore reefs.

It clouds the water and blocks sunlight from reaching the photosynthetic algae that gives coral its vibrant colours. The algae depends on the sun to survive.

It can also kill or damage sea grasses, which are important food for mammals and fish because they also need the sun to survive.

The study shows that large river flood events during the wet season are washing sediment into the ocean, which is having a significant impact on water quality around the reef.

The sediment reaches far off the coast and lasts several months.

Sediment impact worse than first thought

The Australian Institute of Marine Science (AIMS) has led the study of 10 years of satellite data, provided by NASA's Ocean Biology Processing Group.

AIMS research program leader Dr Britta Schaffelke says the study used new techniques to examine the long-term data.

She says it showed the sediment had a greater impact than previously thought.

"If we have large flood events, there are really visible brown flood blooms that are reaching into the coastal area and we always knew that the water clarity is then reduced for probably several weeks or perhaps even months at a time.

"The study showed that water clarity was affected not only in the inshore area, but actually at a lower level it was visible quite a way off shore, which is something that we didn't know before."

The study focused on run-off from the Burdekin River.

By removing the variability in water clarity caused by waves or other forces stirring up the ocean bottom, the researchers could identify the specific impact of river run-off.

Dr Schaffelke says the research has also given scientists a better understanding of how long the impact lasts.

"We now have a better understanding of how far the reach is of the river water, how far the influence goes and also how long that influence lasts," she said.

"So in big flood years that inference actually lasted pretty much into the dry season, whereas in the years where there wasn't that much river flow was still turbid in the summer but then cleared up towards the winter dry season."

She says that will allow scientists to better analyse the impact of the sediment on the health of the reef.

"We can now compare that to what we know about the ecology of the reef and get a better understanding of how water clarity affects the organisms and how long the effects are lasting."

Calls for better land management practices

The fine sediment in the river is caused by land erosion.

Dr Schaffelke says modelling studies show the amount of sediment in the water has increased significantly since human settlement and the beginning of agricultural activities.

"With better land management practices, which are currently going on in the catchment, we can probably reduce the amount of that erosion and reduce the amount of load the rivers are carrying into the reef."

She says better land use practices are a win-win situation.

"The retention of nutrients, clays and fine silts in the catchments near the Burdekin River would not only safeguard the long-term productivity of farms but also improve water clarity and ecosystem health in the central Great Barrier Reef during the wet and dry seasons."

Work on reducing the amount of sediment entering the ocean has already begun.

"Under the Government's reef rescue initiative, these things have been going on for the last 10 years," she said.

"Landholders are improving their practices, but it takes a long time to fix up the land issues."

The research has been published in the international journal Marine Pollution Bulletin.

Topics: great-barrier-reef, environment, marine-parks, science-and-technology, water-pollution, water, land-management, qld, mackay-4740, townsville-4810, cairns-4870

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