How does sediment affect the Great Barrier Reef?

Sediment has a significant impact on the health of a coral reef, although the link between it and the health of the Reef may not always be obvious.

Sediment is just what you think it is – some kind of solid matter that moves from one place to another.

When sediment particles being carried in water (suspension) eventually drop out and settle to the ground, the process is called sedimentation. Really fine sediment is made up of silt and clay.

Nutrients and other pollutants attached to sediments have the potential to be released in the marine environment and create problems for coral and seagrass.

Much of the sediment that is washed to the Reef is very fine and can stay suspended in the water for a long time, often travelling a great distance away from or along the coast. It causes turbidity (the water loses its transparency and looks murky or muddy) which leads to reduced light for seagrasses and coral and can reduce their growth if present for extended periods.

Scientists now know that high concentrations of suspended sediment can interfere with filter feeding by organisms such as clams, reduce coral recruitment, alter the quantity and quality of light available for photosynthesis –– essential for growth of coral and plants such as seagrass –– and even smother corals.  In some areas, turbidity can reduce light penetration for days, weeks or even months.

Fine sediment can remain in inshore areas for decades, making it available for wind-driven resuspension which causes ongoing coastal turbidity.

Sediment can also affect the reproductive cycle and early development of coral and some species of fish, and damage gills.

The main source of the problem

Most of the soil that becomes sediment is the result of erosion on the mainland. It is carried to coastal areas by river and streams as run-off, usually during the wet season when heavy rain and flooding are common.

Monitoring and scientific modelling have shown the main source of sediments from the Great Barrier Reef catchments is from agricultural land use, with grazing including gully and hillslope erosion accounting for nearly half of the fine sediment generated by human activity. The second biggest contributor is streambank erosion. Sugarcane cropping, non-irrigated dryland cropping and other land uses, such as urban, mining and industrial, also contribute but to a smaller degree.

Graziers manage 31.1 million hectares of land and over 100,000 kilometres of streambank in the Great Barrier Reef catchments.

Overgrazing and resulting loss of ground cover makes land vulnerable to excessive erosion. Without vegetation to hold soil together, rain can easily wash large amounts of soil into creeks and rivers. Best management practices such as excluding cattle from gullies and maintaining ground cover promote healthy soil while minimising sediment run off.

Both the Australian and Queensland governments are investing millions of dollars and working with landholders on a range of pilot gully remediation projects that are delivering promising early results. Industry and agricultural producers are also taking action to change the way they use their land, including introducing innovative management practices, to minimise the loss of sediment from their properties.

Last updated
13 February 2019