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The impact of catchment-derived sediment on the Great Barrier Reef

A new paper uses IMOS data in the model simulations that were developed as part of the eReefs project.

The Great Barrier Reef is the world’s largest coral reef system and a Marine World Heritage site. A general decline in ecosystem health of the Great Barrier Reef (GBR) in recent decades has been attributed to a number of factors including increased terrestrial loads of sediment and nutrients into the GBR lagoon over the last 150 years.

These increases have been linked primarily to the altered land-use practices on catchments that translated into a reduced vegetation cover and elevated rates of the sediment erosion particularly during large flood events. A multi-year Water Quality Improvement Plan has been established and partly implemented in recent years by the Australian Government and the community to reduce run-off of pollutants through the improved management of catchments.

Demonstrating changes in marine systems that result from these management actions, however, is a challenge, because of the high natural variability of sediment processes on the shelf and limited understanding of the role of catchment sediment in maintaining suspended sediment levels over the GBR region.

According to one school of thought, for example, changes in sediment loads from catchments will have a minor impact on chronic turbidity over coral reefs since the sediment store in the lagoon is already large. Another vision, based on recent measurements, suggests that newly imported materials from catchments can cause significant changes in water clarity inshore as well as mid-shelf.

This new paper by Margvelashvili et al  describes a numerical study aiming at better understanding of the distribution and fate of fine sediment delivered from catchments to the GBR shelf. Numerical experiments are conducted using a 3D fine resolution sediment transport model of the GBR region developed through the eReefs project. This model, tested through the extensive calibration and validation studies, represents the first 4 km resolution sediment transport model of the entire GBR shelf coupled to hydrodynamic and wave models.

This study uses IMOS data for model assessment as part of the eReefs project, and demonstrates that investment in sustained ocean observing enables research that delivers benefits across Australian society, its environment and its economy.

The numerical experiments using the 3D model indicate the deposition of the bulk mass of catchment sediments from river plumes within a few tens of kilometres from river mouths. A very fine fraction of easily resuspended catchment sediment has a capacity to propagate over much greater distances reaching out into the mid-shelf and outer-shelf regions. The model suggests such particles, instrumental to the development of low density flocs in the marine environment, can play a critical role in altering optical properties of water masses over the shelf during wet years.

The mid-term (4 year) impact of Great Barrier Reef catchments on the probability of suspended sediment concentration exceeding the ecologically significant trigger value of 2 mg/L is confined to inshore regions adjacent to river mouth locations.

The sediment transport model, coupled to hydrodynamic and biogeochemical models, runs in near-real-time. Output from these models is available online at (https://research.csiro.au/ereefs/).

To read the full paper:

https://www.sciencedirect.com/science/article/pii/S0025326X18305824#s0010 …

eReefs is a collaboration between the Great Barrier Reef Foundation, Bureau of Meteorology, CSIRO, AIMS and the Queensland Government supported by funding from the Australian Government's Caring for our Country, Queensland Government, BHP Billiton Mitsubishi Alliance and the Science Industry Endowment Fund.

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Bathymetry map truncated to 100 m depth (left) and map of the distribution of mud (right) interpolated from GA MARS database. Blue dots on the left plot indicate river discharge points. Enlarged coastal regions represent coastal embayments receiving inputs from two major rivers in GBR – Burdekin and Fitzroy. Figure from Marine Pollution Bulletin. Click on image to enlarge.

Four-year mean probability for Total Suspended solids (TSS) to exceed 2 mg/L according to baseline model (left) and the difference between this probability and that calculated from the model scenario with no river loads during the simulation (right). Red on the left plot indicates areas where TSS is more likely to exceed the threshold. Red on the right plot indicates areas sensitive to river loads. Arrows on the right plot indicate a coastal area enlarged in this figure to enhance visibility. Figure from Marine Pollution Bulletin.