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by Debora De Freitas last modified 2007-07-12 13:42

There has been a long running debate in the GBR as to the influence of river suspended sediment loads on regional turbidity values in the GBR lagoon. One school of thought, typified by the work of Larcombe, Woolfe, Ridd and colleagues, claims that turbidity in the GBR lagoon is not limited by fine sediment supply i.e. there has already been, for the last few thousand years, sufficient fine sediments in the shallow parts of the lagoon (in generally less than 10m water depth) to produce the turbidity we see produced by wind-driven resuspension. Thus increased suspended sediment delivery from the rivers due to agricultural and urban developments on their catchments will have no effect on regional turbidity (and hence not be a major issue for GBR reefs). The other school of thought, typified by the work of Wolanski, Fabricius and McCulloch, claims that as each new delivery of fine suspended sediments occurs to the GBR lagoon an ‘easily resuspendable’ layer of benthic sediment is laid down. During the following 12 month period this layer is resuspended by wave action but due to its nature it produces higher concentrations of turbidity than would be the case without this ‘easily resuspendable’ layer. Thus increases in the delivery of suspended sediments from the rivers will lead to increased above historic levels of turbidity (and thus be a major issue for the health of GBR reefs).

Resolution of this debate is critical to the priority given to managing erosion on GBR catchments in comparison to managing other pollutants e.g. nitrate from fertiliser or herbicide residues. Preliminary results from Ian Webster (CLW) off the Fitzroy River and Eric Wolanski (AIMS, MTSRF) off the Tully River using turbidity loggers suggests that the second hypothesis (above) may have some validity. I propose that this is a question able to be answered using sensor technology as the sensors for turbidity are available, relatively low cost and work well. The project would have to extend over a number of years to check a number of river discharge events and include a good representation of variable weather conditions.


Jon Brodie is a research scientist specialising in water quality at the Australian Centre for Tropical Freshwater Research (ACTFR) and the Marine and Tropical Science Research Facility (MTSRF). Jon's training was in environmental chemistry at James Cook University in the 1970s examining agricultural chemical residues in groundwaters of the Burdekin River delta. He spent some years as a lecturer in chemistry at Queensland University of Technology (Brisbane) and the University of the South Pacific (Suva, Fiji) and as an analytical chemist with the CRA Metallurgical Research Section (Newcastle). For the last 20 years his interests have been in environmental research and consultancy and the management of marine and freshwater pollution. He spent ten years as an environmental researcher/consultant with the University of the South Pacific (Fiji and other Pacific island states) and the ACTFR. For 11 years he managed the Water Quality Research and Management Program of the Great Barrier Reef Marine Park Authority. He has also been recently involved in a number of environmental projects overseas in the Middle East (Yemen), the Pacific islands (American Samoa, Samoa, Kiribati and PNG) and the ASEAN states.