Identification of key pollutant sources in Great Barrier Reef catchments
Project Leader: Jon Brodie, JCU
The key findings from this MTSRF-funded project are:
- Elevated concentrations of dissolved inorganic nitrogen in streams are clearly linked to the runoff of fertiliser residues in areas of intensive cropping (particularly sugarcane and bananas).
- Some specific soil types may also contribute a disproportionate phosphorus load.
- Grazing lands of the Dry Tropics (i.e Burdekin and Fitzroy) contribute the vast majority of the suspended sediment load to the Great Barrier Reef lagoon.
- Pesticides detected in streams are sourced to intensive cropping lands, with the exception of tebuthiuron which is sourced to grazing land use.
The total key pollutant loads from each region have been calculated based on a combination of long-term monitoring data and recent SedNet/ANNEX model outputs. ‘Hot spot’ areas or catchments have been identified, as well as the key land uses in each region which would benefit from further management consideration. An overall risk assessment of pollutant runoff from each basin of the Great Barrier Reef catchment area (Wet Tropics, Burdekin, Mackay Whitsunday, Fitzroy and Burnett Mary) was conducted using current ‘anthropogenic loads’ (current load minus pre-European settlement load), reef condition score and reef exposure score, to produce an ‘overall relative risk score’. This risk assessment rates the Mackay Whitsunday and Wet Tropics regions as the areas with the highest overall risk. The Burdekin and Fitzroy regions are ranked as medium-high, while the relative risk to the Burnett Mary region is medium. Results from this project have been instrumental in the development of regulatory thresholds for water quality on the Great Barrier Reef.
Connectivity and risk: tracing contaminants from the catchment to the reef
Project Leader: Jon Brodie, JCU
This MTSRF-funded project has been successful in tracing materials, particularly sediment and pesticides, from the terrestrial environment to the marine environment through the processes of generation, transport, transformation, trapping and risk. Inshore-offshore sediment transport in the Wet Tropics has also been investigated by identifying the relationships between sediment input and transport and regional turbidity regimes. Researchers have shown that the movement of sediment from the Burdekin catchment is dependent on the size and also the location of flood events. The Burdekin Dam plays an important role in trapping a high proportion of incoming suspended sediment, with the bulk of the suspended sediment delivered to the Great Barrier Reef lagoon coming from the lower catchments below the Dam (Bowen/Bogie catchment). The project has now started to produce data on suspended sediment behavior in flood plumes by describing the pathway of the finer sediment and its potential impacts on the reef. This exciting work has shown that the finer sediment signature is still present in secondary flood plume waters (though masked through organic flocculation processes), and further work can potentially start to connect plume behaviour and potential impacts with specific soil types/sources in the sub-catchments of the Burdekin River. Geochemical records from coral cores have been successfully used as proxies to demonstrate long-term changes in catchment sediment loads, and to develop long-term histories of regional trends in agricultural and urban activities. Finally, the sediment budget work in the Tully and Cleveland Bay areas has been successful in developing our understanding of the sediment pathways, and the long-term impact of increased sediment supply. Sediments sourced during high flow events are deposited in the bay, and are re-suspended during the dry season through the trade winds. This has the effect of increasing the long-term turbidity of the bay, with consequences for local coral and seagrass ecosystems. Science-based sediment reduction rates which may potentially promote seagrass and reef growth in this area have been proposed.
Report Series No. 39 - Lewis, S. E., Bainbridge, Z. T., Sherman, B. S., Brodie, J. E. and Cooper, M. (2009) The trapping efficiency of the Burdekin Falls Dam: Estimates from a three-year monitoring program
Article submitted to Estuarine and Coastal Shelf Science, 2007. Approval to upload to website provided by Dr K. E. Fabricius, Australian Institute of Marine Science.