An independent compilation of knowledge about the effects of dredging and sediment disposal on the Great Barrier Reef World Heritage Area has found the impacts will differ, depending on the location, timing, size and type of dredging and disposal activity.
The Dredge Synthesis Report was produced by a 19-member panel of experts, brought together through a joint initiative of the Great Barrier Reef Marine Park Authority (GBRMPA) and the Australian Institute of Marine Science (AIMS).
The technical and scientific experts – with a range of skills from oceanographic modelling to coral ecology – were asked to review and synthesise existing studies and data on the biophysical effects of dredging and disposal, while also identifying key knowledge gaps.
The outcome is an overview of the potential impacts of dredging and disposal on habitats such as coral reefs and seagrass meadows, and on fish and species of conservation concern such as dugong or marine turtles.
Among its key findings, the panel concluded:
- In terms of direct effects, dredging and burial of seafloor habitats during disposal can have substantial impacts at a local level, but have only a small impact on the broader Great Barrier Reef and its biodiversity as a whole.
- In terms of indirect effects, sediments released by dredging and disposal have the potential to stay suspended in the water and move. This may be contributing significantly to the long-term chronic increase in fine suspended sediments in inshore areas, however there wasn’t consensus among the panelists on the extent to which this happens and its impact on biodiversity.
- Dredging and disposal may be a significant source of fine sediments in the World Heritage Area, in addition to other sources, such as land run-off. A general comparison shows past large dredging projects produced amounts of fine sediment similar in magnitude to natural loads coming from land run-off in the same region.
- The recent policy commitments to ban disposal of capital dredge material in marine environments will mean future disposal, which will be limited to maintenance dredging, will contribute much less fine sediment. This reduced amount will still need to be considered in the context of other cumulative impacts on the Great Barrier Reef.
Project leader Dr Laurence McCook said the report acknowledged the challenges in assessing effects. “Understanding the significance of dredging impacts compared to other natural and human pressures remains difficult,” Dr McCook said.
“For example, it’s difficult to compare sediment from dredging and the amount of sediment that flows off the land and into the ocean because there’s limited data and many differences in the physical and chemical make-up of different types of sediments.
“In many locations, the extent to which sediments are resuspended and transported by waves and ocean currents, or are stable in the long-term, is also poorly understood.”
Australian Institute of Marine Science research program leader Dr Britta Schaffelke said the panel proposed a number of ways to reduce the knowledge gap.
“A better understanding of how sediment moves, settles or disperses over the long term in the Great Barrier Reef can be achieved through more extensive, long-term monitoring and better integration of current monitoring and data,” Dr Schaffelke said.
“Further efforts are also needed to understand how increased turbidity and suspended sediments affect a wide range of marine species.
“The report also found the disposal of dredge material on land involves a different set of challenges to be considered, ranging from technical feasibility to the assessment of environmental impacts.”
The publication is part of ongoing efforts by GBRMPA and the Australian Institute of Marine Science to improve understanding of the effects of dredging and dredge disposal, enabling better management of the risks associated with these activities.
The work of the panel will help in updating best practice guidelines, and in assessing proposed developments that involve dredging and disposal in the World Heritage Area.