David McB. Williams (CRC Reef / AIMS), Christian H. Roth (CSIRO Land and Water), Russell Reichelt (CRC Reef), Peter Ridd (James Cook University), George E. Rayment (Sugar CRC / Qld NR&M), Piers Larcombe (JCU), Jon Brodie (JCU), Richard Pearson (Australian Centre for Tropical Freshwater Research / JCU), Clive Wilkinson (Global Coral Reef Monitoring Network), Frank Talbot, Miles Furnas (AIMS/ CRC Reef), Katharina Fabricius (CRC Reef/ AIMS), Laurence McCook (CRC Reef/ AIMS), Terry Hughes (JCU), Ove Hough-Gulberg (University of Queensland), Terry Done (AIMS/ CRC Reef)

Run-off of sediment and nutrients to the Great Barrier Reef has increased several-fold as a result of past and current land-use practices. There is significant concern that coastal ecosystems in the Great Barrier Reef World Heritage Area (GBRWHA) are being adversely affected as a consequence of this increase.
While improvements have been made in sustainable land use, other adverse practices continue, including: expansion of farming into marginal areas; increases in fertiliser application; overgrazing; and loss of riparian vegetation and wetlands. If more effective action is not taken to reduce run-off of sediment, nutrients and other pollutants, the present threat to the World Heritage Area and adjacent freshwater systems will worsen.

Provision of a credible and a comprehensive science base that reflects the general state of knowledge accepted by the scientific community is critical to the public debate. This statement intends to provide a consolidated view of our current understanding of the impacts of terrestrial run-off on the Great Barrier Reef World Heritage Area (GBRWHA).

Current scientific understanding about terrestrial run-off in the GBRWHA has been documented in detail in a recent review undertaken by David Williams on behalf of the CRC Reef. It represents the outcome of a review of existing published scientific literature, complemented by a robust and broad consultation of experts in the fields of marine and terrestrial sciences and whose expertise is relevant to the GBRWHA. The review document is accessible by clicking here. Major conclusions of the review are summarised in the CRC Reef brochure Land use and the Great Barrier Reef World Heritage Area.

These documents clearly outline our present state of knowledge. A key conclusion concerns the area under threat. Areas at risk are near-shore reefs and seagrass beds south of Port Douglas and within 20km of the coast. The areas of most concern are those between Port Douglas and Hinchinbrook as well as Bowen to Mackay. A major proportion of the GBRWHA (including the outer reef) is not likely to be threatened by terrestrial runoff.

The other key conclusion is that while enhanced runoff has severely damaged coral reefs elsewhere, there is a lot of uncertainty about the processes by which runoff may damage the GBRWHA. This is not surprising given the complexity and interactions between the physical and ecological processes involved. Since compilation of the review, more scientific publications have become available or are in the process of being published, indicating that our state of knowledge is in constant flux and as new and sometimes contrasting views are introduced, healthy scientific debate is stimulated. This is normal and rigorous scientific debate is an essential part of the process of transforming scientific information into knowledge or understanding.

In summary, the assessment of the potentially adverse impacts of terrestrial runoff and delivery of pollutants (sediments and associated nutrients, pesticides, heavy metals) to the GBRWHA from land-based activities is not straightforward and will continue to be so for a while. The main reasons for this are:

• We have little or no baseline data to indicate what the GBRWHA looked like before European settlement. Much of our understanding has only emerged over the past decade or two, so that in many instances we have to infer from other reef systems with longer records and from the interpretation of "records" embodied within coral cores.
  
  
• Against the backdrop of strong disturbance from natural processes (e.g. cyclones, inherent climatic variability, natural biological cycles), many of which have "recovery" periods of decades, it is very difficult to distinguish or even quantify the relative contribution of anthropogenic disturbances. It is likely that adverse human impacts from enhanced runoff will be first observed in the reduced capability, or failure, of coral reefs or seagrass beds to recover from natural disturbance rather than as direct impacts.
  
  
• We are dealing with complex ecological processes, where inherent ecosystem buffering capacity makes it difficult to identify clear human-induced trends in change from relatively short-term studies. However, in many ecosystems, apparent resistance to change due to high buffering capacities can be followed by an abrupt ecosystem collapse once critical disturbance thresholds have been overstepped. Predicting these thresholds is extremely difficult, yet they are absolutely critical, as an overstepping can often lead to irreversible changes or to very slow rates of recovery when the "pressure" abates.

All three reasons are important enough to adhere to the precautionary principle in managing the land-based sources of impacts on the GBRWHA until we achieve more certainty in our understanding of impacts. The very real risk is that by not undertaking any significant action now to reduce delivery of elevated levels of nutrients and pollutants to the GBRWHA, we may overstep some thresholds leading to irreversible loss of near-shore reef systems and sea grass beds. Moreover, post-European land use has very significantly altered and in many cases caused significant damage to rivers or loss of wetlands in the majority of the catchment area of the GBRWHA. The direct and indirect impacts of the changes to or loss of freshwater biodiversity and food chain links to the GBRWHA have yet to be fully assessed.

In conclusion, on the basis that:

• available evidence indicates that post-European land use has significantly increased runoff and sediment associated nutrient and contaminant delivery to near-shore regions of the GBRHWA,
  
• runoff has had clear detrimental impacts on freshwater aquatic systems,
  
• there is significant risk that this impact is currently or may in future damage areas of high exposure along the wet tropical and central Queensland coasts of the GBRWHA,

There is a continued urgency to work towards a reduction in the runoff of sediments, nutrients, herbicides and other pollutants into the Great Barrier Reef World Heritage Area.

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