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2013 Scientific Consensus Statement

Land use impacts on Great Barrier Reef water quality and ecosystem condition

Scientific Consensus in 2013

To support the development of the Reef Water Quality Protection Plan 2013 (Reef Plan) a multidisciplinary group of scientists, with oversight from the Reef Plan Independent Science Panel, was established to review and synthesise the significant advances in scientific knowledge of water quality issues in the Great Barrier Reef and to reach consensus on the current understanding of the system.

The overarching consensus is that key Great Barrier Reef ecosystems are showing declining trends in condition due to continuing poor water quality, cumulative impacts of climate change and increasing intensity of extreme events.

The evidence base is synthesised in a series of five supporting chapters and the following conclusions are based on those detailed reviews:

  1. The decline of marine water quality associated with terrestrial runoff from the adjacent catchments is a major cause of the current poor state of many of the key marine ecosystems of the Great Barrier Reef.
  2. The greatest water quality risks to the Great Barrier Reef are from nitrogen discharge, associated with crown-of-thorns starfish outbreaks and their destructive effects on coral reefs, and fine sediment discharge which reduces the light available to seagrass ecosystems and inshore coral reefs. Pesticides pose a risk to freshwater and some inshore and coastal habitats.
  3. Recent extreme weather - heavy rainfall, floods and tropical cyclones - have severely impacted marine water quality and Great Barrier Reef ecosystems. Climate change is predicted to increase the intensity of extreme weather events.
  4. The main source of excess nutrients, fine sediments and pesticides from Great Barrier Reef catchments is diffuse source pollution from agriculture.
  5. Improved land and agricultural management practices are proven to reduce the runoff of suspended sediment, nutrients and pesticides at the paddock scale.

Independent Science Panel remarks

The Independent Science Panel (the panel) was established in 2009 to provide multidisciplinary scientific advice to the Australian and Queensland Governments on implementing Reef Plan. The panel also oversaw and reviewed the 2013 Scientific Consensus Statement.

In reviewing the evidence and conclusions of the Consensus Statement, the panel noted:

  1. There has been excellent progress over the past four years with greater scientific understanding and measurement of ‘catchment to reef’ processes and progress by the farming community towards land management practices that reduce pollutant loads to the Great Barrier Reef.
  2. Water quality modelling, supported by appropriate validation, indicates that early adopters of best practice land management have reduced total pollutant loads - a significant step towards the goal of halting and reversing the decline in water quality to the reef.
  3. The recent relative risk assessment is a major achievement allowing the development of cost-effective, regionally-specific management actions to improve water quality. The leading example is the recommendation to reduce nitrogen loads from northern rivers. This will reduce the frequency and severity of primary outbreaks of crown-of-thorns starfish arising from floods in this area, which propagate to many other reefs in the central Great Barrier Reef over 15 year cycles.
  4. While current management interventions are starting to address water quality in the Great Barrier Reef, sustained and greater effort will be needed to achieve the ultimate goal of no detrimental impact on the health and resilience of the reef. In addition to continuous improvement, transformational changes in some farming technologies may be necessary to reach some targets.
  5. Conditions in terrestrial catchments are most strongly connected with marine receiving waters during floods but the extreme rainfall causing major floods is often episodic and may be separated by decadal droughts. Consequently, there are inherent and complex lags in this system which must be recognised in performance evaluations of Reef Plan. This challenge is best met by investing in continued development of coupled catchment-reef models and the essential collection of adequate data to calibrate and validate the models.
  6. The Consensus Statement has identified new knowledge needed to help achieve the ultimate goal of Reef Plan. These are outlined in the supporting chapters of the Consensus Statement and will assist with identifying future research priorities. Future efforts should focus on synthesising the knowledge gained and communicating the results to landholders and decision makers. The Consensus Statement provides an excellent platform for this work.


Globally, reefs and other coastal marine ecosystems are exposed to a combination of pressures including increased discharge of sediment, nutrients and pesticides, crown-of-thorns starfish outbreaks, increased bleaching associated with global climate change, and increased incidence of and severity of coral diseases, destructive fishing practices, overfishing or loss of herbivorous fish and other grazing organisms. These pressures have led to precipitous declines in coral cover and to persistent shifts away from coral dominance.

On the Great Barrier Reef, recent evidence shows that coral cover has declined from around 50 per cent in the 1960s to around 14 per cent in 2011 with a well-documented decline from 28 per cent in 1985 to 14 per cent in 2013. Coral cover decline in the northern Great Barrier Reef has not shown the consistent downward trend seen along the developed coast of the central and southern Great Barrier Reef. This reflects the limited catchment development in Cape York. The causes of this decline are various and are often region or reef specific for example:

  • cyclones can physically destroy coral reefs
  • runoff of sediment and nutrients predominantly affects coastal and inshore reefs with evidence linking nutrient inputs with crown-of-thorns starfish outbreaks which impact most of the Great Barrier Reef
  • coral bleaching caused by increased temperatures associated with climate change
  • coral disease linked to degraded water quality and climate change.

Recent declines in coral calcification have been linked to the combined effects of increases in temperature and the effects of ocean acidification. Similarly, the extent of coastal seagrass meadows has been in severe decline, particularly in the southern Great Barrier Reef, which has had associated impacts on dugongs and green turtles. Southern Great Barrier Reef dugong populations are much smaller than in the mid-1960s, a situation exacerbated by recent extreme weather events which damaged seagrass meadows. Mortality of dugongs and green turtles in the southern Great Barrier Reef in 2011 was higher than in any year since monitoring started in 1998.

Declining marine water quality, influenced by terrestrial runoff, is recognised as one of the most significant threats to the long-term health of the Great Barrier Reef. Management of this issue will improve ecosystem resilience to other pressures including those associated with a changing climate.


The establishment of Reef Plan in 2003 by the Australian and Queensland Governments was supported by evidence showing a decline in water quality and the associated ecosystem decline on the Great Barrier Reef.

A comprehensive review of the evidence available at the time was prepared by a taskforce of experts. In 2008, an updated Scientific Consensus Statement was prepared as part of the review and updating of Reef Plan 2009. This ensured that Reef Plan 2009 was based on advances in scientific knowledge.

The key finding of the 2008 Scientific Consensus Statement was that water discharged from rivers to the Great Barrier Reef continued to be of poor quality in many locations with land derived contaminants, including suspended sediments, nutrients and pesticides, present at concentrations likely to cause environmental harm. The other main findings were that climate change and major land use change had confounding influences on Great Barrier Reef health and current management interventions were not effectively solving the problem.

This document was prepared by an independent panel of scientists with expertise in Great Barrier Reef water quality. This document does not represent government policy.

Lead authors: Jon Brodie, Jane Waterhouse, Britta Schaffelke, Frederieke Kroon, Peter Thorburn, John Rolfe, Jo Johnson, Katharina Fabricius, Stephen Lewis, Michelle Devlin, Michael Warne, Len McKenzie

Independent Science Panel: Roger Shaw, Eva Abal, Mike Grundy, Peter Doherty, Neil Byron

Summary of evidence to support the Scientific Consensus Statement 2013

  1. Marine and coastal ecosystem impacts
  2. Resilience of Great Barrier Reef marine ecosystems and drivers of change
  3. Relative risks to the Great Barrier Reef from degraded water quality
  4. Sources of sediment, nutrients, pesticides and other pollutants in the Great Barrier Reef catchment
  5. The water quality and economic benefits of agricultural management practices
Last updated:
27 August, 2014
Last reviewed:
8 July, 2013

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