The Conclusions are the highest-level output of the 2022 Scientific Consensus Statement. They represent the consensus view of 35 experts and are derived from the underlying evidence base.
Consensus methods are used to determine the extent to which experts, or a broader audience, agree about a given issue. Adopting formal consensus methods can be particularly useful when scientific evidence is intended to inform policy decisions as such methods can provide a level of confidence and assurance about the extent of agreement about specific findings. Reaching consensus on the main findings of the 2022 Scientific Consensus Statement is important to build confidence among audiences that hold an interest in water quality outcomes for the Great Barrier Reef.
The definition of consensus applied for the 2022 Scientific Consensus Statement is: ‘A public statement on scientific knowledge on Great Barrier Reef water quality and ecosystem condition, drawn from multiple lines of evidence, that is generally agreed by a representative group of experts. The consensus does not necessarily imply unanimity.’ Formal consensus processes were used to produce the 2022 Scientific Consensus Statement Summary and Conclusions.
The development of the Conclusions involved a formal expert elicitation process designed by an expert Consensus Process Working Group. A consensus workshop brought together the Lead Authors of the 30 questions and several contributors with specific expertise to discuss and agree on a final set of Overarching Conclusions and Concluding Statements for each Theme with a clear line of sight to the Summary and underpinning evidence base. The outputs from the consensus process also highlight the strength of the evidence, and areas where further knowledge is needed.
The Overarching Conclusions and Concluding Statements presented here were agreed by 35 experts involved in the 2022 Scientific Consensus Statement consensus process, endorsed by the Reef Water Quality Independent Science Panel (ISP), and reviewed by three independent eminent experts.
The 2022 Scientific Consensus Statement Summary contains more detailed information for each theme. The most detailed evidence can be found in the 30 syntheses of evidence. It is critical that any policy or management decisions are based on consideration of the full synthesis of evidence.
Overarching conclusions
- Pollutant loads from the catchment area to the Great Barrier Reef have increased from pre-development loads by 1.4 to 5 times for fine sediments, and 1.5 to 3 times for dissolved inorganic nitrogen (with variations depending on basins).
- Historical and continuing land management and catchment modification impair Great Barrier Reef water quality through extensive vegetation degradation, changed hydrology, increased erosion, and expansion of fertilised land uses, urban centres and coastal developments.
- Poor water quality, particularly elevated levels of fine sediments, nutrients and pesticides, continues to have detrimental impacts on Great Barrier Reef ecosystems. The greatest impacts are on freshwater, estuarine, coastal and inshore marine ecosystems.
- Human-induced climate change is the primary threat to the Great Barrier Reef and poor water quality can exacerbate climate-related impacts. Good water quality is critical for healthy and resilient ecosystems and supports recovery from disturbances such as mass bleaching and extreme weather events. Meeting water quality improvement targets[1] within the next ten years is imperative.
- While several land management practices and remediation actions are proven to be cost-effective in improving water quality, translating these into more substantial pollutant reductions will require significant scaling up of the adoption of these actions, prioritisation of pollutant hotspots, and greater knowledge of the costs and potential co-benefits of practice adoption.
- Greater focus on locally effective management solutions can encourage faster adoption, especially when designed and delivered using collaborative approaches involving landholders, Indigenous communities, the broader community, policy makers and scientists.
- World-leading monitoring, modelling and reporting programs underpin the Great Barrier Reef ecosystems and provide essential knowledge to inform water quality improvement strategies. These programs could be strengthened and refined by increasing their spatial and temporal coverage to capture regional and local differences, provide more balanced coverage across land uses and ecosystems, improve trend analysis and quantify uncertainties.
- Expanded research effort and more consistent methods are urgently needed to adequately assess 1) the co-benefits and efficiency (including costs) of management solutions across different landscape and climate conditions, 2) the effectiveness of water quality improvement programs and instruments including assessment beyond the life of programs, and 3) ecosystem risks from a wider range of pollutants.
[1] The 2025 targets defined in the Reef 2050 Water Quality Improvement [currently under review] require a 25% reduction in the 2009 anthropogenic end-of-catchment fine sediment loads, 20% reduction of particulate nutrients, and a 60% reduction of dissolved inorganic nitrogen loads. The target for pesticides is to protect at least 99% of aquatic species at end-of-catchments by 2025.
Theme Concluding Statements section
Values, condition and drivers of health of the Great Barrier Reef
The Great Barrier Reef is one of the world’s most complex natural systems, recognised for its considerable ecological, social, economic, and cultural values. Human-induced climate change is the overriding threat to the Great Barrier Reef, and the ability of ecosystems to recover from climate impacts and acute disturbances continues to be compromised by poor water quality.
- The Great Barrier Reef is a highly connected system, and the movement of water, matter and organisms is critical for many ecosystems from the catchment and floodplains to the outer reef. [Q1.4]
- Since the 2017 Scientific Consensus Statement, the condition of inshore coral reef ecosystems has declined, while coral cover on shallow mid- and outer shelf reefs has shown clear recovery following repeated mass bleaching, tropical cyclones and/or crown-of-thorns starfish outbreaks. Partial recovery has been observed in inshore seagrass meadows. Historical loss of some wetland types has been substantial, but condition trends for mangroves and wetlands have stabilised. [Q1.2/1.3/2.1]
- The primary threat to the Great Barrier Reef is climate change. Poor water quality is also a major threat, especially for freshwater, coastal and inshore marine ecosystems, reducing the ability of marine ecosystems to recover from climate-related disturbances. It is therefore imperative that water quality improvement targets are met within the next ten years for Great Barrier Reef ecosystems to persist. [Q2.2, Q2.4]
- Climate change is increasing the intensity of extreme rainfall events and changing the frequency and intensity of droughts, adding challenges to effective water quality management. Other stressors including tropical storms and crown-of-thorns starfish outbreaks and the combined effects of multiple stressors can negatively affect the health of Great Barrier Reef ecosystems. [Q1.2/1.3/2.1, Q2.2, Q2.3, Q2.4]
- The influence of land-based pollutants, particularly fine sediments, nutrients and pesticides, is spatially and temporally variable between pollutants and locations, and is most pronounced in freshwater, estuarine, coastal and inshore marine environments. These environments provide critical ecosystem services across the Great Barrier Reef and maintain high tourism, aesthetic, spiritual, recreational and economic values. [Q1.1, Q1.4, Q2.3, Q3.1, Q4.1, Q5.1]
Sediments and particulate nutrients
Exports of fine sediment and particulate nutrients from catchments to the Great Barrier Reef are 1.4 to 5 times higher than pre-development rates. Increased delivery of land-based fine sediment and particulate nutrients reduces the quality and quantity of light in Great Barrier Reef ecosystems, with impacts on inshore seagrass meadows and coral reefs. Management is most effective when targeted to sediment and particulate nutrient hotspots which can occur in all land uses from gully, streambank or hillslope erosion.
- Flood plumes carrying fine sediment and particulate nutrients elevate turbidity, with the influence decreasing with distance from river mouths. Some particulate nutrients become bioavailable in the marine environment and promote algal growth but these processes are poorly understood. Reduced water clarity can persist for up to six months due to frequent resuspension of the newly delivered sediment following large floods. Ecosystem responses include changes to the abundance, diversity, spatial extent and recovery rates of inshore seagrass meadow and coral reef ecosystems, and their associated communities including fish and dugongs. [Q3.1, Q3.2]
- Land used for grazing is the largest contributor of fine sediments, because it is by far the biggest land use by area (73%); other intensive land uses can generate large amounts of fine sediment locally but cover a smaller area. The main drivers of anthropogenic sediment and particulate nutrient exports are overgrazing, land clearing, tillage and other soil disturbances. Modelling and observations show that about half of the sediment exported comes from gully erosion, with hillslope and streambank erosion accounting almost equally for the other half. Much of the export comes from a relatively small area. For example, in the Bowen catchment, 30% of the sediment from gully erosion comes from just 2% of the gullies. [Q3.3, Q3.4, Q3.6]
- Land management and remediation are most cost-effective when targeted at hotspots of fine sediment and particulate nutrient export. Large-scale remediation of gullies can reduce sediment exports by more than 90% in restored gullies within 1 to 2 years. Maximising revegetation of streambanks and channels is expected to reduce erosion, but the water quality benefits have not been quantified in the Great Barrier Reef catchment area. To reduce hillslope erosion, management should aim to maintain more than 70% ground cover. Retention and protection of existing vegetation is important for reducing erosion. [Q3.4, Q3.5, Q3.6]
Dissolved nutrients
Agricultural development, especially sugarcane, and landscape modification have substantially increased nitrogen and phosphorus (nutrient) exports to the Great Barrier Reef, with dissolved inorganic nitrogen exports 1.5 to 3 times pre-development rates. Excess nutrients can have a negative impact on the health of Great Barrier Reef ecosystems, particularly on inshore coral reefs, and this may be exacerbated further when combined with climate change impacts. Reducing fertiliser application to industry recommended rates is the most effective and profitable practice for reducing nutrient exports. Natural and near-natural wetlands, and healthy rivers have the potential to support water quality improvements but are understudied in the Great Barrier Reef.
- The highest concentrations of nitrogen and phosphorus in the Great Barrier Reef are found in inshore areas. Upwelling and nitrogen fixation can also contribute to nutrient inputs. Excess amounts of dissolved nutrients are detrimental to coral health, recruitment, calcification, and reproduction, and may increase coral susceptibility to bleaching. Excess nutrients can lead to phytoplankton blooms that can increase the food supply for crown-of-thorns starfish larvae, possibly contributing to outbreaks. Other factors such as inherent life history traits and the removal of starfish predators can also influence outbreaks. Combining evidence from these different factors will contribute to a more complete understanding about when, where and how population outbreaks will occur. [Q4.1, Q4.2, Q4.3]
- Basins dominated by sugarcane make the greatest contribution to dissolved inorganic nitrogen exports (42% of total). Grazing lands contribute 22%, and although other land uses contribute smaller amounts, they can be locally important. Increased erosion from grazing and other land uses can contribute to nutrient export, with transformation of particulate nutrients to dissolved form (termed bioavailable nutrients) during transport. The primary drivers of anthropogenic nitrogen and phosphorus export are fertiliser application, changed catchment hydrology and erosion. [Q4.4, Q4.5]
- The most effective management practice to reduce dissolved inorganic nitrogen exports from sugarcane is reducing fertiliser applications. However, optimum application rates are uncertain, and large reductions may impact on productivity and profitability, which impedes adoption of reduced rates. The effectiveness of other management practices for nitrogen in sugarcane is variable and/or not clearly demonstrated. The evidence for improving water quality through the management of phosphorus in crop production, and nitrogen in the production of crops other than sugarcane, is limited. The same is true for cost-effectiveness of most practices. Reducing erosion reduces both particulate and dissolved nutrients, although the transformation processes from particulate to dissolved forms are poorly understood. [Q3.3, Q3.4, Q3.5, Q3.6, Q4.4, Q4.5, Q4.6]
- The global evidence indicates that tropical wetland systems can retain, process and, at times, export nutrients. It also shows that natural and near-natural wetlands are typically more effective at removing some forms of nutrients than constructed or restored wetlands. Hydrology, maintenance and presence of vegetation are important factors for maximising nutrient removal efficiency. There are few studies from the Great Barrier Reef that measure or model the efficacy or cost of wetlands as a water quality improvement tool. The historical loss of natural wetlands, particularly in floodplains, the degradation of those remaining, the presence of vegetation, hydrological characteristics, and the need for ongoing and long-term maintenance of restored wetlands are important considerations for maximising nutrient removal efficiency and determining future protection and management opportunities for Great Barrier Reef wetlands. [Q4.7, Q4.8, Q4.9]
Pesticides and other pollutants
Pesticides frequently occur at concentrations that exceed protection guidelines for freshwater ecosystems of the Great Barrier Reef, particularly during the first flush of each wet season. Exceedances also occur in the inshore marine waters but are less frequent and typically associated with flood plumes. The risk that pesticides pose to aquatic organisms reduces with distance from their source. Management practices have been shown to reduce pesticide losses. Much less is known about the distribution, concentrations and impacts of other pollutants on the Great Barrier Reef. For most pollutants, including some pesticides, there is insufficient monitoring and toxicity data to comprehensively assess environmental risk.
- Pesticide mixtures are present in the vast majority of all monitored fresh, estuarine and marine waters of the Great Barrier Reef. The Mackay Whitsunday region and Barratta Creek in the Burdekin region, consistently record higher concentrations and risk than elsewhere. Long-term analyses indicate that photosystem II herbicide concentrations have increased in some inshore sites of the Great Barrier Reef between 2005 and 2018. Almost all tested pesticides are reported as harmful to non-target aquatic species of the Great Barrier Reef. Pesticides can increase the vulnerability of local aquatic species to other stressors, including heatwaves. [Q5.1]
- Sugarcane remains the main source of pesticides, with other agricultural and urban sources also contributing. Pesticide export is influenced by the timing and rate of application, rainfall, irrigation regimes, and pesticide and soil properties. [Q5.2]
- The most effective management practices to reduce pesticide risk from agricultural land uses include: reducing the total amount of pesticide applied (within label recommendations); optimising application methods; timing application to coincide with periods with a low chance of rainfall runoff; choosing pesticides with lower environmental risk; reducing soil erosion; and improving irrigation efficiency. [Q5.3]
- Other pollutants present in the Great Barrier Reef include: metals; persistent organic pollutants; per- and poly-fluoroalkyl substances; plastics; pharmaceutical, veterinary and personal health care products; and coal dust. There were no local studies on sunscreens, but international studies suggest that they are likely to be present. Limited data indicate that metal concentrations are highest near more industrial and developed areas, although concentrations above national water quality guideline values are rarely found. There are insufficient monitoring data to determine the spatial range, concentrations, temporal trends and the risk that most of these pollutants pose to Great Barrier Reef ecosystems. [Q6.1]
Human dimensions of water quality improvement and emerging science
Trust building and collaborative processes with landholders, stakeholders, scientists, Traditional Owners and policy makers are key factors for enhancing the adoption and impact of land management change for water quality improvements. There is a lack of published information on the effectiveness and efficiency of policies, programs and instruments designed to motivate adoption of land management approaches for improved water quality, the conditions that engender practice change and enable scalability, as well as the broader co-benefits for people and the environment.
- Empowerment of Traditional Owners for decision making and management of the Great Barrier Reef, and its connected catchment area, can be strengthened through better understanding by all partners of Indigenous culture and connection to Country. Integration of Indigenous knowledge and input also requires support for increased capacity and improved capability to become engaged and involved in policy and program design, planning and delivery. [Q7.3]
- Methodologies to assess the effectiveness of programs and instruments to improve water quality through land management change are inconsistent, and usually only refer to estimated pollutant load reductions. There is a need for greater consideration of the broader co-benefits of improved land management, taking into account interconnected social, economic, cultural and human health factors that influence adoption, especially in the context of future climate scenarios. [Q7.1, Q7.2, Q8.1, Q8.2]
- Numerous factors, from high-level elements like program and policy direction to more granular influences such as landholder relationships and characteristics of the practices, influence the implementation and effectiveness of water quality improvement programs, including the adoption of various management practices. Distrust of government, researchers and program delivery providers significantly hinders the adoption of land management practices. Overcoming this barrier through the development, testing, and scaling of management practices via collaborative processes is crucial for accelerating water quality improvements. [Q7.2, Q7.3]
Strength of evidence
A total of 4,480 studies were used to address the 30 questions that form the 2022 Scientific Consensus Statement. The strength of the evidence was assessed taking into account the confidence rating (based on the overall relevance and consistency of the evidence), quantity of evidence items, and diversity of study types. The strength of the evidence is considered to be high for the topics covering the value and connectivity of Great Barrier Reef ecosystems, the spatial and temporal distribution of sediments, nutrients and pesticides, their drivers and sources, and the impacts on most Great Barrier Reef ecosystems (with some exceptions related to wetlands, where there is less evidence available). For questions related to the effectiveness of management practices in reducing pollutant exports, the strength of evidence is considered to be moderate, with limited information on certain land uses (such as bananas/horticulture, urban, and roads) and certain management practices in the Great Barrier Reef context (e.g., streambank rehabilitation, wetland treatment systems). The strength of evidence is limited to moderate for the questions reviewing the effectiveness of programs and instruments aimed at motivating landholders to adopt management practices and the factors influencing adoption of practices that improve water quality due to limited published and peer reviewed information. The questions with the lowest strength of evidence include ‘other pollutants’ (such as per- and poly-fluoroalkyl substances, pharmaceutical, veterinary, and personal health care products, coal dust and fly ash, and sunscreen), with limited local evidence on many pollutant types particularly with regard to ecological risk, and the factors of success in Indigenous involvement in water quality management and decision making, with limited peer reviewed evidence relevant to the Great Barrier Reef.
New knowledge since the 2017 Scientific Consensus Statement
Since the 2017 Scientific Consensus Statement, there has been a shift in the severity of climate-related impacts on Great Barrier Reef ecosystems and associated values, with climate-related impacts projected to intensify rapidly throughout this century. The importance of the role of improved water quality on the health of Great Barrier Reef ecosystems in the context of climate change has become clearer with monitoring data demonstrating the role of good water quality in promoting the recovery of coral reefs and seagrass meadows following disturbance events. Other concepts have been reinforced through multiple lines of evidence, such as the verification that sediment and nutrient exports are well above pre-development rates, quantification of the contribution of different land uses and biophysical drivers such as vegetation degradation and fertiliser inputs to end-of-catchment pollutant loads, and knowledge of the role of catchment development and the historical and ongoing loss of important ecological functions that contribute to poor water quality. The distribution and impacts of mixtures of pesticides in Great Barrier Reef ecosystems is also now better understood with an increased focus on ecological risk and identification of clear hotspot areas for management. The scope of evidence on ‘other pollutants’ has been expanded including recent studies on the distribution of per- and poly-fluoroalkyl substances and plastics. In terms of management options, new evidence has established the effectiveness of the remediation of large-scale gullies in combination with sustainable land management practices to maintain sediment export reduction, as well as further clarification of the potential benefits of Enhanced Efficiency Fertiliser application in sugarcane for reducing dissolved nutrient exports under different conditions. New research has also begun to assess the effectiveness of specific wetland treatment systems for water quality improvement in agricultural areas. Recent scientific findings associated with the human dimensions of water quality management show the need for greater emphasis on social values, the importance of landholder trust in government, scientists and program delivery organisations in engendering management practice change and greater collaboration among everyone involved in water quality management. This should include Indigenous knowledge and greater engagement and involvement of Traditional Owners in management and decision making for the Great Barrier Reef.
Key knowledge gaps
A detailed list of knowledge gaps identified through the 2022 Scientific Consensus Statement process is presented within each synthesis of evidence and collated by Themes within the Summary document. Key knowledge gaps and actions identified by experts are listed below. These gaps need to be considered in the context and uncertainties of a changing climate, and include:
- Greater assessment of the impacts of pollutants on some ecosystems such as wetlands.
- Increased knowledge of the combined and cumulative impacts of multiple stressors on a range of ecosystems and species, especially in the context of climate change projections.
- Better understanding of nutrient sources, transformations and fate (especially for particulate nitrogen and phosphorus) and their interaction with other impacts including bleaching and crown-of-thorns starfish outbreaks.
- Additional information on the ecological risk of emerging alternative pesticides.
- Collection of fundamental data and establishment of guideline values for most of the ’other pollutant’ groups including coal, per- and poly-fluoroalkyl substances, pharmaceutical, veterinary, and personal health care products, and sunscreens.
- Quantification of water quality improvements from a wider range of management solutions to target effective practices that deliver benefits in shorter timeframes.
- Quantification of the cost-effectiveness of management solutions across projects, programs and instruments using consistent and comparable approaches.
- Approaches to facilitate integration of Indigenous knowledge and Traditional Owners into water quality management.
- Quantification of the co-benefits of land management to improve water quality.
- Development of consistent methods to evaluate the effectiveness of programs and instruments applied to motivate land management change.
Ensuring that research outputs are peer reviewed and publicly available is essential for inclusion in future Scientific Consensus Statements.
