Summary statement for Pesticides and other pollutants

The synthesis of the evidence for pesticides included a total of 591 studies extracted and synthesised for 3 questions (with some overlap in evidence between questions). Other pollutants included 92 studies extracted and synthesised for 1 question. The Summary Statement was developed using the evidence base from the four questions in this theme. Convergence was reached for this Summary Statement among all authors within the Expert Group for this theme.

The summary of findings relevant to policy or management action for pesticides and other pollutants are:

• Pesticides are ubiquitous across monitored Great Barrier Reef ecosystems including end-of-catchment waterways, palustrine wetlands (e.g., vegetated swamps) and in estuarine and nearshore marine habitats. Their presence in the offshore marine environment is largely unknown. Concentrations of pesticides (invariably as mixtures) are greatest in wetlands, followed by end-of-catchment then marine locations, with concentrations decreasing with greater distance from the site of application. Exposure of marine ecosystems to pesticides is closely linked to flood plume dispersal. Modelling suggests that pesticide exposure is highly dynamic, changing by orders of magnitude within hours. [Q5.1]
• Sites in the Mackay Whitsunday region, along with Barratta Creek in the Burdekin region, that feature intense cropping and lower discharge (related to rainfall), consistently record higher concentrations of pesticides and higher risk than other locations. [Q5.1]
• Annual trends in pesticide concentrations are difficult to identify over the short term; however, a long-term analysis indicated that photosystem II herbicide concentrations increased in some inshore sites of the Great Barrier Reef. [Q5.1]
• Pesticides are designed to control agricultural pest species and virtually all tested pesticides are potentially harmful to non-target aquatic species of the Great Barrier Reef region. For example, photosystem II herbicides consistently impact all photosynthetic marine organisms of the Great Barrier Reef that have been tested, including corals and seagrass. Other simultaneous pressures, including heatwave conditions and variation in light were shown to increase the sensitivity of Great Barrier Reef species to pesticides, indicating that guideline values applied under some conditions in the field are likely to underestimate the risk to aquatic ecosystems. [Q5.1]
• Monitored pesticides that contribute most to risk in all Great Barrier Reef ecosystems examined include atrazine, diuron, imidacloprid and metolachlor. Photosystem II herbicides dominate the contribution to risk in many waterways; however, non-photosystem II pesticides make substantial contributions to risk at specific locations, and their influence appears to be increasing since 2016. [Q5.1, Q5.2]
• Sugarcane areas are the largest contributor to end-of-catchment pesticide concentrations, and are dominated by photosystem II herbicides. While pesticides are used over large areas of grazing lands, the relative ecological toxicity to aquatic organisms of the dominant pesticide, tebuthiuron, is low compared to other photosystem II herbicides. Other land uses including forestry, horticulture, banana growing, and urban areas can be large users of some pesticides, but their total area within the Great Barrier Reef catchment area is relatively low. Nonetheless, they can contribute to pesticide concentrations. Catchments with minimal agricultural activity, such as the Ross and Kolan basins, have the lowest photosystem II herbicide contributions. Imidacloprid is the most commonly detected insecticide in the Great Barrier Reef catchment area and is associated with banana, sugarcane and urban activities. [Q5.2]
• The key factors that influence export of pesticides to the Great Barrier Reef are pesticide application rates, the timing between pesticide application and rainfall (longer timeframes between application and significant rainfall/irrigation are associated with lower pesticide exports), irrigation regimes, and pesticide properties such as persistence. Other factors that can influence delivery of pesticides to the Great Barrier Reef include soil characteristics, pesticide formulations (more soluble pesticides are more vulnerable to dispersal), climatic conditions and particularly extreme weather events, and catchment characteristics. [Q5.2]
• Most pesticide exported to the Great Barrier Reef is via surface runoff. Pesticide export via groundwater may be a contributor in some basins, although this has only been measured in the Wet Tropics region and Lower Burdekin floodplain. [Q5.2]
• The most effective management practices for reducing pesticide risk from the Great Barrier Reef catchment area vary between land uses. Practices that demonstrably reduce pesticide risk from agricultural land uses include reducing the total amount of pesticide applied through lower application rates (within label recommendations), improving application methods, timing of application in relation to weather risk periods, switching to pesticide products with lower environmental risk and reducing soil erosion through retaining cover, controlled traffic and improved irrigation management for pesticides with greater soil sorption. These findings have remained relatively consistent through time, and across climatic regimes and farming systems of the Great Barrier Reef catchments. A range of non-chemical pesticide control measures (integrated pest management, cultural controls that modify the pest’s growing environment) hold considerable potential for reducing reliance on chemical control measures, but most are yet to be trialled in the Great Barrier Reef catchment area with respect to long-term pesticide use reductions, efficacy and economic outcomes. [Q5.3]
• In the assessment of cost-effectiveness of pesticide management in agricultural industries, economic returns remain variable. However, for sugarcane, progressing from traditional to industry standard herbicide management is generally profitable across sugarcane districts. The adoption of management practices can be driven by a range of factors including costs and is discussed in detail in the theme on human dimensions of water quality improvement. [Q5.3]
• For non-agricultural lands, pesticide management options largely rely on non-structural controls such as regulations, and improved wastewater treatment processes (e.g., membrane bioreactors, reverse osmosis). The emergence of significant pesticide resistance across multiple industries has started to cause considerable changes in pesticide use and other alternative pest control measures. [Q5.3]
• Other pollutants detected in the waters, sediments and biota of the Great Barrier Reef include metals, persistent organic pollutants, per- and poly-fluoroalkyl substances, plastics, pharmaceutical, veterinary, and personal health care products, coal and fly ash and sunscreens. Assessment of spatial patterns, temporal trends and ecological risks for ecosystems and individual biota is severely limited by data availability. There are very few routine monitoring programs for these pollutant groups, with the exception of some monitoring within the Regional Report Cards (e.g., Gladstone, Dry Tropics and Mackay Whitsunday) for which the raw data are not publicly available. [Q6.1]
• Metal concentrations in water and sediments are higher in more industrial and developed coastal environments compared to less developed catchments and offshore areas. Concentrations of metals in water above national water quality guideline values are rarely documented, but have been recorded in some studies including copper (associated with legacy mining in the Fitzroy basin), mercury (associated with sugarcane in the Tully catchment) and aluminium (from acid sulfate soils in Trinity Bay, Cairns). Biota found inshore (e.g., seagrass, algae, turtles, corals) have higher concentrations of metals in their tissues than those found offshore. Although ecotoxicological studies indicate that the ecological risks from metals in the Great Barrier Reef are relatively low and constrained to a few small locations, the combined risks from mixtures have not been assessed. [Q6.1]
• Persistent organic pollutants are associated with industry, oil spills, coal, and urbanisation. They are detectable in Great Barrier Reef sediments (generally below guidelines values, with exceptions such as following oil spills) and biota, and from the limited data available, decrease across an inshore to offshore gradient. Persistent organic pollutants can affect fish physiology and behaviour, coral reproduction and trophic food webs. [Q6.1]
• Plastics, including microplastics and fibres, are extensively distributed in coastal and marine environments. Coastal sites are influenced by surrounding land use, river and stormwater inputs, while offshore sites are mostly influenced by recreational activities, tourism, commercial shipping and fishing. Plastics have been recorded in zooplankton, crustaceans, fishes, birds and turtles from the Great Barrier Reef, with ecological risks varying across species, feeding behaviour and life stages. [Q6.1]
• A more cohesive and coordinated approach to examine the interaction of multiple pollutants and stressors, including climate change, is required. Ecotoxicological studies that employ multiple lines of evidence are urgently required for all pollutant groups identified in the Great Barrier Reef to understand the risks that these pollutants pose to Great Barrier Reef biota and ecosystems. [Q6.1]