Summary statement for Dissolved nutrients

The synthesis of the evidence for dissolved nutrients included a total of 1,272 studies extracted and synthesised for 9 questions (with some overlap in evidence between questions). The Summary Statement was developed using the evidence base from the nine 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 dissolved nutrients are:

• Nutrients, especially nitrogen and phosphorus, play a crucial role in the water quality and overall health of Great Barrier Reef ecosystems, supporting corals reefs, seagrass meadows and fisheries. Nitrogen is generally considered the major limiting nutrient in marine waters, both globally and in the Great Barrier Reef, however, phosphorus can limit primary productivity at certain times and locations. Multiple lines of evidence demonstrate that nutrient concentrations originating from land-based activities follow a cross-shelf gradient with the highest concentrations found in estuaries and inshore waters, and lower values in midshelf and offshore waters. Peak concentrations are usually found during the wet season (December to May), between Cooktown and Gladstone, adjacent to areas of more intensive catchment development and in waters influenced by river discharge. Weather patterns, river discharge and associated land-based inputs, as well as marine processes such as upwelling and nitrogen fixation are among the key drivers of nutrient variability, with oscillations over time. [Q4.1]
• Excessive amounts of dissolved inorganic nutrients can have detrimental effects on Great Barrier Reef ecosystems. In coral reefs, the most severe impacts may be indirect, for instance, excess nutrient availability on inshore reefs is generally (but not always) positively correlated with increased fleshy macroalgal abundance which can reduce coral settlement and recruitment, outcompete corals, reduce coral cover and negatively affect coral calcification. Excess nutrients might also contribute to coral-eating crown-of-thorns starfish outbreaks (see below). Direct effects of elevated nutrients include reduced coral calcification, negative impacts on coral reproduction, and potentially reducing thermal tolerance to bleaching. [Q4.2]
• There is no clear evidence of direct negative impacts of increased dissolved inorganic nutrients on seagrass ecosystems, and although elevated nutrients may be beneficial for mangrove growth, they can interact with climate stressors such as drought (low rainfall and low humidity) causing mangrove decline. There is limited evidence of the impact of dissolved inorganic nutrients on Great Barrier Reef wetland ecosystems. [Q4.2]
• During high river discharge events, elevated nutrient levels cause phytoplankton blooms that sometimes coincide with developing crown-of-thorns starfish larvae. These blooms increase the amount of food available for larvae which can increase survival, growth, and development rates, though it has not yet been shown that crown-of-thorns starfish outbreaks are limited by larval supply. Crown-of-thorns starfish outbreaks in the Great Barrier Reef start on midshelf reefs between Cairns and Lizard Island –the crown-of-thorns starfish ‘initiation area’, which occasionally receives nutrient-enriched flood plumes from rivers. Although land-based nutrient runoff may contribute to outbreaks, other factors such as marine upwelling, life history traits including high fecundity, and the effect of predator removal (e.g., fishing) may also be important. Combining evidence from these different factors will contribute to a more complete understanding about when, where and how population outbreaks will occur. [Q4.3]
• Overall, exports of anthropogenic dissolved inorganic nitrogen are twice as high as pre-development rates, mainly as a result of fertiliser-adding land uses. The Herbert, Burdekin, Fitzroy, Johnstone, Mulgrave-Russell, Tully and Haughton basins are the largest exporters of total dissolved inorganic nitrogen to the Great Barrier Reef (exporting over 500 tonnes per year, each). [Q4.4]
• Anthropogenic exports of dissolved inorganic nitrogen are greatest in basins dominated by sugarcane including those in the Wet Tropics, Burdekin and Mackay Whitsunday Natural Resource Management regions. Increased erosion from grazing and other land uses can also contribute to nutrient export, with transformation of particulate nutrients to dissolved form (termed bioavailable nutrients) during transport. Other land uses including urban, bananas and other horticulture contribute smaller amounts but can be locally important. Surface runoff, subsurface movement and groundwater are all significant transport pathways of dissolved nutrients to the Great Barrier Reef. Most export occurs in the wet season, with chronic and continuously high exports in wet tropical catchments. [Q4.4, Q4.5]
• Primary biophysical drivers of anthropogenic dissolved nutrient exports to the Great Barrier Reef are fertiliser application, altered catchment hydrology leading to changed (typically shorter) water residence times in rivers and reduced interaction of surface and subsurface runoff with floodplains, and erosion. Increased rates of fertiliser application and low nutrient use efficiency, increased cultivation area, low efficiency irrigation systems and heavy rainfall can lead to increased nutrient export, especially nitrogen, in surface and subsurface runoff and groundwater. [Q4.5]
• During river transport, nutrients can be transformed by a range of processes such as denitrification, desorption of nutrients from soil particles, plant uptake and burial, remineralisation, and deposition to sediment. However, the majority of nutrient loads are delivered from the source to the Great Barrier Reef World Heritage Area. The Reef Water Quality Report Card 2020 estimates that ‘Moderate’ overall progress has been made toward meeting the dissolved inorganic nitrogen load reduction targets. The monitoring program should be able to start detecting these improvements to export loads; however, for some management actions it may be several years until the benefits of management are fully realised. [Q4.4, Q4.5]
• The most effective and profitable management practice for reducing dissolved nitrogen exports from the Great Barrier Reef catchment area is reducing nitrogen fertiliser applications to industry recommended rates, with consistent results across different land uses, climates, and management contexts. Further rate reductions give consistent water quality benefits, but productivity and profitability varies. In sugarcane, other management practices include the use of Enhanced Efficiency Fertiliser, mill mud application, improved irrigation, crop residue management, improved farming systems (such as growing legumes in between sugarcane crops) and burying fertiliser. For many of these management practices, the effectiveness and profitability is not clearly demonstrated and, for some, varies depending on climate, soil and seasonal characteristics. The effectiveness and profitability of practices for reducing dissolved phosphorus exports are less clear, as is the situation for crops other than sugarcane. The adoption of management practices can be enhanced by a range of factors as discussed in detail in the theme on human dimensions of water quality improvement. [Q4.6]
• For urban and non-agricultural land uses, structural measures that include vegetation or biological components such as constructed wetlands, biofilters, algal ponds and existing riparian zones have considerable potential for removal of diffuse runoff nutrients and may also be important for management of wastewaters. Improvements in technologies for wastewater management also show that systems such as membrane filtration and chemical addition are likely to perform well. Non-structural controls such as policy, planning, regulation and compliance appear to work best when applied as part of an integrated approach, and recycling and reuse show considerable potential. [Q4.6]
• The global evidence indicates that tropical wetland systems can retain, process and in some cases, export nutrients, sediments and pesticides with a wide-ranging capacity for pollutant retention. The evidence also shows that natural and near-natural wetlands are typically more effective at nutrient removal (certain forms) and pesticide removal than constructed or restored wetlands, and sediment is often retained in wetlands but can be remobilised in large flow events. There are few peer reviewed studies that comprehensively measure or model wetlands’ efficacy or costs of water quality improvement in the Great Barrier Reef catchment area. Critical factors for optimising the efficacy of water quality improvement include: the presence and maintenance of vegetation communities; hydrological characteristics including the wetland size relative to the contributing catchment area, flow rate, loss pathways and water residence times; and the type, form and input concentration of the targeted pollutant. Available evidence indicates that the costs of treatment wetlands are highly variable. These factors, and the need for ongoing maintenance, need to be carefully considered when planning for the use of wetland systems as a water quality improvement tool in the landscape. [Q4.7, Q4.8]
• Natural and near natural wetlands provide many benefits to society and the environment including regulating ecosystem services such as improved water quality and carbon sequestration, supporting services such as nutrient cycling and provision of habitat, cultural services such as aesthetics and recreation and provisioning services including food, water and other resources. In the Great Barrier Reef catchment area, these wetlands and services are under pressure due to expansion of coastal agriculture, urban and industrial development. Wetlands can be restored to enhance water quality benefits but without a long-term plan of maintenance, and clear definition of restoration goals, restoration project sites have a high risk of returning to a degraded state, reducing the services they provide. The historical loss of natural wetlands across floodplains and the degradation of those ecosystems remaining are important considerations for determining future protection and management opportunities for wetlands in the Great Barrier Reef catchment area. [Q4.7, Q4.9]