Summary statement for Sediments and particulate nutrients

The synthesis of the evidence for sediments and particulate nutrients included a total of 850 studies extracted and synthesised for 6 questions (with some overlap in evidence between questions). The Summary Statement was developed using the evidence base from the six 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 sediments and particulate nutrients are:

• The export of sediments and particulate nutrients to the Great Barrier Reef have increased for most river basins over the last 170 years since the arrival of Europeans. There are distinct patterns of sediment composition across the continental shelf of the Great Barrier Reef. The inner shelf (up to 20 metres water depth) is dominated by land-derived ‘terrigenous’ sediment shifting to predominantly marine sediment derived from corals, shells and other carbonates, in the outer shelf (>40 metres depth). In situ monitoring and remote sensing data show a clear spatial gradient of decreasing turbidity levels with increasing distance from river mouths, likely related to the depth of the water column (i.e., degree of wave resuspension) and the availability of sediment on the seafloor to be readily resuspended. [Q3.1]
• While most sediment-laden flood plumes are confined to the inner shelf of the Great Barrier Reef, at times during periods of large riverine discharge and low wind speeds, fine (<20 µm) terrigenous sediment and associated particulates can be carried to the middle shelf and even the outer shelf (particularly for the areas northwards of Bowen where the middle and outer shelf is closer to the coast). Reduced water quality can persist for up to six months due to frequent resuspension of this newly delivered sediment following large discharge events. Increases in sediment loads since the arrival of Europeans greatly influences the area of the inner and middle shelf affected by diminished light. Studies on particulate nutrients in flood plumes and river estuaries highlight the potential for rapid transformation of particulate nutrients to bioavailable forms (dissolved nutrient forms that are readily consumed by algae) within coastal areas. [Q3.1]
• Increases in the loads of fine sediments and particulate nutrients affect the quantity and quality of light (i.e., modified light spectrum towards ‘less usable’ light) reaching coral reefs, seagrass and other benthic organisms and can influence the behaviour and condition of animals including fish, particularly in inshore areas. This has resulted in persistent impacts on reef composition (including changes in the abundance and diversity of species at different depths, and increases in macroalgal growth), and variability in the distribution, abundance and composition of seagrass meadows. These direct effects can result in indirect effects on other taxa including seagrass-dependent dugong which can extend across the whole Great Barrier Reef. The greatest impacts of fine sediments and particulate nutrients occur in the inshore central and southern Great Barrier Reef (Wet Tropics to Burnett Mary Natural Resource Management regions). [Q3.2]
• Sedimentation, the settling of sediments and particulate nutrients onto surfaces, can also have negative direct effects on a variety of taxa including corals, causing tissue damage, reducing growth rates and altering microbial communities. Importantly, settled and suspended particulate matter can suppress recovery from other disturbance events (e.g., heatwaves, cyclones) and affect recruitment and early life stages of corals and fishes. [Q3.2]
• Overall, exports of anthropogenic fine sediment to the Great Barrier Reef are estimated to be 1.4 to 5 times higher than pre-development loads (depending on the basin). These estimates are supported by a range of evidence including modelling, geochemical and isotope tracing of the sources of exported material, and proxy records of exports over time in sediments and coral cores. The Burdekin and Fitzroy basins are the largest exporters of total fine sediment and particulate nutrients to the Great Barrier Reef (including anthropogenic exports), each exporting an annual average load of over 3,000 kilotonnes and 1,300 kilotonnes of fine sediment per year, respectively, and more than 3,000 tonnes of particulate nitrogen per year. [Q3.3]
• Grazing land use is the largest contributor of fine sediment export to the Great Barrier Reef (especially in the Burdekin and Fitzroy regions), estimated to generate 60% of the total export and a larger proportion of the anthropogenic export, with all other land uses each contributing much smaller amounts. Intense land uses such as mining and developing urban areas can generate large sources of fine sediment locally but cover a relatively small area and overall exports are relatively small. It is estimated that gully erosion contributes around 50% of the total fine sediment load exported from the Great Barrier Reef catchment area, with the remainder comprised of almost equal contributions from streambank erosion and hillslope erosion. Each process can dominate in particular basins. [Q3.3]
• The most important primary biophysical drivers of anthropogenic sediment and particulate nutrient exports to the Great Barrier Reef include vegetation degradation (e.g., land/tree clearing, low ground cover, and changes in structure and function of grass species including a shift to non-native pastures, typically in response to long-term grazing pressure), surface disturbance from cattle trampling, feral animals, tillage in cropping areas, historical surface mining, unsealed roads and construction earthworks. While these drivers vary between erosion types, in different locations and at different times, gully erosion is strongly driven by surface disturbance and vegetation degradation / low ground cover, and streambank erosion is accelerated by removal or degradation of riparian vegetation and grazing pressure. Soil degradation, spatial concentration of runoff by roads, tracks and fence lines are also important drivers in some locations. Runoff detention by large dams has reduced exports significantly but have been outweighed severalfold by the other drivers, and dams are less effective at trapping fine-grained sediments of most ecological risk to downstream Great Barrier Reef ecosystems. [Q3.4]
• The Reef Water Quality Report Card 2020 estimated that ’Moderate’ overall progress has been made towards meeting the Reef 2050 Water Quality Improvement Plan fine sediment load reduction target and ‘Very Good’ progress for the particulate nutrient load reduction targets. In some basins, targets are assumed to have been met (including several basins in the Cape York region) while in others, especially those that were not given management priority, there has been little progress. For some management actions it may take many years until the benefits of management are fully realised, and it may take decades to detect reduced exports in monitoring programs due to the high annual variability of exports controlled by river discharge. [Q3.3]
• The most effective management practices for reducing fine sediment and particulate nutrient export from the Great Barrier Reef catchment area vary between land uses and erosion types. Common practices for hillslope erosion include the use of moderate and adaptive grazing stocking rates, maintenance of at least 40% ground cover (but preferably >70%), regular periods of strategic rest from grazing (especially in the early wet season), cattle exclusion from fragile land-types, maintaining or reintroducing vegetation into landscapes (including pasture management and vegetation buffers), management of sediment delivery pathways within catchments (e.g., via management of roads, drains and gullies), and other practices that minimise soil runoff (such as green cane trash blanketing, zero/minimum tillage and controlled traffic farming). There is high variability in the cost-effectiveness of practices at the farm/project scale which is driven by several factors including a wide range of different practices and economic returns, location within the landscape and factors relating to sediment mobilisation and delivery. The way that cost-effectiveness is assessed between projects and programs is also inconsistent (i.e., different metrics and considerations). 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. [Q3.5]
• While a range of gully and streambank projects have been undertaken in the Great Barrier Reef catchment area, the vast majority of works have not been quantitatively monitored for sediment and particulate nutrient reductions, with the exception of studies published within the National Environmental Science Program Tropical Water Quality Hub (2014 to 2021). Remediation of large alluvial gullies has been demonstrated to be a highly effective strategy to significantly reduce fine sediment loads delivered to the Great Barrier Reef, achieving over 90% fine sediment reductions within one to two years when a combination of treatments is applied. Ongoing maintenance (including livestock exclusion) of the remediation sites is required to retain these benefits. Few studies focus on measuring the water quality benefits of streambank restoration, however, it is evident that bank erosion generally occurs at lower rates on vegetated streambanks than non-vegetated streambanks. [Q3.6]
• Large-scale remediation of high sediment yielding gullies is 26 – 60 times more cost-effective in achieving the same cumulative fine sediment reduction than lower-cost options for lower-yielding gullies. The limited available peer reviewed evidence shows that hillslope gully remediation, that is based on the use of low-cost check dams in gully floors and fencing, is less cost-effective compared to large high-yielding gullies. Alluvial gully treatments typically have shorter response times in terms of fine sediment reductions and can be treated at larger scales and in fewer locations, resulting in additional logistical efficiencies. Of relevance to management prioritisation, the available studies also indicate that a small number of gullies (~ 2% of the total number) contribute a substantial proportion of the sediment yield (30%), highlighting the need for targeting the high yielding gullies as a means of efficiently reducing fine sediment exports to the Great Barrier Reef. [Q3.6]
• Areas downstream of dams and closer to the coast lead to higher rates of sediment and particulate nutrient delivery to the coast, therefore the location of sites for targeting management is important when estimating management practice effectiveness and translating load reductions between the site, and what is delivered to ecosystems downstream. [Q3.4, Q3.5]