Northern Australia contains diverse and important river and coastal systems with high natural values including species and habitats of national and international significance. These rivers are also of increasing interest for agricultural expansion due to their water and soil resources. As part of ongoing research in northern Australia, CSIRO has been commissioned by the Australian Government to explore the opportunities and risks of irrigated agriculture and associated infrastructure in the Roper River, Victoria River and Southern Gulf rivers’ catchments.
The CSIRO water assessments in northern Australia are a large, multidisciplinary research program that continues and builds upon work delivered in 2018 under an initial Northern Australia Water Resource Assessment (NAWRA) program, and encompasses a broad range of integrated activities, including groundwater hydrology, digital soil mapping, land suitability mapping, surface water hydrology, Indigenous water values and catchment-to-coast ecology.
“The program is a significant scale of work, both in terms of the breadth of research and the spatial extent of the assessment” says Dr Danial Stratford, a senior researcher with CSIRO Land and Water who is currently leading the Ecology team in CSIRO’s current NAWRA portfolio.
“It’s a really vast area, and very diverse. Our team is working to understand the ecological context of each catchment, knowing that in the context of risks and opportunities of water resource development, ecology is a significant part of the equation on the risk side.”
Critically important river and estuarine habitats
The catchments, rivers, estuaries and near-shore marine zones of northern Australia contain rich biodiversity, with at least 170 fish species, 150 waterbird species and 30 aquatic and semi-aquatic reptiles. In addition, floodplain and riparian vegetation is supported by river flows, and vine and palm forest can rely upon groundwater in habitat patches scattered within an often-dry landscape.
The region’s unique estuarine and riverine habitats are critically important in supporting productive commercial fisheries, and species of national and global conservation significance including the freshwater sawfish, and many species of important cultural value.
“The remarkable thing about the northern Australian rivers is that they are largely unspoiled compared to many other parts of the world,” says Dr Osmar Luiz, an aquatic ecologist at Charles Darwin University whose allied research focus is fish ecology and conservation in northern Australia.
“Comparatively, there is a small human population, and no major development – no big dams on nearly all of these rivers. That is largely unique in the world and renders northern Australian rivers global refugia for rare and unique species with life-habitats that rely on natural river flows. Worldwide, there are very few, if any, rivers that have unmanaged flows to this extent, so as researchers in northern Australia we feel very fortunate that we can document the state of those river catchments and establish what is as close to a pristine ecosystem baseline as we have anywhere.”
However, in recent years, a number of existing and potential pressures on northern Australia’s ecosystems continue have become more apparent. These include climate change, invasive species, a changing fire regime, mining, and agricultural intensification – all of which may impact inland and marine ecosystems and may further exacerbate impacts associated with increased human development.
While water extraction and agricultural development have the potential to create a range of benefits and opportunities for the region, there are also a number of associated risks. While not relegating the importance of the natural ecosystem and iconic species, commercial activities such as fishing have operated sustainably across Australia’s north for up to 50 years.
Many of their target species rely on unregulated catchment flows to sustain their current harvests. The water resource assessments seek to build a deeper understanding of the changes and trade-offs that may arise from across a range of potential water resource development scenarios.
“CSIRO is not proposing any particular development, or any particular pathway to development,” says Dr Stratford.
“We recognise that any potential development in the region could take many different forms. What we are seeking is to scientifically assess what is possible using a range of realistically plausible development scenarios, and to generate a robust ecological knowledge base that can be used to inform and support future decision making.”
An ecological assets-based approach
Although the Roper, Victoria and Southern Gulf catchments straddle different regions of northern Australia, and are subject to quite different climate and ecological conditions, what all three have in common is that they are complex, finely balanced systems. The ecology of these regions is a result of long-term relationships to the flow regimes of these rivers driven by both wet and dry cycles across scales of time.
“For nine months of the year, northern Australia ecosystems are highly stressed because it’s so hot and dry,” says Rob Kenyon, a marine ecologist with CSIRO Oceans and Atmosphere.
“Then during the monsoon season, rivers flow and the catchments are inundated with water, often to the point of flooding, and that’s when a lot of the fish and crustaceans take the opportunity to move to new habitats and to reproduce; from estuaries to the river upstream, or from freshwater habitats to the coast. The system is cued to those natural, seasonal rhythms and the animals have evolved to live with those cycles.”
In order to undertake the assessments, the researchers are characterising the ecological context of each catchment taking an assets-based approach, in which the diverse species and habitats are categorised into groups depending upon their ecological characteristics.
By collating and building upon knowledge and work undertaken by NESP, university, industry and government researchers, as well as investing in future research needs and capability through research partnerships, we are developing an understanding of the differing ecology of these important assets across northern Australia. These assets are then used with modelling to investigate what impacts might result from changes in flow regimes within the catchments.
The categories of ecological assets include:
- species or species groups that have a conservation value, value to industry, human or cultural value. Examples include barramundi, prawns and catfish
- functional groups: groups that are potentially unrelated but have significant habitat and ecological similarities between them, such as waterbirds that use similar foraging behaviour and habitat
- habitats that support a broad range of species. Examples include mangroves, floodplain wetlands, and in-channel refuge waterholes.
“Once we have identified and prioritised our ecological assets, we can then profile their relationship with the environment to understand what their needs are in terms of habitat and river flow,” says Dr Stratford.
“We need to know where our ecological assets are located in order to understand how a change in flow regime might impact them. If a potential development was to occur in a specific part of a catchment, we want to understand what species and habitats would be affected, and how.”
Dealing with complexities
To develop an understanding of how water-use or infrastructure placement may impact ecological assets, the researchers use modelling to explore an extensive range of different scenarios.
Building a dam or undertaking water extraction would affect the flow of a river in different ways, depending on where in the catchment it was to occur and also how it could be managed. Additional factors like changes in river flow due to climate change might also intensify the impacts of any human-led development and is also part of our consideration and modelling.
“Ecological systems are very complex,” says Dr Luiz.
“But we know that impacts from different activities can interact. If we’re getting less rainfall in the wet season due to climate change, and there’s also less river flow due to water extraction, these changes have a compound effect. The human impacts can make the climate impacts worse.”
An additional layer of complexities the researchers explore is the ways in which the diverse habitats within a catchment may be affected by different development scenarios.
As an example, floodplain wetlands are supported by higher flows in the wet season with floods connecting and inundating large areas of important habitat, so any changes to these parts of the flow regime will have an impact on the connectivity, extent and productivity of that habitat and the species that rely on it.
In contrast, during the dry season, in-channel waterholes that persist are important refuge habitat for many species and would be more impacted by changes to the low flows including changes that extend periods of no-flow occurring before the next wet season.
“For some critical species like the freshwater sawfish, northern Australia is really their last safe haven,” says Mr Kenyon.
“They move up to 400 kilometres upstream into riverine pools as juveniles and they do that during floods. They use the high flows – probably the upper 10% of flows – to minimise their risk of stranding and navigating obstructions such in-stream rockwalls to reach permanent upstream pools.”
“Many of the commercial species need those wet season high flows too,” continues Mr Kenyon.
“Juvenile banana prawns live in estuaries, and if it’s a poor wet season – if they don’t sense that change in salinity – then the population is not cued to emigrate out to sea and that year’s fishery catch is low. Barramundi move into flood plain habitat during flooding and grow very quickly. After a big flood, that year’s population can contribute to a large percentage of the catch five or six years afterwards. Even fully estuarine species like mud crabs benefit from flows. All these animals have evolved to use those pulsed, seasonal flows to optimise their habitat use.”
With ever-increasing interest in development in northern Australia, the research being undertaken by CSIRO is crucial for closing knowledge gaps about the region’s unique biodiversity.
Understanding what potential changes in the flow regime might mean for a range of key species and habitats will provide important ecological context and support informed decision making over the coming years and decades.