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By Éva Plagányi Laura Blamey Rob Kenyon 21 March 2022 5 min read

Rivers flowing into the Gulf of Carpentaria provide critical ecosystem services to riverine and estuarine reaches of the waterway. Natural annual and seasonal patterns of high and low river flows are crucial in the functioning of these relatively pristine ecosystems. Photo credit: Queensland Department of Agriculture and Fisheries

Water is essential for life. Balancing complex water needs between users and the environment is one of our biggest ongoing challenges. Water needs for urban consumption, agriculture, mining and other activities need to be balanced with ensuring natural aquatic ecosystems can still function and support healthy populations of fish and plants.

Australia is still considering major river modification in some parts of Australia to support economic development, and science can help understand the trade-offs.

Extracting water from rivers has downstream impacts on estuarine and marine ecosystems that are not always well recognised. We’ve recently completed a novel ecosystem modelling study to estimate the effects of water extraction in 3 important northern Australian river catchments.

Our study challenges notions that flood water runoff out to sea is wasted water. To the contrary, we demonstrated how river flows fuel the productivity of the entire Gulf of Carpentaria ecosystem and the fish, prawns and crabs that support valuable industries.

Iconic marine species of northern Australia

Australia’s Gulf of Carpentaria is a relatively pristine and remote place but, like other marine regions around the world, many of the fisheries that operate out at sea rely on natural annual and seasonal patterns of river flows. If these patterns of high and low flow are disrupted by human activities, it can impact the survival, growth and catch rates of marine species that depend on rivers and coastal estuaries for part of their life cycle.

Marine species in northern Australia that also rely on rivers include barramundi, mud crabs, common banana prawns and largetooth sawfish. Each of these species have different commercial, recreational, cultural and conservation value, but they all interact with natural cycles of river flow.

For example, iconic species like barramundi and the threatened largetooth sawfish spend their first few years of life in rivers before heading to coastal waters. Largetooth sawfish return again to the same river mouth several years later once they are old enough to breed.

Largetooth sawfish (Pristis pristis) are an iconic but threatened species. They are reliant on rivers and estuaries for parts of their life cycle and water extraction from rivers is predicted to have serious consequences for remaining populations. Photo credit: Rich Pillans, CSIRO

Downstream impacts on tropical marine species

So, what is the effect of building a dam or extracting water from a river and thus reducing natural flows into estuarine systems?

To answer this question, CSIRO in conjunction with colleagues from the Northern Prawn Fishery (NPF Industry Pty Ltd), Griffith University and Queensland Department of Agriculture and Fisheries used an ecosystem modelling approach to estimate what effects different water extraction types and amounts would have on riverine, estuarine and marine ecosystems.

Our 2.5-year study was funded by Fisheries Research and Development Corporation (FRDC) and CSIRO and focussed on three important Gulf of Carpentaria catchments; the Mitchell, Gilbert and Flinders Rivers.  We held several workshops with a range of fishing, landholder and water resource management stakeholders to provide inputs into how best to describe the system functioning.

Building the picture with novel modelling approaches

Our study built on considerable previous ecological and catchment management research. We used river flow modelling scenarios to accurately depict how different illustrative water development scenarios will alter river flows in the lower reaches of the river. These lower reaches critically influence the marine and estuarine environments.

The novel tool we used was a Model of Intermediate Complexity - otherwise known as MICE. This rigorous, data-based, state-of-the-art ecosystem modelling method is a reliable tool for understanding and quantifying complex ecosystem interactions. It does so without needing to capture the full complexity of an ecosystem with 100’s of interacting species.

We used ecological and fisheries data stretching back some 50 years, together with MICE to understand how past natural changes in river flows have influenced the recruitment and abundance of key marine species. We then used the tested-and-verified relationship as a basis for predicting what the likely impacts would be under a range of water extraction scenarios.

Water development will affect all species in some way

Changes in river flows will impact all the species we considered, with some species more sensitive than others. There are likely bigger impacts if water is removed from more than one river catchment, and if water is removed from low-level flows in drier years. In many cases, plausible water extraction scenarios are predicted to have ‘substantial’ to ‘extreme’ impacts on the marine and estuarine ecosystems.

The largetooth sawfish emerged as one of the most sensitive species to water extraction, but also one of the most uncertain as there are big gaps in our knowledge of these large and unusual animals.

On the other hand, catches of the well-studied common banana prawn, a mainstay of the Northern Prawn Fishery and a popular item on Aussie barbeques, are predicted to decrease from 4 to 40% depending on the extent of water extraction from each of three northern rivers.

Our study also quantified how flows from different rivers combine to mitigate or exacerbate impacts on banana prawn catches throughout the Gulf region. And we looked at economic risks to the fishery under a range of water extraction scenarios, highlighting situations when the balance of river flow interruption tips too far and could lead to the fishery becoming economically unviable.

Within the commercial barramundi fishery, predicted catch decreases under hypothetical water-use  scenarios ranged from 4 to 61%. For mud crabs, under these scenarios catches were predicted to decrease up to 83% in some years.

Understanding impacts and how to reduce them can help support more ecologically friendly and equitable water resource development.

Barramundi (Lates calcarifer) have considerable commercial, recreational and cultural importance in the Gulf of Carpentaria. Good river flows are critical for food availability, growth and habitat connectivity. Photo credit: Queensland Department of Agriculture and Fisheries

Seagrass beds and mangroves respond differently

The Gulf of Carpentaria is also one of the few places in the world that still supports extensive mangroves and seagrass beds. These habitats are increasingly recognised for stabilising coastlines, cycling nutrients and as critical for carbon storage pools.

Mangroves and seagrass beds have long been known as vital habitats for countless fish and crustacean species. They play roles in nutrient and food provision, shelter from predators, maintaining water quality, stabilising habitat substrate and trapping sediments.

We did not predict substantial impacts of water extraction on seagrass (unless water development activities lead to increases in chemical and nutrient loads which were not modelled). However, mangroves are predicted to decline under some scenarios. This is of concern and merits further investigation.

So, can we extract water and not affect ecosystems?

This study shows that water removal will affect the downstream estuarine and marine ecosystems. Take too much water, and the impacts will be large. Lower levels of extraction, particularly when river flow is naturally low is important.

Comprehensive studies such as our MICE modelling will undoubtedly be valuable in informing ongoing science-based decision-making around who gets access to shared water resources and how much is allocated for extractive use while keeping coastal ecosystems in balance.

The Final Report and Fact Sheet are now available on the FRDC website.

Giant mud crabs (Scylla serrata) are found in mangrove and estuarine habitats. They are of commercial, recreational and cultural importance in the Gulf of Carpentaria. Fluctuations in catch rates and abundance have been linked to river flow and other environmental variables. Photo credit: Queensland Department of Agriculture and Fisheries

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