WHEN water spills out on to the floodplains of the Murray–Darling Basin, it’s a shot in the arm for the ecosystems that have long depended on floodwaters for their survival. Shrublands of golden goosefoot, grasslands of curly Mitchell grass, forests of river red gum—plants of all shapes and sizes take a deep drink and burst into life.
But as the floodwater recedes, carrying with it much of the decaying leaf litter that has accumulated on the ground since the previous flood, it can lose oxygen and turn black in colour. Flowing back into the rivers, this ‘blackwater’ is known to kill aquatic species, from the tiniest shrimp to the apex predator of the river system—the Murray cod.
Now new research has shown that not all blackwater is bad. Rather, during cooler times of the year, in the right dose at the right time, it is fuelling the food web in much the same way that algae do, creating more sustainable populations of bugs and fish.
The perception that floodplain water is bad
The microscopic side of rivers and wetlands is the domain of CSIRO scientist Dr Gavin Rees, based in Wodonga. The term ‘blackwater event’ is familiar to many people in the Basin, he says. “Certainly, fishers understand the term, and people downstream do. It’s an emotive issue. Because of past fish kills there’s a perception that all water that comes off floodplains is bad.”
While blackwater is not necessarily harmful to humans, he adds, if it is being used for drinking water it must be treated, which drives up the cost. Local businesses also suffer when tourists and recreational fishers steer clear of the river.
A suffocating brew
Making blackwater is like making tea, explains Rees. “If you put dried eucalypt leaves in water, in 20 minutes you’ll start to see it. You extract the organic matter and the water goes tea-coloured.”
The more leaf litter you add, and the more decomposed it is, the stronger the brew. Now rich in dissolved organic carbon, the water attracts microbes that feed on the carbon. But microbes consume oxygen and, especially during warmer weather, they can become too much of a good thing. “It’s when the microbes use oxygen faster than algae can replace it that the problem starts,” says Rees.
If oxygen levels fall below 2 mg/litre, the water is termed ‘hypoxic’ and fish struggle to breathe. If it’s localised, they may be able to find refuge but if it’s largescale they are left with nowhere to go—while crayfish may be seen crawling onto riverbanks, desperately seeking oxygen, fish simply suffocate.
Following the floodplain’s fingerprints
Rees set out to determine whether the carbon in blackwater is of value ecologically. His study began in the Barmah-Millewa Forest, located on the border of Victoria and New South Wales. It’s Australia’s largest red gum forest, and like all our red gum forests, has been the source of large volumes of blackwater flowing into the Murray River.
“The reason the forest is there is because it gets flooded,” explains Rees. “It is where it is because of the river’s narrow channel. When the water reaches the narrow channel, it pushes out over the floodplain and that creates an environment where red gums flourish.”
Since regulation was introduced in the Basin many years ago, flooding has become a highly managed process and much less regular. The volume and timing of managed water releases is tailored to achieve specific ecological outcomes such as maintaining breeding or feeding grounds.
“In the Barmah-Millewa Forest, for example, the Moira grasslands get watered at certain times, to provide nesting areas for waterbirds,” says Rees. “But the red gums don’t need flooding every year.”
With more time between floods, leaf litter can really build up. Rees found that the amount of dissolved organic carbon in the river had increased three-fold after the forest was flooded, triggering a lot more microbial activity.
Using a biochemistry technique known as isotope analysis, he was able to identify the unique ‘signature’ of the carbon that came from the forest and trace it down the river to see what became of it.
“It’s like fingerprinting,” he explains. “Grinding up eucalyptus leaves reveals their specific signature. If I grind up a water bug and get the same signatures, I know the water bug got that signature from eating the floodplain carbon, not from eating algae. We can then track that carbon up into bigger animals in the food web and tell if their food came from the floodplain carbon or another source.”
Bugs, beetles and freshwater shrimp were some of the river creatures found to have indirectly eaten carbon that came from the forest. “We stopped at bugs because, by inference, we know what fish eat.”
The conclusion, says Rees, is that dissolved organic carbon carried into the river from the floodplains can fuel the river’s food web. The implication is that the river’s ‘productivity’—the number of tonnes of fish it can support, for example—may be understated. Productivity is usually calculated based on algae content, algae being the base source of food in the food web.
Flushing out the leaf litter
Flushing the leaf litter off the floodplains more regularly can minimise the risk of hypoxic blackwater forming, but deciding when and how to do this is complex, with each floodplain having a unique combination of ecology and hydrology.
CSIRO scientist Dr Klaus Joehnk is working with the Murray–Darling Basin Authority to develop a modelling tool that will help water managers find the best way to do this by safely exploring what-if scenarios before they release water down the river.
“The intention is to reduce leaf litter as much as possible with regular watering to prevent long-lasting hypoxic events later,” says Dr Joehnk. “A lot of recreational fishers and people living along the river are fearful of managing leaf litter with water, but it’s only bad if the water becomes hypoxic. And fast-flowing floodwaters can sometimes self-aerate, delivering carbon to other parts of the river in a ‘good’ blackwater event.”
The tool combines known data such as the extent of the floodplain; the amount of accumulated leaf litter; how much water is being released down the river, and when; and the temperature of the water. It is initially being tested for the Barmah-Millewa floodplain and Dr Joehnk is confident it can be generalised for all floodplains in the Basin.
Eagerly awaiting its launch later this year is Dr Tapas Biswas, program manager of the River Murray Water Quality Program at the Murray–Darling Basin Authority which is funding the development: “We have learnt from the past that there are some areas where blackwater is generated but we don’t know how much and what type of organic litter is coming from what floodplain. CSIRO is working out those unknowns and this decision-support tool will help us to inform river operations so we can manage the blackwater risk.
“The aim is to improve water quality by reducing fish kills due to hypoxic blackwater, and to reduce disruption to businesses when the water is black in colour. If we can improve water quality, we can also reduce the cost of water treatment.”
Don’t throw the carbon out with the bath water
Eliminating blackwater is not an option, stresses Dr Joehnk: “If we get an extreme natural flood, like we did in 2016 and 2017, the extent of the flooding will make it near impossible to prevent blackwater events.” Extreme floods can reach higher parts of the floodplain which managed floods can never reach, and where leaf litter will have accumulated over long periods.
But neither is prevention desirable—a certain amount of microbial activity is necessary for all river life. Dr Rees: “What we found is that floodplain water is not always bad and can at times be beneficial. With some changes to how water should move across floodplains, we can manipulate it to return what would have been a normal part of the system, for the benefit of the ecology. Let’s not throw the carbon out with the bath water.”