WWhen heavy rains eventually extinguish the flames that ravage southeastern Australia, another ecological threat will arise. Sediments, ashes and debris that are washed in our waterways, particularly in the Murray-Darling Basin, can decimate aquatic life.
We have seen this before. After the 2003 forest fires in the alpine region of Victoria, water filled with sediments and debris (known as sediment slugs) flowed to rivers and lakes, greatly reducing fish populations. We will probably see it again after the emergence of forest fires this season.
Large areas of northeastern Victoria have been burned. While this region represents only 2% of the entire land area of the Murray-Darling basin, water flowing from the Victorian streams of the Northeast (also known as in-flow) contributes 38% of the total inflows to the Murray-Darling basin.
The remains of fire that flow into the Murray-Darling basin will exacerbate the risk of fish and other aquatic lives dying in mass, as observed in previous years.
What will flow to the waterways?
In general, bushfire ash comprises organic carbon and inorganic elements such as nitrogen, phosphorus and metals such as copper, mercury and zinc.
Sediments that precipitate on waterways can also contain large amounts of soil, since the fire has consumed the vegetation that once united the soil and prevented erosion.
And carcinogenic chemicals, which are found in the soil and ashes in greater quantities after forest fires, can contaminate streams and deposits during the first year after the fire.
How they harm aquatic life
Immediately after the forest fires, we expect to see an increase in the flow of the current when it rains, because the burned soil repels, does not absorb, the water.
When large amounts of carbon are present in a waterway, such as when sediments and carbon-laden waste are carried away, bacteria quickly consume oxygen from water. The remaining oxygen levels may fall below what most invertebrates and fish can tolerate.
These high sediment loads can also suffocate aquatic animals with a thin layer of silt that covers their gills and other breathing structures.
Habitats are also at risk. When the sediment is suspended in the river and the light cannot penetrate, the proper habitat for the fish decreases. More turbid water also means that there are fewer opportunities for aquatic plants and algae to make photosynthesis (convert sunlight into energy).
In addition, many of Australia’s water bugs, the cornerstone of river food networks, need pools with rubbish and debris to cover themselves. They depend on the silt on the surface of the rocks and the hooks that contain algae, fungi and bacteria to feed on.
But the heavy rains that follow the fire can lead to the pools and the spaces between the cobbles to fill with silt, causing bed bugs to starve and lose their homes.
This is also bad news for fish. Any fish that eats insects and manages to avoid dying due to lack of oxygen can face an immediate food shortage.
We saw this in 2003 after the sediment slug penetrated the Ovens River into the northeast Murray Basin. The researchers observed dead fish, stressed fish swallowing saliva on the surface of the water and freshwater crayfish coming out of the stream.
Long term damage
Forest fires can increase the amount of nutrients in the streams 100 times. The effects may persist for several years before nutrient levels return to pre-fire conditions.
More nutrients in the water may sound like a good thing, but when there is too much (especially nitrogen and phosphorus), along with warm temperatures, they can lead to an overgrowth of blue-green algae. These algae can be toxic to people and animals and often close recreational waters.
Large parts of the upper Murray River basin over Lake Hume have been burned, with the risk of increasing the nutrient load within the lake and causing blooms of blue-green algae that can flow downstream. This can affect communities from Albury to the mouth of the Murray River in South Australia.
Some aquatic species are already reeling at the edge of their preferred temperature as the temperature of the current increases due to climate change. In places where forest fires have burned to the edge of the stream, decimating the vegetation that provided shade, there will be less resistance to temperature changes and fewer cold places for aquatic life to hide.
Colder hiding places are particularly important for popular fishing species such as trout, which are very sensitive to rising water temperatures.
But although we can expect an increase in the flow of burnt water repellent soil, we know from previous forest fires that, in the long term, the flow of the current will fall. This is because in the upper basins, younger regenerating forests use more water than older forests that replace evapotranspiration (when plants release water vapor to the surrounding atmosphere and evaporation of the surrounding land surface).
It is particularly worrying for the Murray-Darling basin, where large areas are already suffering from continuous drought. Forest fires can exacerbate existing dry conditions.
So what can we do?
We need to act as soon as possible. Understandably, the priorities lie in eliminating the immediate and continuing threat of forest fires. But after that, we must improve sediment and erosion control to prevent debris from being dragged into bodies of water in areas affected by fires.
One of the first things we can do is restore the areas used for forest fire control lines and minimize soil movement along the access roads used to suppress forest fires. This can be achieved using sediment barriers and other erosion control measures in high-risk areas.
In the longer term, we can restore vegetation along waterways to help cushion extreme temperatures and sediment loads entering streams.
It is also important to introduce strategic water quality monitoring programs that incorporate real-time detection technology, providing an early warning system for poor water quality. This can help guide the management of our rivers and reservoirs in the coming years.
While our current approach is to put out fires, as it should be, it is important to start thinking about the future and how to protect our waterways. Because inevitably, it will rain again.
• This story was first published in the conversation.