How bushfires and rain turned our waterways into ‘cake mix’, and what we can do about it



The Murray River at Gadds Reserve in north east Victoria after Black Summer bushfires.
Paul McInerney, Author provided

Paul McInerney, CSIRO; Anu Kumar, CSIRO; Gavin Rees, CSIRO; Klaus Joehnk, CSIRO, and Tapas Kumar Biswas, CSIRO

As the world watched the Black Summer bushfires in horror, we warned that when it did finally rain, our aquatic ecosystems would be devastated.

Following bushfires, rainfall can wash huge volumes of ash and debris from burnt vegetation and exposed soil into rivers. Fires can also lead to soil “hydrophobia”, where soil refuses to absorb water, which can generate more runoff at higher intensity. Ash and contaminants from the fire, including toxic metals, carbon and fire retardants, can also threaten biodiversity in streams.




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As expected, when heavy rains eventually extinguished many fires, it turned high quality water in our rivers to sludge with the consistency of cake mix.

In the weeks following the first rains, we sampled from these rivers. This is what we saw.

Sampling the upper Murray River

Of particular concern was the upper Murray River on the border between Victoria and NSW, which is critical for water supply. There, the bushfires were particularly intense.

Sludge in Horse Creek near Jingellic following storm activity after the fire.
Paul McInerney/Author Provided

When long-awaited rain eventually came to the upper Murray River catchment, it was in the form of large localised storms. Tonnes of ash, sediment and debris were washed into creeks and the Murray River. Steep terrain within burnt regions of the upper Murray catchment generated a large volume of fast flowing runoff that carried with it sediment and pollutants.

We collected water samples in the upper Murray River in January and February 2020 to assess impacts to riverine plants and animals.

Our water samples were up to 30 times more turbid (cloudy) than normal, with total suspended solids as high as 765 milligrams per litre. Heavy metals such as zinc, arsenic, chromium, nickel, copper and lead were recorded in concentrations well above guideline values for healthy waterways.

Ash and sediment blanketing cobbles in the Murray River.
Paul McInerney/Author Provided

We took the water collected from the Murray River to the laboratory, where we conducted a number of toxicological experiments on duckweed (a floating water plant), water fleas (small aquatic invertebrates) and juvenile freshwater snails.

What we found

During a seven-day exposure to the bushfire affected river water, the growth rate of duckweed was reduced by 30-60%.

The water fleas ingested large amounts of suspended sediments when they were exposed to the affected water for 48 hours. Following the exposure, water flea reproduction was significantly impaired.

And freshwater snail egg sacs were smothered. The ash resulted in complete deaths of snail larvae after 14 days.




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These sad impacts to growth, reproduction and death rates were primarily a result of the combined effects of the ash and contaminants, according to our preliminary investigations.

But they can have longer-term knock-on effects to larger animals like birds and fish that rely on biota like snail eggs, water fleas and duckweed for food.

What happened to the fish?

Immediately following the first pulse of sediment, dead fish (mostly introduced European carp and native Murray Cod) were observed on the bank of River Murray at Burrowye Reserve, Victoria. But what, exactly, was their cause of death?

A dead Murray Cod found on the banks of the Murray River following storms after the bushfires.
Paul McInerney/Author Provided

Our first assumption was that they died from a lack of oxygen in the water. This is because ash and nutrients combined with high summer water temperatures can trigger increased activity of microbes, such as bacteria.

This, in turn can deplete the dissolved oxygen concentration in the water (also known as hypoxia) as the microbes consume oxygen. And wide-spread hypoxia can lead to large scale fish kills.




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But to our surprise, although dissolved oxygen in the Murray River was lower than usual, we did not record it at levels low enough for hypoxia. Instead, we saw the dead fish had large quantities of sediment trapped in their gills. The fish deaths were also quite localised.

In this case, we think fish death was simply caused by the extremely high sediment and ash load in the river that physically clogged their gills, not a lack of dissolved oxygen in the water.

These findings are not unusual, and following the 2003 bushfires in Victoria fish kills were attributed to a combination of low dissolved oxygen and high turbidity.

So how can we prepare for future bushfires?

Preventing sediment being washed into rivers following fires is difficult. Installing sediment barriers and other erosion control measures can protect specific areas. However, at the catchment scale, a more holistic approach is required.




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One way is to increase efforts to re-vegetate stream banks (called riparian zones) to help buffer the runoff. A step further is to consider re-vegetating these zones with native plants that don’t burn easily, such as Blackwood (Acacia melanoxylin).

Streams known to host rare or endangered aquatic species should form the focus of any fire preparation activities. Some species exist only in highly localised areas, such as the endangered native barred galaxias (Galaxias fuscus) in central Victoria. This means an extreme fire event there can lead to the extinction of the whole species.

Ash and dead fish on the banks of the Murray River near Jingellic following Black Summer fires.
Paul McInerney/Author Provided

That’s why reintroducing endangered species to their former ranges in multiple catchments to broaden their distribution is important.

Increasing the connectivity within our streams would also allow animals like fish to evade poor water quality — dams and weirs can prevent this. The removal of such barriers, or installing “fish-ways” may be important to protecting fish populations from bushfire impacts.

However, dams can also be used to benefit animal and plant life (biota). When sediment is washed into large rivers, as we saw in the Murray River after the Black Summer fires, the release of good quality water from dams can be used to dilute poor quality water washed in from fire affected tributaries.




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Citizen scientists can help, too. It can be difficult for researchers to monitor aquatic ecosystems during and immediately following bushfires and unmanned monitoring stations are often damaged or destroyed.

CSIRO is working closely with state authorities and the public to improve citizen science apps such as EyeOnWater to collect water quality data. With more eyes in more areas, these data can improve our understanding of aquatic ecosystem responses to fire and to inform strategic planning for future fires.

These are some simple first steps that can be taken now.

Recent investment in bushfire research has largely centred on how the previous fires have influenced species’ distribution and health. But if we want to avoid wildlife catastrophes, we must also look forward to the mitigation of future bushfire impacts.The Conversation

Paul McInerney, Research scientist, CSIRO; Anu Kumar, Principal Research Scientist, CSIRO; Gavin Rees, Principal Research Scientist, CSIRO; Klaus Joehnk, Principal research scientist, CSIRO, and Tapas Kumar Biswas, Senior scientist, CSIRO

This article is republished from The Conversation under a Creative Commons license. Read the original article.

How is oxygen ‘sucked out’ of our waterways?



File 20190114 43544 kywlhi.jpg?ixlib=rb 1.1
Dead fish are a source of food for bacteria, which then extract oxygen from the river.
AAP

Stuart Khan, UNSW

A million fish have died in the Murray Darling basin, as oxygen levels plummet due to major algal blooms. Experts have warned we could see more mass deaths this week.




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Fingers have been pointed at poor water management after a long period of drought. However, mass fish deaths can also be caused by floods, and even raw sewage.

So what’s going on when oxygen gets “sucked out of the water”?

The phenomenon is very well known to water quality engineers; we call it “biochemical oxygen demand”. To understand it, we need to talk about a little bit of biology and a little bit of chemistry.

When oxygen meets water

Oxygen molecules are soluble in water in the same way that sugar is soluble in water. Once its dissolved, you can’t see it (and, unlike sugar, oxygen is tasteless).

The maximum amount of oxygen that you can dissolve in water depends on a number of factors, including the water temperature, ambient air pressure, and salinity. But roughly speaking, the maximum amount of dissolvable oxygen, known as the “saturation concentration” is typically around 7-10 milligrams of oxygen per litre of water (7-10 mg/L).

This dissolved oxygen is what fish use to breathe. Fish take water in through their mouths and force it through their gill passages. Gills, like our lungs, are full of blood vessels. As water passes over the thin walls of the gills, dissolved oxygen is transferred into the blood and then transported to the fish’s cells. The higher the oxygen concentration in the water, the easier it is for this transfer to occur.

Once in the cells, the oxygen molecules play a key role in the process of “aerobic respiration”. The oxygen reacts with energy-rich organic substances, such as sugars, carbohydrates and fats to break them down and release energy for the cells. The main waste product from this process is carbon dioxide (CO₂). This is why we all need to breathe in oxygen and we breathe out carbon dioxide. Fish do that too. A simple way to express this is:

Organic substances + Oxygen Carbon dioxide + Water + Energy

The Hunter River in NSW suffered a ‘blackwater’ event in 2016 when floodwaters washed organic matter into the river.
Andrew S/Flickr, CC BY-SA

What is the biochemical oxygen demand?

Just like fish and people, many bacteria gain energy from processes of aerobic respiration, according to the simplified chemical reaction shown above. Therefore, if there are organic substances in a waterway, the bacteria that live in that waterway can consume them. This is an important process of “biodegradation” and is the reason our planet is not littered by the carcasses of animals that have died over many thousands of years. But this form of biodegradation also consumes oxygen, which comes from dissolved oxygen in the waterway.

Rivers can replenish their oxygen from contact with the air. However this is a relatively slow process, especially if the water is stagnant (flowing creates turbulence and mixes in more oxygen). So if there is a lot of organic matter present and bacteria are feasting on it, oxygen concentrations in the river can suddenly drop.

Obviously, “organic substances” can include many different things, such as sugars, fats and proteins. Some molecules contain more energy than others, and some are easier for the bacteria to biodegrade. So the amount of aerobic respiration that will occur depends on the exact chemical nature of the organic substances, as well as their concentration.

Therefore, instead of referring to the concentration of “organic substances”, we more commonly refer to the thing that really matters: how much aerobic respiration the organic substances can trigger and how much oxygen this will cause to be consumed. This is what we call the biochemical oxygen demand (BOD) and we usually express it as a concentration in terms of milligrams of oxygen per litre of water (mg/L).

Like us, bacteria don’t consume all of the food which is available to them instantly – they graze on it over time. Biodegradation therefore can take days, or longer. So when we measure the BOD of a contaminated water sample, we need to assess how much oxygen is consumed (per litre of water) over a specified period of time. The standard period of time is usually five days and we refer to this value as the BOD5 (mg/L).

Murray cod pull oxygenated water through their gills, transferring it to their bloodstream. Without oxygen in the water, they die.
Guo Chai Lim/Flickr, CC BY-NC-SA

As I mentioned earlier, clean water might only have a concentration of dissolved oxygen of up to around 7-10 mg/L. So if we add organic material in a concentration which has a higher BOD5 than this, we can expect it to deplete the ambient dissolved oxygen concentration during the next five days.




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This phenomena is the main reason for which biological sewage treatment was invented. Raw (untreated) municipal sewage can have a BOD5 of 300-500 mg/L. If this were discharged to a clean waterway, the typical base-level of 7-10 mg/L of oxygen would be consumed, leaving none available for fish or other aquatic organisms.

So the purpose of biological sewage treatment is to grow lots of bacteria in large tanks of sewage and provide them with plentiful oxygen for aerobic respiration. To do this, air can be bubbled through the sewage, or sometimes surface aerators are used to churn up the sewage.

By supplying lots of oxygen, we ensure the BOD5 is effectively consumed while the sewage is still in the tanks, before it’s released to the environment. Well treated sewage can have a BOD5 as low as 5 mg/L, which can then be further diluted as it’s discharged to the environment.

In the case of the Darling river, the high BOD load was created by algae, which died when temperatures dropped. This provided a feast for bacteria, lowering oxygen, which in turn killed hundreds of thousands of fish. Now, unless we clean the river, those rotting fish could become fodder for another round of bacteria, triggering a second de-oxygenation event.The Conversation

Stuart Khan, Professor of Civil & Environmental Engineering, UNSW

This article is republished from The Conversation under a Creative Commons license. Read the original article.