How is oxygen ‘sucked out’ of our waterways?



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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.




Read more:
Explainer: what causes algal blooms, and how we can stop them


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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More of us are drinking recycled sewage water than most people realise


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.

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Explainer: what causes algal blooms, and how we can stop them


Michele Burford, Griffith University

Outbreaks of algae have killed up to a million fish in the Murray Darling Basin over the last two weeks. The phenomena of “algae blooms”, when the population of algae in a river rapidly grows and dies, can be devastating to local wildlife, ecosystems and people. But what are algae blooms? What causes them, and can we prevent them?

Microscopic algae are fundamental to life on earth. These tiny plants provide the fuel that drives marine and freshwater foodwebs, and via photosynthesis, they gobble up carbon dioxide to help counteract emissions, and provide us with oxygen to breathe. Besides rivers, streams, lakes, estuaries and the coast, they can also be found in diverse environments such as snow, soil, and in corals.




Read more:
Are toxic algal blooms the new normal for Australia’s major rivers?


But when humans channel agricultural run-off, sewerage and stormwater discharge into waterways, we dramatically increase the amount of nutrients such as nitrogen and phosphorus. This creates an imbalance, because some microscopic algae are supremely effective at mopping up nutrients and can grow very quickly, dividing up to once a day and quickly overtaking other species. The result is an algal bloom.

Cyanobacteria (blue-green algae) under a microscope.
Author provided

So why don’t we have algal blooms all the time? This is because algae don’t just require nutrients to grow. Like any plant, factors such as temperature and light availability are also important in determining how quickly algae grow and whether they form blooms. Blooms also need slow moving or still water to become established.

In Australia, our algal blooms are typically in freshwaters. The main group of algae responsible for this are known as blue-green algae, or more accurately, cyanobacteria. They regularly bloom in warmer weather in our reservoirs, lakes and slow flowing rivers. In 2016, for example, 1,700km of the Murray River was affected by an algal bloom.

There are many ways they impact the environment and economy. Some algal blooms are toxic, requiring expensive water treatment and – in extreme cases – shutdown of water supplies. This isn’t just a problem in Australia. In 2014, some 500,000 people in the US were left without drinking water due to a toxic algal bloom in Lake Erie.

The toxins can also affect domestic animals, such as dogs, when they drink contaminated water, and limit use of lakes and rivers for swimming, boating and fishing. Even when algal blooms are not toxic, they unbalance the food web, reducing the number of species of animals and plants.

They can also reduce oxygen levels at night, as they switch from photosynthesis (producing oxygen) during the day, to a process called respiration at night where they use oxygen. Low oxygen can stress and even kill fish and other animals if they cannot escape this.

At some point, algal blooms crash when conditions become unsuitable. The resulting dead algae break down, providing an ideal food source for bacteria. This is when waters can become smelly, often with a rotten egg smell. As the bacteria multiply, they suck the oxygen out of the water. At this point, oxygen levels become low both day and night.

If the area of low oxygen is extensive, such as a whole lake or many kilometres of a river system, fish and other animals may not be able to escape to more suitable oxygen levels, and major fish deaths typically occur.

In other areas of the world, algal blooms have caused such severe oxygen conditions that thousands of square kilometres of ocean around the world are now known as dead zones, where no animals can live. These vast dead zones are not something we ever want to see in Australia.

So what can be done about blooms?

There are a wide range of treatments that can be used to control blooms, for example, aerating the water, and adding clays and chemicals, but the catch is they are very expensive on a large scale.

Ideally, the problem should be tackled at the source. This means reducing nutrient loads to our waterways. There has already been progress on this in our cities where sewage treatment plants have been upgraded to reduce nutrient loads to waterways. But tackling nutrients coming from agriculture – erosion, fertilisers, animal waste – is much more challenging and expensive because of the vast areas involved. So this remains work in progress.

It’s also very difficult to predict when blooms will occur; despite being simple plants, algae have an amazing range of strategies to grow and survive. But as we learn more about their complexity our ability to model and predict blooms will improve. This is crucial to managing risks to water supplies and preventing major environmental effects, such as fish deaths.




Read more:
Toxin linked to motor neuron disease found in Australian algal blooms


Ultimately there are no quick fixes to algal blooms. Given the pressure we put on our waterways, they are here to stay. In fact they are likely to increase due to increasing temperatures and more extreme conditions, such as droughts. We know what we need to do to reduce the scale and likelihood of blooms: the challenge is devoting the resources to achieve it.The Conversation

Michele Burford, Professor – Australian Rivers Institute, and Dean – Research Infrastructure, Griffith University

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

Toxin linked to motor neuron disease found in Australian algal blooms



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Cyanobacterial blooms and algae are common in water bodies around the world. However, Australia is yet to monitor the growth of neurotoxins in our algae.
from http://www.shutterstock.com

Brendan Main, University of Technology Sydney

Algal blooms in major Australian rivers are releasing a toxic chemical that may contribute to the development of motor neuron disease (MND).

My colleagues and I tested algae from waterways in New South Wales, and found that a neurotoxin called BMAA was present in 70% of samples, including those from crucial water sources such as the Darling and Murrumbidgee rivers.

This compound is well known overseas, and has been found in waterways in the United States, Europe, Asia, and the Middle East. But this is the first time it has been detected in Australia. Although its presence has been suspected, it was never specifically tested until now.

Two samples containing BMAA were collected from the Murrumbidgee River, which runs through the NSW Riverina, a hotspot for MND in Australia. Positive samples were also collected in Centennial Park and Botany wetlands in central Sydney, as well as Manly Dam on Sydney’s Northern Beaches.




Read more:
What we know, don’t know and suspect about what causes motor neuron disease


In the past 30 years, Australian rivers have had the dubious honour of hosting some of the largest algal blooms in history. In 1991 a bloom stretched along more than 1,200km of the Darling River, prompting the New South Wales government to declare a state of emergency. The army was mobilised to provide aid to towns.

Since then, southeast Australia has had four large blooms, most recently in 2016. The future isn’t promising either. Rising water temperatures mean blooms are likely to increase in frequency and duration in the future.

Multiple state agencies monitor populations of types of bacteria in Australia, regularly testing water quality and issuing alerts when blooms are present. This testing is necessary because of the impressive number of toxins that cyanobacteria can produce, ranging from skin irritants to liver and neurological toxins. Most of these compounds are relatively fast-acting, meaning that their effects take hold rapidly after exposure.

The neurotoxic compound BMAA, however, is not currently part of regular testing, despite links between long-term exposure to algal blooms and the development of diseases such as MND. BMAA is known to be produced by a type of freshwater and marine bacteria, as well as some species of algae.




Read more:
Watch out, Australia: a red-hot summer means blue-green algae


How BMAA affects our health

Research in America found that regular participation in water-based recreational activity resulted in a threefold increase in the risk of developing MND. Satellite mapping also revealed that lakes prone to algal blooms were often surrounded by clusters of MND patients.

Southwestern NSW has become a focus for MND researchers since 2014, due to the presence of a hotspot for MND cases around the Riverina. The town of Griffith has reported a prevalence of this disease that is nearly seven times higher than the national average of 8.7 cases per 100,000 people. Hotspots like these can help researchers identify environmental factors that contribute to diseases.

This is particularly important in MND, in which only 5-10% of patients have a family history. The other 90-95% of cases are sporadic, occurring without warning. It is possible that BMAA exposure, in association with genetic, or other environmental risk factors, contributes to the high incidence of MND in the Riverina.




Read more:
Exposure to algae toxin increases the risk of Alzheimer’s-like illnesses


BMAA also has a similar structure to the amino acids that make up the proteins in our body. We hypothesise that this contributes to its toxicity and ability to build up in animal tissue and in plants that are exposed to contaminated water.

Similar to mercury, BMAA can accumulate in the food chain, which means that people could be consuming relatively large amounts of it through their diet. A US animal study found that dietary exposure to BMAA resulted in the formation of plaques and protein tangles in the brain, which are hallmark features of neurodegeneration.

Research now needs to focus on tracking and monitoring algal blooms to detect the presence of BMAA, and determining how long it remains in the ecosystem after these blooms occur.

The ConversationThis can potentially help to reduce human exposure to BMAA. Although the factors that cause MND are many and varied, we hope this understanding could ultimately help to reduce the number of people who develop the disease.

Brendan Main, PhD Candidate, University of Technology Sydney

This article was originally published on The Conversation. Read the original article.