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.
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
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).
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.
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.
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.
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.
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?
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.
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 directors of most Australian companies are well aware of the impact of carbon emissions, not only on the environment but also on their own firms as emissions-intensive industries get lumbered with taxes and regulations designed to change their behaviour.
Many are getting out of emissions-intensive activities ahead of time.
But, with honourable exceptions, Australia’s tourism industry (and the Australian authorities that support it) is rolling on as if it’s business as usual.
This could be because tourism isn’t a single industry – it is a composite, made up of many industries that together create an experience, none of which take responsibility for the whole thing.
But tourism is a huge contributor to emissions, accounting for 8% of emissions worldwide and climbing as tourism grows faster than the economies it contributes to.
Tourism operators are aiming for even faster growth, most of them apparently oblivious to clear evidence about what their industry is doing and the risks it is buying more heavily into.
If tourism destinations were companies…
If Australian tourist destinations were companies they would be likely to discuss the risks to their operating models from higher taxes, higher oil prices, extra regulation, and changes in consumer preferences.
Aviation is one of the biggest tourism-related emitters, with the regions that depend on air travel heavily exposed.
But at present the destination-specific carbon footprints from aviation are not recorded, making it difficult for destinations to assess the risks.
A recent paper published in Tourism Management has attempted to fill the gap, publishing nine indicators for every airport in the world.
The biggest emitter in terms of departing passengers is Los Angeles International Airport, producing 765 kilo-tonnes of CO₂ in just one month; January 2017.
When taking into account passenger volumes, one of the airports with the highest emissions per traveller is Buenos Aires. The average person departing that airport emits 391 kilograms of CO₂ and travels a distance of 5,651 km.
The analysis used Brisbane as one of four case studies.
Brisbane’s share of itineraries under 400 km is very low at 0.7% (compared with destinations such as Copenhagen which has 9.1%). That indicates a relatively low potential to survive carbon risk by pivoting to public transport or electric planes, as Norway is planning to.
The average distance travelled from Brisbane is 2,852 km, a span exceeded by Auckland (4,561 km) but few other places.
As it happens, Brisbane Airport is working hard to minimise its on-the-ground environmental impact, but that’s not where its greatest threats come from.
The indicators suggest that the destinations at most risk are islands, and those “off the beaten track” – the kind of destinations that tourism operators are increasingly keen to develop.
Queensland’s Outback Tourism Infrastructure Fund was established to do exactly that. It would be well advised to shift its focus to products that will survive even under scenarios of extreme decarbonisation.
They could include low-carbon transport systems and infrastructure, and a switch to domestic rather than international tourists.
Experience-based travel, slow travel and staycations are likely to become the future of tourism as holidaymakers continue to enjoy the things that tourism has always delivered, but without travelling as much and without burning as much carbon to do it.
An industry concerned about its future would start transforming now.