Toxic algal blooms – a sign of rivers under stress

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Toxic algal blooms pose a serious threat to Australia's already embattled waterways and are the subject of much scientific research.

A blooming disaster

Australia holds one world record it could do without: in November 1991 we scored the largest river toxic algal bloom in history. An estimated 1000-kilometre stretch of the Barwon and Darling rivers in New South Wales was affected; from the air it looked like a long ribbon of pea soup.

But it wasn't so drinkable. Toxic algal blooms are just that: toxic. The government declared a state of emergency as livestock perished and residents who normally took their drinking water from the rivers were forced to rely solely on rainwater tanks and emergency water-filtration equipment.

Luckily, the small number of people living in the area meant that this problem was manageable. But imagine if a water supply for a major urban centre was affected by a toxic algal bloom. This is not as unlikely as you may think (Box 1: Is Newcastle's water vulnerable to toxic algal blooms?).

What is a toxic algal bloom?

Blue-green algae, more correctly known as cyanobacteria, are naturally occurring components of all freshwater environments (Box 2: Cyanobacteria: the simple things of life). They occur as single microscopic cells or in colonies that form slimy strands visible to the naked eye. When conditions are favourable, blue-green algae populations can 'bloom', multiplying at such a rate that they dominate the local aquatic environment.

At this point, problems for other organisms start to occur. The water begins to stink and a green scum may appear on the surface. Some species of blue-green algae produce toxins which are dangerous – sometimes fatal – to livestock, wildlife, marine animals and humans.

The decomposition of dead blue-green algal cells by bacteria consumes oxygen. When billions of such cells die during a bloom, the water becomes oxygen-depleted. This can lead to the death of other marine organisms, including fish, which need oxygen to survive. As well, the blue-green algae contain toxins that affect human and animal health (Box 3: Harmful effects of blue-green algae on human health).

A fair-dinkum problem

Toxic algal blooms are not unique to Australia. They have been reported in many different river, lake and estuarine systems throughout the world. Nevertheless, the problem is particularly acute in Australia: current land and water management practices combine with our generally arid climate to create conditions in which blue-green algae thrive.

First, blue-green algae like abundant phosphorus and nitrogen. These two nutrients enter Australia's waterways in large amounts from factories and sewerage outlets, and as run-off from farms and suburban parks and lawns. This process is known as eutrophication. Much of the phosphorus and nitrogen comes from ordinary household detergents.

Blue-green algae are favoured by long periods of sunlight which provide warm temperatures and the energy for photosynthesis. Sunlight is rarely in short supply in Australia.

They also enjoy still, calm conditions. Such conditions are usually present in dams and reservoirs, but they can also occur in rivers during drought or when their flow is reduced by irrigation and household use. The flow of many river systems in Australia can sometimes be too low to prevent algal blooms.

When an ecosystem is functioning properly, population explosions can often be kept in check by natural processes. For example, many aquatic animals feed on blue-green algae. But many of Australia's aquatic ecosystems have been affected by such things as the damming of rivers and the introduction of pests like European carp. A damaged ecosystem may be less able to cope with the effects of algal blooms.

Watering down the resource

Toxic algal blooms are symptomatic of a broader land and water management problem in Australia. Although our waterways are our most precious resource, for decades we have been using them as a waste disposal unit. At the same time, exotic pests have invaded almost every pond, dams and drainage systems have altered natural flow patterns, and river banks have become severely eroded.

Can the problem be solved?

Science can certainly help. We are still learning, but already scientists have developed techniques to help deal with the toxic algal blooms.

Research into the primary sources of excess phosphorus and nitrogen in our waterways has led to the development of improved land management practices. The detergent industry has also reduced the amount of phosphorus in its products. CSIRO scientists have developed a modified clay product that can remove dissolved phosphorus from water bodies, starving the algae.

Stirred, not shaken

Other techniques to minimise algal blooms have been surfacing. For example, a team led by CSIRO's Dr Ian Webster has discovered that blue-green algal blooms can be reduced by 'stirring'.

During calm, sunny periods, a warm surface layer develops in water-bodies. This effect is known as 'thermal stratification'. An experiment conducted by Dr Webster and his team showed that thermal stratification was the key factor in the establishment of blue-green algal blooms: the blue-green algae floated up to this warm layer where they absorbed light and bred rapidly. The team concluded that if thermal stratification could be minimised by mixing the cold and warm layers, blue-green algal blooms might be prevented. Many dams now have propeller or aeration systems to help circulate the water.

But how can big dams be 'stirred'? Different techniques include increasing 'environmental flows' from dams and releasing water in 'pulses' rather than at a steady rate.

Don't bank on our rivers

Another research effort led by Professor Bob Wasson then at the Australian National University discovered that river bank erosion is a much more significant contributor to the sedimentation of our waterways than was previously thought. Erosion, in turn, brings phosphorus into the waterways, thereby helping to create favourable conditions for blue-green algae.

Such erosion could be prevented by some simple, though sometimes costly, land management practices. These include fencing off rivers, providing alternative watering points for livestock away from river banks, and revegetating small channels and gullies.

More than science is required

Scientists will not solve the problem by themselves. Restoring our waterways will take a concerted effort by all Australians.

We need to implement land and water management practices that treat our arid continent more sympathetically. We also need to reduce our water consumption and the rate at which we dump phosphorus, nitrogen and other wastes into rivers, lakes and streams. Coordinated efforts along these lines are already underway.

Perhaps most importantly, we need to learn how to live with our environment, not against it. World records for environmental disasters? Gold medals aren't given out for those.

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Page updated February 2012.