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Blue-green algae

Blue-green algae occur naturally in rivers, lakes and dams. Under certain conditions, blue-green algae multiply quickly, creating an ‘algal bloom’ that can decrease water quality. This affects the environment, human health and industry.

To see if there is an algal bloom alert in your area, check the Murray–Darling Basin Authority's (MDBA) water quality alerts map

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Why blue-green algae is a problem in the Murray–Darling Basin

Too much algae in the rivers of the Basin is bad for water quality. Algal blooms can make water unsafe for humans and animals to drink, and it can even be unsafe for humans to come into contact with it, making swimming and even boating dangerous. Treating affected water is expensive.

The death of algal blooms can also have harmful effects.

About blue-green algae

Blue-green algae are a type of microscopic bacteria known as Cyanobacteria. They are common all over the world. Blue-green algae photosynthesise, like plants, so they need sunlight, carbon dioxide and nutrients to grow.

With the right conditions blue-green algae can reproduce very quickly, particularly when it’s warm and sunny and there are high levels of nutrients like nitrogen and phosphorus in the water. When this happens, the blue-green algae form what is called an ‘algal bloom’. This can look like a scum on the water, and is usually green.

Algal blooms and their impact

Algal blooms can affect the colour and taste of water, as well as how safe it is to drink. Some blue-green algae produce toxins that are dangerous to humans and animals. This can potentially mean water affected by an algal bloom:

  • is not safe for humans to drink
  • can poison wildlife, livestock and domestic animals
  • is not safe for recreational activities such as swimming and boating
  • is difficult and expensive to treat to make it safe for drinking.

This has serious consequences for Basin communities and farmers, as well as the tourism industry.

Algal blooms can also contribute to fish deaths. Like plants, blue-green algae do not photosynthesise at night. Instead, they use oxygen in a process called respiration. When large numbers of blue-green algae take oxygen out of the water, there may not be enough left for fish and other aquatic life to breathe.

Are blooms getting worse?

A 2011 report on blue-green algae in the River Murray looked at long-term trends. The report found that the amount of algal blooms has not changed much within the river system between 1980 and 2008. However, higher frequencies were recorded at upstream sites and lower frequencies at downstream sites.

How the MDBA responds to algal blooms

The MDBA does what it can to help local authorities manage algae. To try and reduce the likelihood and intensity of algal blooms, river operators consider how water quality will be impacted when making decisions on water delivery and management.

It can be difficult to stop a blue-green algal bloom once it has started. Sometimes environmental water holders can help manage algae by changing where water is released into the system or releasing new water. However, there is not always enough water available to reduce algal blooms. Even when water is available, flushing the blooms may not reduce them and may even spread the problem downstream.

Under the Basin Plan, Basin state governments must keep water quality targets in mind, including blue-green algae. The MDBA reports performance against water quality targets every year and evaluates water quality outcomes every 5 years.

Water quality targets are set out in the Murray–Darling Basin Plan, and the Water Quality Guidelines under the National Water Quality Management Strategy.

Other activities that help to manage algal blooms include:

  • regularly taking water samples to monitor algae levels
  • alerting the public when there is a bloom
  • putting up signs at affected sites to warm people to avoid contact with the water.

Recognising and reporting algal blooms

Algal blooms usually start as small green floating dots and can develop into a thick scum on the water surface. They are often green, but they can also be white, brown, blue, yellow or red.

If you think you have seen blue-green algae, avoid contact with the water and report it to your local council or water supplier.

Report water quality issues or fish deaths

Find out who to contact if you need to:

  • report a suspected water quality issue (including blue-green algae or blackwater)
  • get more information about water quality in your area
  • use water after a water quality incident
  • report dead or dying fish in your area.
    Updated: 23 Dec 2022