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Blue-green algae

Blue-green algae are actually a type of bacteria known as Cyanobacteria. Despite being called algae, they only have some things in common — they photosynthesise using light to produce oxygen and they need sunlight to grow. Blue-green algae are a natural part of the freshwater environment. Six species are generally found in the Basin. Of these, two species, Dolichospermum and Microcystis, are the most common types in the River Murray.

If conditions are favourable, they can reproduce at very high rates to form 'blooms' which produce sometimes toxic scums and impact river users and environments.

A motorboat rounds a bend on the Murray River where algal bloom is evident.
A motorboat rounds a bend on the Murray River where algal bloom is evident. Image taken in April 2009.

Problems caused by algae

When blooms of blue-green algae occur they interfere with other uses of the water, can affect people’s health and have consequences for the environment and the economy.

While blooms can be harmful, water is often safe for:

  • irrigation purposes, except for those vegetable crops consumed raw
  • recreational activities, including fishing, boating and sightseeing, as long as you avoid skin contact or swallowing water
  • drinking water where suppliers treat affected water by using activated carbon and other techniques.

For anyone planning to be out on the water, they should make a habit of checking for any local warnings of blooms.

Water quality and environments

Algae blooms affect water quality by changing tastes, odours, and appearances. Water supplies can also be affected. If you are concerned you should seek guidance from your local water supply authority. Large blooms can affect water-based leisure activities such as fishing and swimming.

Once blooms subside, decaying algae can reduce the oxygen levels in the water, damaging environments and the animals they support. During periods of drought, algal blooms can be particularly harmful to already struggling environments.

People and animal health

Some species produce toxins, that when ingested, can cause liver damage, stomach upsets and disorders of the nervous system in people. Direct contact with lots of blue-green algae can cause skin and eye irritations.

Algae can also have effects on livestock causing illness or occasional death and there is evidence it can poison wildlife and domestic pets.

Why do blue-green algae bloom?

Low levels of blue-green algae are present in freshwater all the time. However a series of favourable environmental factors including warm water temperatures, sunny days and nutrients can lead to a blue-green algae bloom.

Different types of blue-green algae favour different kinds of water conditions. Some prefer flowing water, while others prefer still water. Many types do well when water column settles into layers, warmer water on top with cooler deeper water resulting in stratification or layering of the water column.  Stratification can also result from differing densities through the water column, where saltier, denser water at the bottom remains unmixed with the fresher surface water above.

When the combination of above factors persist for several days, blue-green algae cells can multiply a great deal, resulting in a bloom.

Although blooms within a river may move downstream in the direction of its flow, the actual rate of movement is hard to predict and will generally be considerably slower than the rivers actual flow rate. 

During favourable conditions, blooms can appear downstream at any time, independent of upstream blooms, and can persist for as long as the favourable conditions remain in place.

How do we know there is blue-green algae?

Although the algal scum can sometimes be visibly detected, water samples must be collected and sent to a laboratory for analysis. Under the microscope, algae species are identified and counted to determine the alert level status and any required management responses thereof.

Visit the NSW Department of Primary Industry Water for more information about recreational guidelines for freshwater toxic algae

Can blue-green algae be removed?

Blue-green algae are naturally occurring microscopic bacteria and cannot be removed from the River Murray and its tributaries. Chemicals that kill or disperse blue-green algae cannot be used in natural waterbodies due to the impact on other organisms that live in and use the water. See our 'how blue-green algae blooms are handled' page for more information. 

Updated: 06 Sep 2019