Expand links In this section

Blue-green algae (cyanobacteria) and water quality

​​'Blue-green algae' or cyanobacteria are a type of microscopic, algae-like bacteria that inhabit freshwater, coastal and marine waters.

Cyanobacteria photosynthesise like plants and have similar requirements for sunlight, nutrients and carbon dioxide to grow and produce oxygen. There are many different varieties of blue-green algae. While often a green or blue-green colour, they can also be white, brown, blue, yellow-brown or red.

If conditions are suitable, blue-green algae can increase to excessive levels and form visible ‘blooms’ which can adversely affect water quality. Poor water quality and the potential for toxicity means that blue-green algae can cause environmental problems, disrupt drinking water supplies, recreational activities and water-dependent industries, and pose a risk to livestock, wildlife and human health.

Lake Burley Griffin, Australian Capital Territory
Black swan on a blue-green algae bloom on Lake Burley Griffin, Australian Capital Territory

Photo by Bruce Gray

Causes of blue-green algae blooms

Blue-green algae can reproduce quickly in favourable conditions where there is still or slow-flowing water, abundant sunlight and sufficient levels of nutrients, especially nitrogen and phosphorus. In still conditions, surface water may form a separate warm top layer (‘stratification’) in which blue-green algae is able to access sunlight and nutrients. If these combined factors are present for several days, algae multiply and form large ‘blooms’. The process of excess nutrients causing rapid growth of aquatic plant and bacterial life in a water body is known as ‘eutrophication’.

Nutrients are either naturally present in sediments or are washed into water systems. In particular, phosphorus may be stored in significant amounts in sediments and released by normal bacterial activity. External sources of nitrogen and phosphorus are agricultural fertilisers, household products, sewage effluent and stormwater runoff, all of which can enter receiving waters either directly or during rainfall events. The availability of varying levels of nitrogen and phosphorus can affect which species of blue-green algae dominate and form blooms.

Blooms can form in response to increased temperatures and phosphorus levels even if nitrogen in water remains low, as some blue-green algae species can obtain nitrogen from the atmosphere.

Effects of a blue-green algae bloom

The main effects of blue-green algae blooms are deterioration of water quality and production of toxins by some species. Exposure to algal toxins has been linked to fatalities of livestock, wildlife and pets.

Decaying algae can reduce dissolved oxygen levels in the water column which can severely degrade aquatic ecosystems and lead to the death of aquatic organisms and, hence, a decline in biodiversity.

Outbreaks of blue-green algae may have economic consequences from restrictions to the consumptive use of water and recreational activities due to health and aesthetic concerns. Some species of blue-green algae can produce neurotoxins, hepatotoxins, allergens or irritants to the skin and eyes, as well as compounds that affect the taste of water and produce unpleasant odours. In severe cases, the toxins can cause damage to the liver and nervous system and there have been human deaths associated with non-routine exposure to algal toxins through dialysis.

When a bloom is detected, alternative sources of water should be sought for human consumption and domestic purposes until specialised treatment processes can be introduced. Boiling water does not destroy algal toxins and can, in fact, release more toxins as the blue-green algae are killed. Irrigators are usually advised to avoid using contaminated water on edible crops or, if this is not possible, to avoid direct spraying.

Blue-green algae management response cycle
Blue-green algae management response cycle showing a continuous cycle: blue-green algae blook is detected alert is triggered public warnings are issued water and/or equipment are treated: chlorine, coagulation, filtration monitoring regime.

Management of blue-green algae blooms

Algal blooms are most effectively managed at the local level because of the varying circumstances in each case. Local councils and state water authorities are best placed to investigate suspected outbreaks and alert the public of any unsafe waters.

The most effective method of preventing blue-green algal blooms is to minimise the nutrient load entering waterways through actions such as planting or maintaining riparian vegetation, conserving soil, and implementing appropriate treatment and disposal of stormwater, agricultural, industrial and sewage effluent.

Flow manipulation can also be used to prevent the build-up of blue-green algae or to disperse blooms. In reservoirs used for drinking water supplies, various mechanical methods are often used to mix water and prevent conditions of stratification which could bring about blue-green algal blooms.

Comprehensive monitoring programs and emergency plans can reduce impacts when blooms do occur.

Larger water supply authorities generally conduct regular water sampling which allows the public to be quickly notified of any potentially harmful blooms and to obtain drinking water from alternative sources. Unfortunately, there are currently few ways of dispersing blooms without adverse side-effects, and toxins can remain in the water column even when algae are not visible.

Copper-based algicides may be effective in controlling blue-green algae if administered appropriately. Other solutions range from avoiding the extraction of water near blooms, to treating water with chlorine, although residual toxins may need to be removed with a combination of coagulation and filtration.

Australian Government action on blue-green algae

The Australian Government cooperates with state and territory agencies to provide policy guidance on controlling blue-green algal outbreaks.

The National Water Quality Management Strategy is endorsed by all states and territories and provides guidance on blue-green algae monitoring actions, alert systems and treatment options, and general drinking water guidelines.
The Australian Government indirectly helps control blue-green algae with strategies to minimise nutrients entering waterways, investing in effective stormwater management and delivering water back into the environment.

In the Murray-Darling river system the Murray-Darling Basin Authority is working closely with the relevant states and territories to develop a Basin Plan which will return more natural flow regimes to the system.

As part of Murray-Darling Basin reforms, the Australian Government is acquiring water entitlements with the objective of returning more water to the environment. These entitlements become part of the Commonwealth environmental water holdings and are managed so that increased flows are provided to rivers and wetlands.

When environmental water is released at appropriate volumes and times, it may help reduce the likelihood and extent of algal blooms. It is especially important to maintain water flows in summer months when conditions are most favourable for blue-green algae, as moving water can help prevent or disperse blooms.


Bacteria: single-celled micro-organisms. Most are harmless and very important to the environment.
Dissolved oxygen: concentration of oxygen dissolved in water.
Ecosystem: a specific composition of animals and plants which interact with one another and their environment.
Eutrophication: the process where an accumulation of nutrients in water bodies leads to rapid growth of aquatic plants.
Nitrogen and phosphorus: chemical nutrients essential for growth and added to many fertilisers.
Stratification: the formation of separate water layers.

Further information