Managing blue-green algae in farm water supplies

The following signs can indicate blue-green algae in a water supply:

  • A sudden change in water colour overnight due to a mass of vivid green algae floating to the surface of the water.
  • The formation of scum — which often looks like green acrylic paint and may leave sky-blue marks on rocks or plants — around the edge of the dam, particularly on the leeward side of the dam or backwater of a stream.
  • The scum may be green, blue-green or khaki green, and can turn brown or green or white when dying off.
  • Scums may appear at dusk or dawn and disappear during the day.
  • Scums may produce a strong earthy smell, or if the bloom is breaking down, it may produce a strong rotting smell.
  • In the early stages of a 'bloom', small green flecks may appear in the water.

Why blue-green algae are a problem

Blue-green algae are a problem for these reasons:

  • Some species of blue-green algae can produce toxins that are poisonous to humans, livestock and other animals.
  • Blue-green algae in large numbers or 'blooms' can seriously reduce water quality, producing odours and thick scums on the surface of water supplies.
  • When algae decompose they may deplete oxygen in the water, causing fish to die.

Not all blooms are toxic but they should be treated as toxic until the water has been tested.

There are also a range of tools and resources, including an online farm water calculator, in the Farm water solutions section.

Effects on livestock

The effects of blue-green algae on livestock vary, depending on the strains of blue-green algae present and the level of toxin accessed by livestock.

If you suspect contamination, provide alternative drinking water supplies.

Contact a vet if livestock show signs of poisoning.

Monitor and test for algal bloom

Inspect farm dams and water troughs regularly (at least 2 or 3 times a week) during hot, dry times of year.

If you suspect you have a blue-green algae bloom:

  • isolate stock from the dam or water supply
  • ensure stock have alternative water supplies wherever possible
  • send samples for testing at a water quality testing laboratory as soon as possible (it's best to use a laboratory accredited by the National Association of Testing Authorities)

If no other water supply is available, where stock water is piped, foot valves or inlet pipes could be moved as far below the surface of the water source as possible, where concentrations of blue-green algae might be lower. This can reduce the amount of algae pumped into stock troughs. This alternative may continue to pose a potential risk to stock and they should be monitored very closely for signs of toxicity.

If no other water supply is available and stock are drinking from low-risk affected dams (determined by water testing), animals should be allowed to drink only from shore areas kept free (by prevailing winds) of dense surface scums of blue-green algae.

Monitor stock carefully for signs of ill-health, as there still might be a potential risk of exposure.

Managing water contaminated with blue-green algae

There are several options to manage algae in farm dams and farm storages:

  • Barley straw applied in plastic mesh bags at a rate of 100 grams over 1000 litres (1 cubic metre) of water can help break up algal rafts. It can take one month to start working but lasts for up to six months.
  • The use of an outboard motor or fire pump to circulate oxygen through the water supply (the farm dam) to aerate the water source and disperse the algal scum.
  • The selective withdrawal of water from different depths in a water source may minimise the intake of high surface accumulations (scum) of blue-green algae.
  • In addition to registered products (outlined below), Phoslock™ is available for use as a water conditioner, meaning that it reduces the amount of nutrients present in the water column. As blue green algae rely on these nutrients for growth, a secondary result of use as a water conditioner may be a reduction in algae.

Chemical treatment

Chemical water treatments that kill algae are an option but are generally not recommended as these treatments allow the release of pre-formed toxins into the water, potentially making the water even more toxic.

There are several agricultural chemicals registered for the control of blue-green algae in specific situations such as farm dams, tanks and troughs. The following copper products are specifically registered for use in Victoria:

  • Coptrol Aquatic Algicide
  • Cupricide Algicide
  • Cupricide 110 Algicide
  • Copcide Algicide

Search the Australian Pesticides and Veterinary Medicines Authority PubCRIS database for other registered chemicals.

Always read the label

The product labels for these chemicals contain a number of important instructions or directions that address the potential risks posed by the use of the products. Users must read, understand and follow the directions contained in the product label to minimise any risks.

For example, some products contain the following prohibitive label statements:

  • DO NOT treat drinking waters used by farm animals grazing on heliotrope or ragwort
  • DO NOT discharge treated water into rivers or lakes without the authority of the Environmental Protection Authority
  • DO NOT use on areas where aquatic birds are feeding

Only use products for registered use

It is important to note that the products are registered for limited situations such as use in farm dams, tanks and troughs.

Use outside of these situations can result in undesirable environmental effects for which the user may be held legally responsible.

Don't use water for 28 days after treatment

Any agent that kills blue-green algae will result in an initial increase in toxin levels as the algae die and more toxin is released into the water supply.

If an algicide is used, water should be monitored for toxin levels, or all animals should be excluded from the water supply for at least 28 days after its use (use as per manufacturer guidelines), because the water can remain toxic for that period.

For this reason, algicides do not necessarily provide a short-term solution to the lack of an alternate water supply.

Copper-based algicides

Copper-based algicides should be used with extreme caution due to the potential to cause copper toxicity in livestock. Animals that may have compromised liver function (for example, those that have been grazing plants such as heliotrope or ragwort) are particularly susceptible to copper toxicity and must not drink water that has been treated with copper-based algicides.

Off-label chemical use

Any person considering the off-label use of an agricultural chemical to control blue-green algae is responsible for the risks associated with off-label use and must ensure that their use complies with the requirements of the Agricultural and Veterinary Chemicals (Control of Use) Act 1992.

Other resources

Download the fact sheet:

Page last updated: 11 Jan 2021