Minimising algal growth in farm dams

Periods of low rainfall are common in the Australian environment.

Warm weather and low surface flows during such times can provide ideal conditions to stimulate excessive growth of algae both in water storages and in slow moving streams. These growths result in coloured masses of algae in the upper parts of the water and are commonly called 'blooms'.

Farm dams are quite susceptible to algal blooms.

Managers can find it difficult to recover dams from algae for the following reasons:

  • Flushing flows are generally not available to clear algae from the dam.
  • It is technically difficult to deal with algae and the associated issues like taste and toxins if dams become contaminated.
  • Treatment of contaminated dams may in itself cause another form of contamination.

It is better to prevent the build-up of predisposing conditions rather than trying to reclaim a contaminated dam.

Excessive algal growth

The general problems caused by excessive algal growth are:

  • unacceptable taste, odour and appearance of water used for the house and garden
  • clogging of filters, meters, valves and trickle irrigation lines
  • toxin release by some (blue-green algae in particular) may poison animals
  • corrosion of metal tanks, pipes
  • production of unsightly slimes on channels, tanks and troughs, and scums on the surface of dams
  • deoxygenation of water resulting from the decay of algae causing damage and death to fish and invertebrates
  • it is not always possible to draw off uncontaminated water from beneath the bloom (as can be done in large reservoirs).

What are algae?

Algae are a diverse group of water-living primitive plants. They multiply either through vegetative propagation or through the production of spores. They may be individual cells, single chains of cells or branching chains of cells.

Algae are a normal part of the aquatic ecosystem and require sunlight, water, carbon dioxide, and nutrients (including phosphorous and nitrogen) to grow.

They occur in most natural waters and in the sea. Freshwater algae reproduce in a wide range of temperatures. They can multiply rapidly at moderately high temperatures if there is ample nutrient in the water.

Types of algae

Freshwater algae are divided into 4 main groups. A microscope is usually necessary to be sure of identification.


Flagellates are of many shapes, sizes and colours and are capable of independent movement by means of flagella (whip like tails). In large numbers they are often responsible for the tastes and odours in drinking water.

Green algae

Green algae are of various shapes and sizes. They are recognised by the long, green ribbons often seen in rivers and channels, on the sides of tanks and drinking troughs, and in thick tangled masses in low-lying swampy areas.


Diatoms are microscopic and unicellular. They may exist as single cells or clump into filaments or colonies. They may be seen attached to the stems of other plants coating them with a brown slime.

Some members of this family are planktonic – they are suspended in the water rather than attached to anything. They are not usually a problem in farm water supplies.

Blue-green algae

Blue-green algae occur as single filaments or as clumps. They are capable of very sudden, explosive blooms and can produce thick scums on the down-wind surface of a water body.

These scums have been described as being like a suspension of greenish paint, or curdled greenish milk.

Colour can range from pale green through blue green, dark green to brown. There is frequently a foul odour which has been compared to either pig sties or raw sewerage.

Some species of blue-green algae can produce harmful toxins and be a serious management problem. If such toxins contaminate a water supply both livestock and wildlife can fall ill, or die. If a blue-green algae outbreak is suspected, do not use the water.

See Blue-green algae and irrigation water for more information about what to look for and how to treat algae affected water.

Conditions favouring algal blooms

Conditions that encourage the growth of algae in water are:

  • high levels of nutrients (nitrogen, carbonates and particularly phosphorus)
  • warm temperatures
  • still shallow water, or a dam where the surface water is strongly stratified relative to the lower waters (no mixing)
  • high levels of organic matter
  • direct exposure to sunlight.

Nutrient availability to algae is regarded as a major influence on blooms. An increase in availability of nutrients may be caused by:

  • nutrients concentrated in runoff water (from over-use of fertiliser or from nutrients attached to eroded soil particles)
  • evaporation from storage causing concentration of nutrients in remaining water
  • disturbance of dam sediments causing the release of stored nutrients
  • contamination with animal dung and other organic materials.

Prevention principles

There are measures that can be taken to minimise algal blooms. These include:

  • minimise nutrient level of water flowing into the dam
  • manipulate the concentration or cycling of nutrients within the dam water. Encourage diluting and flushing during wetter months.
  • minimise sunlight load on the dam
  • minimise temperature of dam water
  • encourage gentle mixing of dam water to avoid stratification of water into discrete temperature layers
  • prevent stock physically disturbing dam sides and sediments, or defecating and urinating in the water.

Planning and management practices

Land management

Keep a comprehensive vegetative cover on the catchment area:

  • maintain a vigorous cover of plants of reasonable height (ideally above 5mm)
  • avoid acidification, soil structural damage or fertility, which will reduce vegetative growth
  • prevent the baring of soil as the result of overgrazing or preferential grazing
  • minimise the time cultivated soils remain bare and time operations to enable rapid revegetation.

Control soil erosion:

  • repair and reclaim eroding areas
  • keep soil disturbance operations (roading, building sites, drains) to a minimum
  • use judicious design, construction and maintenance of tracks and roads
  • avoid cultivation of steep slopes.

Use strategic fertiliser programs:

  • match fertiliser application soil and plant needs
  • apply fertiliser at times of strongest plant growth
  • do not spread close to drainage lines or dams.

Locate and manage intensive animal operations to carefully control waste production and runoff:

  • minimise the production and concentration of animal wastes
  • recycle wastes (extensively) onto pastures or appropriate crops
  • use storage ponds of adequate capacity to hold liquid wastes before recycling
  • keep intensive operations away from streams and drainage lines.

Drainage line management

  • Keep drainage lines well vegetated. Dense grassy swards are suitable for drainage lines in the upper part of catchments but tree based vegetation is needed around streams and waterways.
  • Employ properly constructed drainage line crossings for stock and vehicle movement.
  • Restrict stock access to beds and banks of streams. Fence as appropriate.
  • Provide off-stream watering points.
  • Do not allow roads, tracks or intensively cropped areas to drain directly into streamside areas or natural drainage lines.

Dam design and construction

  • Use large deep dams to give lower water temperatures and reduce the intensity of temperature stratification of the water.
  • Locate and size dams to encourage the regular dilution and flushing of stored water from catchment flows.
  • Locate dam to minimise sunlight load where possible.
  • Construct water entry path into excavation of the dam such that it does not cause excessive turbulence and scour.

Dam management

  • Employ buffer or filter zones of thick vegetation immediately upstream of the dam to minimise entry of sediment and nutrients.
  • Fence dam and reticulate water to troughs.
  • Minimise erosion caused by wave action, but encourage a reasonable amount of wind action over the surface of the water to mix surface layers of water with lower layers. Do not allow surrounding vegetation to block too much of the wind.
  • Occasionally de-sludge dam to avoid excessive build up of sediments and organic material.
  • Minimise disturbance of bottom sediments by stock (including geese and ducks). The addition of filter alum (aluminium sulphate) to dam waters at the beginning of the summer season is another option for helping keep sediments on the bottom.
  • Grow trees to give some shade to the dam. Do not plant trees on the embankment.

If all else fails

Even with the best planning and good management practices, there is still a chance of getting an algal bloom.

If an algal bloom does occur there are a few essential steps:

  1. Stop using the water and use an alternative uncontaminated source. Only return to using the original supply if testing indicates it is safe to do so.
  2. Get professional identification of the algae and advice on particular action needed.
  3. Analyse why the bloom occurred and plan actions to avoid future recurrence.

Treating with chemicals

Chemical treatments:

  • may have side effects such as killing aquatic life that feed and control the levels of beneficial algae
  • will cause plants to rot, which in turn will cause a sudden decrease in oxygen levels in the dam that will affect aquatic life.

Some blue-green algae produce toxins which can persist in the water following treatment of the algae. These toxins can be poisonous to animals, humans and aquatic life.

There are registered chemicals to kill blue-green algae but chemical treatment should not be attempted unless absolutely necessary and after seeking professional advice.

Whenever using any chemicals, make sure the directions on the label are followed and all requirements for withholding periods are observed.

Page last updated: 27 Dec 2023