Blue-green algae problems in dams
Published: April, 1999
Reviewed: August 2013
What are the signs to look for?
- A sudden change in water colour overnight due to a mass of vivid green algae floating to the surface.
- the formation of scum which looks like green acrylic paint and leaves 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.
- scums can be green, blue-green or khaki green, and can turn brown/green or white once it is dying off.
- scums may appear at dusk or dawn and disappear during the day.
- there may be 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.
What is blue-green algae and what conditions favour its growth?
Blue-green algae is the common name for a group of algae which have similar characteristics. When they encounter the right combination of conditions (such as still water that is clear enough for light penetration, the right temperature range (warm) and adequate nutrients (phosphorous and nitrogen) in solution in the water) they can multiply rapidly. When the mass of algae float to the surface, a vivid green 'bloom' can appear overnight.
Why are blue-green algae a problem?
Blue-green algae in large numbers or 'blooms' can seriously reduce water quality by producing odours, thick scums and of particular concern are some species of blue-green algae that can produce toxins which are poisonous to humans and livestock. When algae decompose they may use up oxygen in water and cause fish to die.
Not all blooms are toxic but they should be treated as toxic until the water has been tested.
What are the effects on human and livestock health?
Some species of blue-green algae produce toxins which can have serious health implications for humans, animals and birds drinking or contacting the water. Although humans are less likely to drink water heavily polluted with blue-green algae, symptoms seen in those who have include gastroenteritis, diarrhoea and vomiting.
Water affected with blue-green algae usually smells and tastes so unpleasant that people are unlikely to drink it, but should nevertheless take care to avoid skin contact. However, in areas where there are no alternative sources of fresh water (such as along the River Murray), steps have to be taken (e.g special carbon filtration) to make the water safe for drinking.
Avoid skin contact
Contact with contaminated water can cause:
- skin irritations and rashes
- swollen lips
- eye and ear irritation
- sore throat
- hay fever symptoms and asthma
Livestock are at real risk of poisoning by blue-green algae unless alternative drinking water supplies are provided, although livestock deaths are relatively rare. In extreme cases death can occur minutes after drinking.
Of far more serious concern to farmers are the productivity losses associated with milder cases of algal poisoning which in dairy cattle has been shown to cause loss of appetite, and consequently a decline in milk yield. But, if swallowed by livestock in sufficient quantities, blue-green algae may cause convulsions, paralysis, liver damage and skin sensitisation.
What can be done about an algal bloom?
Inspect farm dams and water troughs regularly (2 or 3 times a week) during hot, dry times of year.
If you suspect you have a blue-green algae bloom:
- Isolate all people and stock from the dam or water supply
- Ensure stock have alternative water supplies
- If no alternative water supplies are available contact your local Water Authority. See below for information on water treatment
- Collect water for testing (follow sampling instructions below)
- Contact a veterinarian if livestock show symptoms of poisoning
- Send sample for testing (see below) as soon as possible
- For diverters, contact the manager of the water body from which the water is diverted.
- Drink or swim in it.
- Rinse fruit and vegetables or cook with it
- Wash clothes in it if other sources of water are available. If not, use rubber gloves to avoid skin contact
- Eat shellfish (mussels, snails, yabbies etc) or fish caught in affected water
- Spray or flood irrigate pastures, crops, vegetables, grapes or fruit with it.
Please note that boiling algal-water will not make it safe.
Contact the Customer Service Centre 136 186.