Potentially toxic algae in rivers

Benthic cyanobacteria Benthic cyanobacteria (better known as blue-green algae) are present in many lakes and rivers in New Zealand.

Not all species of cyanobacteria are toxic but under favourable conditions some species produce natural toxins. Toxin levels are normally low but if cyanobacteria are present in large numbers in the water it may be unsafe for contact and drinking and should be avoided.

In flowing waters, cyanobacteria can form thick growths that may appear as dark brown, green, black or even purple mats on stones.

The more brightly coloured green algae commonly found in rivers and streams are harmless.

Why are they a problem?

Some cyanobacteria species produce natural toxins that can harm humans and animals.

What are the possible health effects?

Contact with toxins can cause irritation of the skin, eyes, nose and mouth. Ingestion of toxins can lead to vomiting, diarrhoea, cramps, convulsions and in extreme cases paralysis.

Who should I call if I experience any symptoms of toxin exposure?

If you experience a serious reaction, seek medical attention urgently.

Who should I call if I think my animal is sick?

If you are concerned about your animal contact a veterinarian immediately and report any animal illness resulting from contact with the cyanobacteria to your local council.

Is it safe to drink water containing high levels of toxin-producing cyanobacteria?

No. Boiling the water or using normal filtration or disinfection systems will not remove the toxins.

Should I eat fish or shellfish from water with toxin-producing cyanobacteria?

No. Eating mussels and other shellfish from affected areas should be avoided as they can concentrate the cyanotoxins.

For further information

Contact Community and Public Health with queries relating to human health,
Ph 03 364 1777 for Christchurch
Ph 03 688 6019 for Timaru

Contact Environment Canterbury for further information on algae status in Hurunui Riversor or queries related to animal health,
0800 324 636