Nelson City Council

Take care when swimming or walking dogs near rivers

What are blue-green algae?

 Blue-green algae (or cyanobacteria) are microscopic organisms that are naturally present in many New Zealand waters, including relatively ‘clean’ waters. Like other algae, growth of blue-green algae is encouraged when river flows are low and stable and temperatures consistently warm.

 What do they look like?

Blue-green algal mats are actually dark brown or black and grow attached to rocks on the river bed. Mats that come loose from the river bed can wash up on the river bank or form floating ‘rafts’ in shallow areas. Where exposed, the mats may dry out and turn a light brown or white colour. They may also produce a strong musty odour.Blue-green algae differ from harmless bright green algae, which often form long filaments.Mats of light brown or olive green algae are also harmless.

 What is the problem with blue-green algae?

Some algal mats produce toxins that can be harmful to humans, dogs, livestock and wildlife. Several dogs have died in Wellington, Nelson and Canterbury after ingesting algal mats.

 What are the possible health effects?

Swallowing water containing blue-green algae toxins can lead to vomiting, diarrhoea, abdominal pain, cramps, nausea and other effects in humans. Skin contact can cause irritation of the skin, eyes, nose and mouth. Exposure to high levels of toxins can result in serious illness or death. Dogs are particularly susceptible to poisoning from blue-green algae as they love to scavenge and play near water. Livestock are also at risk from poisoning.

 What should I do if I find blue-green algae?

Rivers users, particularly those with dogs, or those taking water for livestock or human consumption, should avoid contact with any thick dark brown-black algal mats (see photo), particularly those that are easily accessible, exposed on river edges or floating in shallow areas of riverbanks or near rocks. If blue-green algal mats are widespread in a river you should presume that the water may be unsafe for bathing or drinking.

 Who should I call if I experience a reaction?

If you think you are experiencing a serious reaction, seek urgent medical attention. Advise your doctor of your potential exposure to toxic algae. Your GP has been asked to notify Public Health of any people with possible reactions.

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

If you are concerned about your animal(s), contact a vet immediately (the toxins can affect dogs within minutes). You or your vet can report any animal illness resulting from contact with the blue-green algae to the council.

Who should I call if I think I’ve seen toxic blue-green algal mats?

If you see thick blue-green algal mats, please contact Nelson City Council on 546 0200.

Dowload this information - including photos - as a pdf. (1.9MB PDF)

Don't swim after heavy rain

Contamination of river water can occur during or shortly after rainfall from storm water runoff. To minimise the risk of exposure to harmful bacteria it is best not to swim for the first 24 hours at least after rainfall that leads to any discolouration of the water.