Media release

Media release: Stay clear - toxic Cyanobacteria in Butchers Dam

Wednesday 16 November 2022

The potentially toxic Cyanobacteria has been found in Butchers Dam in Central Otago, prompting the Otago Regional Council to advise people to stay out of the dam and keep dogs well away from the water and dam edges.

Cyanobacteria, a blue-green algae (Anabaena lemmermannii), can potentially produce toxins which are harmful to humans and pets if swallowed, or through contact with skin, says ORC’s Water Quality Scientist Rachel Ozanne.

Butchers Dam Aerial View


The warning to keep clear of the dam will remain in place until weekly testing shows that the cyanobacteria is no longer a health risk, she says.

Algae in lakes and dams often begin blooming at this time of year from a combination of rising water temperatures, the increase of sunlight hours and nutrients in the waterways.

Ms Ozanne says a member of the public alerted ORC that an unusual algae was present at Butchers Dam, and subsequent testing identified it was cyanobacteria.

“What’s happened at Butchers Dam is an indication that other lakes are likely to experience algae blooms too and the public should inform themselves of what to look for,” she says.

Planktonic cyanobacterial blooms are generally green in colour and can give lakes a “pea soup” appearance.

“They can also form visible green to red films or scums on the water’s surface, especially at the water’s edge,” she says.


Can persist beyond summer

If the presence of likely cyanobacteria cells is reported, ORC takes a composite water sample and that material is analysed for cyanobacteria identification and cell counts, says Ms Ozanne.

“Cynobacteria can be present throughout summer, but some persistent blooms will hang around through winter too,” she says.

Other “hot spots” for cynobacteria around Otago incluce Lake Hayes and Waihola, Falls Dam, Tomahawk Lagoon in Dunedin and some other reservoirs.

There have been instances in the past where dogs, birds, cattle and other wildlife have died from exposure to cynobacteria. Symptoms can include lethargy, tremors, paralysis, foaming at the mouth or vomiting.

“Affected animals should be taken to a vet immediately,” she says.


What to do

Avoid contact with water in Butchers Dam, even parts which look clear, as toxins can persist after an algal bloom has dissipated or moved.

Anyone who suspects they are experiencing a reaction due to contact with toxic algae should seek medical attention.

If you spot green algae in lakes, or dark mats in streams and rivers, contact ORC on our 24/7 pollution hotline, 0800 800 033.

For more information on potentially toxic algae, see LAWA’s factsheet


Phormidium algae in Silver Stream

Last week dog owners were warned to keep their canines leashed to prevent them ingesting potentially lethal Phormidium algae, which occurs in many Otago rivers and streams during summer as water temperatures rise.

Phormidium, also known as Microcoleus, was again, last week, identified as present at one of the ORC’s water monitoring sites at Silver Stream near Mosgiel.

Phormidium will likely be around in parts of Otago all summer, and high rivers flows will dislodge blooms at times, and wash them onto banks, where dogs are attracted to the musty smell.