Media release

Potentially toxic algae confirmed in Cardrona River

Wednesday 25 October 2017

The presence of the potentially toxic algae Phormidium has been confirmed in the Cardrona River, prompting a warning from the Otago Regional Council for people (and dogs) to avoid contact with waters in the river.

ORC has found the naturally-occurring algae Phormidium at the Regional Council’s Mt Barker sampling site on the river.

Signs have been put up at popular locations on the river today to warn the public of the risks associated with the algae.

ORC environmental resource scientist Rachel Ozanne said high river flows during the winter kept the region’s rivers relatively free of Phormidium.

However, warmer temperatures since then and the resulting stable flows meant Phormidium has the opportunity to flourish, certainly in the Cardrona.

Where Phormidium is obvious in a river, people should assume the water is unsafe for their animals to swim in and exercise caution, Ms Ozanne said.

In flowing rivers, Phormidium forms thick dark brown or black mats typically found on large rocks, stones, and cobbles. 

“It’s important that people using waterways are aware of the risks associated with its possible presence, and are alert to this,” Ms Ozanne said.

Some cyanobacteria produce toxins that are a possible health risk to humans and animals if eaten, and they can also cause irritation to the skin and eyes. Phormidium mats can indicate that other cyanobacteria posing a risk to human health may also be present. There have been cases in New Zealand of dog deaths associated with toxic algae where dogs have eaten the mats formed by Phormidium, having been attracted by their deep earthy odour.

The mats can detach from the river bed and accumulate along the water’s edge where it is readily accessible to dogs. Dogs seem to like the smell of Phormidium and, should the material be toxic at the time it is ingested, severe poisoning and often death can result in dogs that are particularly sensitive to the toxins.

“If the bed of a river is covered in thick dark brown or black mats that have a velvety texture and a musty smell, it is wise to be cautious and avoid that river site,” Ms Ozanne said.

Pets should be taken to a vet immediately if they show signs of illness after coming into contact with algal mats. The greatest risk is to dogs, which are more likely to consume the mats in the water.

“The most common signs that a dog might have consumed toxic algal material are lethargy, muscle tremors, fast breathing, salivation, twitching, paralysis and uncontrolled shaking, convulsions, or frothing at the mouth soon after being in the water. Anyone concerned their pet may have consumed toxic algae should contact their vet immediately.”

People are advised to avoid contact with waterways where the mats are present. If anyone becomes unwell after contact with affected waterways, they are advised to see their doctor.

For more information contact:

Dean Olsen
Manager Resource Science
Ph 03 470 7408 or 027 602 2737

Southern District Health Board (public health issues):
Moira Finn
Senior Communications Advisor
Ph 03 476 9830 or 027 836 4718

A large mat of Phormidium in the Lindis River