Potentially toxic algae found in Cardrona River

Toxic algae warning sign on the Cardrona River (January 2017) Photo: Nikki Heath / Wanaka Sun

Otago Regional Council (ORC) has warned people and dogs to avoid contact with waters in Cardrona River after the presence of a potentially toxic algae has been confirmed.

ORC has found the naturally-occurring algae Phormidium at its Mount Barker sampling site on the river. Signs were put up at popular locations on the river on Wednesday 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, Rachel 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,” Rachel 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,” she said.

“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.

Toxic algae warning sign on the Cardrona River (January 2017) Photo: Nikki Heath / Wanaka Sun

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