Media release

Alert to dog walkers as toxic algae reappears in Silver Stream

Thursday 10 November 2022

Dog owners are being warned to keep their canines leashed to prevent them ingesting potentially lethal Phormidium algae, which occurs in many Otago rivers and streams during summer.

Phormidium, also known as Microcoleus, has this week again been identified as present at one of the Otago Regional Council’s water monitoring sites at Silver Stream near Mosgiel.

ORC’s Water Quality Scientist Rachel Ozanne says it’s important for dog owners learn to identify toxic algae, in order to keep dogs away from it.

“It’s crucial that people can identify the toxic algae and not let their dogs near it. If eaten, Phormidium can induce severe poisoning or death,” she warns.

Phormidium is identified by its thick, dark brown mats on riverbed rocks in the riverbed, which can look like black tar. The Phormidium mats can easily detach from the riverbed and wash up on riverbanks.

Ms Ozanne says it is the musty smell of Phormidium which is attractive to dogs.

“Keep dogs on a lead and well clear of any Phormidium mats washed up,” she says.


Seek medical attention

The signs that a dog may have consumed Phormidium include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions and should be treated as an emergency and referred to a vet immediately, Ms Ozanne says.

“Any human experiencing a reaction from contact with toxic algae should seek urgent medical attention,” she says.

The algae generally growing at this time of year because of lower flows, rising water temperature and sunlight hours increasing. There is no known treatment to rid rivers of the natural algae, she says.


Blooming in rivers over summer

Ms Ozanne expects the Phormidium will likely be around all summer, and that high rivers flows will dislodge blooms at times.

It’s been recently noticed at Riccarton Road. But in previous years there’s been pockets of Phormidium present along the entire length of the Silverstream; which people need to be aware of,” Ms Ozanne says.

The toxic algae blooms in many of Otago’s rivers and lakes during the warmer months, and there are “hot spots” with permanent signage, at Ophir, for the Manuherekia River and the Waianakarua River, at State Highway 1.

There have not yet been reports of Phormidium in other Otago catchments, she says.

However, she says with water warming up Phormidium is likely to start blooming in North Otago waterways, particularly the Waianakarua and in Central Otago, particularly in the Cardrona and Manuherekia Rivers.

Ms Ozanne says Phormidium is not becoming more prolific and has always been present, but its reporting has become more widespread as people get more familiar with it.

To learn more about potentially toxic algae, including how to identify algal blooms in rivers and lakes, see the LAWA factsheet.

To report algal blooms in Otago’s lakes and rivers, contact us: 0800 474 082.