Watch: Cardrona River check identifies toxic algae

The Otago Regional Council has issued a warning for people and dogs to avoid the Cardrona River following the appearance of a potentially toxic algae in the water.

ORC manager resource science Dean Olsen said a council staff member noticed the cyanobacteria phormidium at the council's Mt Barker sampling site on the river on Wednesday.

Signs have been erected at popular sites along the river, including the Ballantyne Rd bridge, the SH6 bridge, the end of Halliday Rd, the start of the Upper Clutha track and at the Albert Town footbridge.

Dr Olsen said phormidium was a naturally occurring algae and part of the stream eco-system but it could produce toxins that posed a possible health risk to humans and could be fatal to dogs if eaten.

Dr Olsen said the algae looked like a very dark brown or black velvety smooth mat that coated the surface of rocks or stones and had a musty or earthy smell.

He said dogs were attracted to the algae by the deep, earthy odour and there have been cases of dogs dying after consuming the mats.

The algae is normally kept in check by high river flows and colder temperatures but rising water temperatures, longer daylight hours and stable flows allowed the algae to flourish, Dr Olsen said.

He said dog owners should be aware the mats could detach from the bed of the river, flow downstream and fetch up on gravel at the side of the stream or be washed downstream to areas where dogs may come across them.

"The main message for the public is to be aware of what phormidium looks like and to take steps to keep your dog from being at risk of eating it."