Phormidium algae, also known as Microcoleus, has been detected in the stream through council monitoring.
The first report of Phormidium from the council last spring came one day earlier, on November 9 last year, when the council issued a similar warning for the stream.
Council water quality scientist Rachel Ozanne said today that dog owners should learn to identify the river-borne algae and keep their animals away from it.
If eaten, the algae could induce severe poisoning or death, she warned.
Symptoms included lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.
If symptoms were evident, owners should treat it as an emergency and contact a vet immediately.
The musty smell of Phormidium was attractive to dogs, she said.
The algae appeared on riverbed rocks as thick, dark brown mats, which could resemble black tar. Mats could easily detach from the riverbed and wash up on riverbanks, she said.
The algae generally grew at this time of year because of lower flows, rising water temperature and sunlight hours increasing, Ms Ozanne said.
There was no known treatment to rid rivers of the natural algae, she said.
She said she expected the Phormidium would likely be around all summer, and that high rivers flows would dislodge blooms at times.
Phormidium had been detected at Riccarton Rd, but in previous years there had been pockets of it present along the entire length of Silver Stream, Ms Ozanne said.
The toxic algae blooms in many of Otago’s rivers and lakes during the warmer months, and there were “hot spots” with permanent signage, at Ophir, for the Manuherekia River and the Waianakarua River, at State Highway 1, she said.
There had not yet been reports of it in other Otago catchments.
However, she said with water warming up Phormidium was likely to start blooming in North Otago waterways - particularly the Waianakarua River and in Central Otago and in the Cardrona and Manuherekia Rivers.
Ms Ozanne said Phormidium was not becoming more prolific and had always been present, but its reporting has become more widespread as people became more familiar with it.
