Otago Regional Council science team leader for water Helen Manly said people should keep dogs on a lead while walking along the stream, keep clear of the water and be cautious of washed-up algal mats.
The phormidium at the bridge was ‘‘very concentrated" and extended a few metres downstream.
Algal mats could become detached from the rocks and float along the waterway, so people should take a cautious approach anywhere along Silver Stream, she said.
‘‘We are erecting signs at access points to Silver Stream to advise dog owners of the risk.’’
Phormidium, which forms thick dark brown mats on rocks in the riverbed and can sometimes look like black tar, thrives in Otago rivers during summer - particularly the Cardrona, Hawea and Manuherekia rivers.
It had also previously been spotted in other parts of the region, such as the Waianakarua River and Silver Stream.
Any person or dog who may have had contact with toxic algae, should rinse off with fresh water as soon as possible.
Anyone experiencing a reaction from contact with toxic algae should seek urgent medical attention, and any signs a dog has been poisoned by toxic algae - including lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsion - should be treated as an emergency and referred to a vet immediately.
• People can report algal blooms in Otago lakes and rivers by contacting the pollution hotline on 0800 800 033.
