The Otago Regional Council has warned people to avoid swimming in the river at the Ophir bridge and upstream of there, near Booths Rd, and advised dog owners to keep their animals out of the water, until the algae's toxicity drops to safe levels.
The algae - Phormidium- usually formed a thick brown-black slimy mat and was found on large rocks and stones. Mats of the algae had been detaching from rocks and floating downstream of the Ophir bridge towards Alexandra, council engineering, hazards and science director Gavin Palmer said yesterday.
When the mats dried out or died, they became light brown or white and were highly toxic either fresh or dried, and potentially harmful to people and animals, he said.
Dogs were particularly susceptible to poisoning from the algae.
''Dogs can eat these algae intentionally or by accident, when drinking from the river or retrieving sticks, with tragic results,'' Dr Palmer said.
Animals which had been poisoned from the type of toxins in Phormidium might display symptoms such as lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions. In extreme cases, death could occur within 30 minutes after signs first appeared.
If humans became ill or were suspected of being ill after swallowing water containing algal bloom, medical advice should be sought immediately, he said.
The same toxic algae was discovered in the Silver Stream, in Mosgiel, in 2010 and was believed to have contributed to the death of at least one dog.
