Blue-green algae (also known as Phormidium) occurs naturally in waterways throughout Otago and was more likely to be present during the summer months than at any other time of the year, Otago Regional Council director of environmental information and science, John Threlfall, said.
This was because of favourable environmental conditions, including warm temperatures, more sunlight, and low, stable river flows, he said.
Last summer Phormidium was confirmed in the Silver Stream near Mosgiel and in several North Otago rivers.
Phormidium was typically black and shiny, and often stuck to rocks or attached itself to stream debris and weeds.
It could produce toxins which might prove fatal to dogs.
In flowing rivers, Phormidium formed thick dark brown or black mats typically found on stable substrate such as large rocks, stones and cobbles.
The mats might separate from the riverbed and float downstream to become caught up in other debris in the river.
When the Phormidium mats die and dry out they bleach to light brown or white.
Brightly coloured long green and brown algae commonly found in rivers and streams were harmless and did not produce toxins, Dr Threlfall said.
In Otago toxic algal blooms in lakes did not occur regularly.
The last documented bloom was in Lake Waihola in 2002.
There were several species of blue-green algae that might or might not be toxic, depending upon prevailing environmental conditions, he said.
"However, if toxic algae are obvious in the river or lake, people should presume that the water may be unsafe for contact recreation or consumption."
Otago-Southland medical officer of health Dr Marion Poore said people swimming in water with increased levels of algal bloom have been known to develop allergic reactions - asthma, eye irritations, rashes, blistering around the mouth and nose, and gastro-intestinal disorders.
Other river users, including fishermen and boat users, should also exercise caution.
Dogs were particularly susceptible to poisoning.
Livestock were also at risk of poisoning from cytotoxins, and should be provided with alternative drinking water.
Symptoms of poisoning in animals exposed to the type of toxins present in Phormidium mats included lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.
In extreme cases, death could occur within 30 minutes.
"In the case of illness or suspected illness after swallowing water containing algal bloom, seek advice from your doctor. If your animals become sick, contact your veterinarian."
