Warm weather, low river flows may produce toxic algae growths

Recent periods of warm weather and declining river flows in Canterbury have prompted warnings about rivers developing potentially toxic algal growths. Environment Canterbury’s field staff have noticed algal mats - likely to be Phormidium - in the lower Ashley River/Rakahuri, North Canterbury, and the Opihi and Pareora rivers in South Canterbury.

The risk of toxic algal growths is likely to increase in the summer when a La Nina weather pattern is predicted to produce warm, stable temperatures with low rainfall.  

“The toxins produced during these warm conditions by river algae like Phormidium can be particularly dangerous to dogs or stock and in some cases can cause human health problems,” said Dr David Kelly, Environment Canterbury surface water quality scientist. 

Regular blooms of the toxin-forming black algae, Phormidium, in the Ashley/Rakahuri and Selwyn Rivers over the past few summers have been linked to dog illness and deaths.  “This algae is found in rivers throughout the country, but only forms problematic thick algal blooms in some sites under the right flow conditions,” said Dr Kelly. 

People are warned to look out for areas of rivers where black mats of algae are growing or forming scums, keep dogs out of these waters and avoid swimming in heavily infested areas. 

From mid-November, Environment Canterbury staff will be conducting regular sampling as part of the recreational health water quality programme to check for the presence of potentially toxic algal blooms. If mats are present, samples may be sent to the Cawthron Institute in Nelson for toxin testing.

Phormidium is a naturally-occurring, freshwater, brown-black algal species. Although district or city councils may place warning signs, these may not be seen at the numerous river access points, hence the need for people/ dog-walkers to treat every low-flowing river cautiously. Contact by skin or swallowing can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset, and other effects in humans.