The recent warm dry weather in Canterbury is prompting warnings about rivers developing potentially toxic algal growths.

Environment Canterbury's freshwater sampling field staff have noticed algal mats likely to be Phormidium in South Canterbury's Pareora River and downstream reaches of the Opihi River, also North Canterbury's Ashley River/Rakahuri, in recent days. People living at Pareora huts have been spoken to by Environment Canterbury staff.

This year, following new Ministry for the Environment guidelines, there will be additional freshwater monitoring for Phormidium. Last summer this was linked with the death of a dog at a North Canterbury river, the Ashley/Rakahuri. Dogs at the Selwyn River were also reported as becoming seriously ill through licking the blackish-brown algal mats on river rocks.

Staff taking freshwater samples as part of the summer swimming water quality programme will be keeping an eye out for algal mats. If mats are present, samples may be sent to Cawthron Institute in Nelson for toxicity testing.

Dog owners should keep their animals on a leash in any areas where they can see the blackish/brown algal mats and report any sightings and locations to the regional council in Timaru or Christchurch.

Phormidium is a naturally-occurring, freshwater, brown-black algae. Although district or city councils may place signs warning of Phormidium, these may not be seen at the numerous river access points, hence the need for people to treat every low-flowing river cautiously.

The algae forms dense dark brown/black mats on river stones. When the mats break away, particularly after a downpour, they accumulate at the river edge. Contact by skin or swallowing can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset, and other effects in humans.

Fishermen and boat users should exercise caution. For further information or in the case of illness or suspected illness, seek advice from your doctor. If animals become sick, contact your veterinarian.