Environment Canterbury warns a toxic algae suspected of having caused the death of a dog last summer could be reoccurring in some rivers after recent warm, dry weather.
Environment Canterbury (ECan) field staff have noticed algal mats, likely to be phormidium, in South Canterbury's Pareora River and downstream reaches of the Opihi River, as well as North Canterbury's Ashley River, in recent days.
People living at Pareora huts have been spoken to by ECan staff.
ECan water-quality analyst for South and Mid-Canterbury, Graeme Clarke, said this year, following new Ministry for the Environment guidelines, there would be additional freshwater monitoring for phormidium.
Last summer, the toxic algae was linked with the death of a dog at the Ashley River.
Dogs at the Selwyn River were also reported to have become 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 the 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 ECan in Timaru or Christchurch, he said.
Phormidium is a naturally occurring, freshwater, brown-black algae.
Although district or city councils may put up warning signs, these may not be seen at the many river-access points, hence the need for people to treat every low-flowing river with caution.
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's edge.
Contact with the skin or through swallowing can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset and other effects in humans.
Fishermen and boat users should also exercise caution.
In the case of illness or suspected illness, advice should be sought from a doctor or veterinarian.
david.bruce@odt.co.nz