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Swimmers and other rivers users have been warned to watch for toxic algae growths in Canterbury rivers following recent periods of warm weather and declining river flows.
Environment Canterbury (ECan) field staff have already noticed algal mats - likely to be phormidium - in the Opihi and Pareora Rivers in South Canterbury and lower Ashley/Rakahuri River in North 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," ECan surface water quality scientist David Kelly said this week.
The toxins produced during these conditions by river algae could be particularly dangerous to dogs or stock and in some cases could cause human health problems.
Regular blooms of the toxin-forming black algae, phormidium, in the Ashley/Rakahuri and Selwyn Rivers over the past few summers had been linked to dog illness and deaths.
The algae was found in rivers throughout New Zealand, but only formed problematic thick algal blooms in some sites under the right flow conditions, Dr Kelly said.
People were warned to look out for areas of rivers where black mats of algae were growing or forming scums, keep dogs out of the water and avoid swimming in heavily infested areas.
From mid-November, ECan would 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 were 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 councils may erect warning signs, these may not be seen at the numerous river access points, hence the need for people 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.