A toxin that can irritate skin and potentially kill dogs has been identified in three Manawatu rivers.
Caused by blue/green algae, cyanobacteria has been found in the Oroua River near Feilding, at Horseshoe Bend near Tokomaru and in the Mangatainoka River at State Highway 2.
Horizons water quality spokesman Barry Gilliland said it was not yet known whether this particular strain of cyanobacteria was toxic but preliminary testing was under way.
"The slimy, black algae can bloom very quickly in warmer weather and will appear as a shiny black mat on the rocks," Gilliland said.
Warning signs would be erected at the two spots and swimmers were advised to check for the algae before swimming in other areas.
"If you are concerned your dog may have eaten the algae it is recommended you contact your veterinarian immediately," he said.
Blue/green algae is present in small quantities in most of the region's rivers year round. In favourable conditions, such as during periods of warm water temperature and low flow, it can become a health risk at some sites.
On Thursday, MidCentral District Health Board health protection officer Peter Wood warned conditions were right for the algae to grow.
Horizons yesterday confirmed the algae at the three locations and was monitoring other swimming spots.
Wood said the algae often had a strong, musty smell.
"Swimming and contact with the water is not advisable when these mats are visible and covering more than half the river bed. Boiling water does not remove toxins produced by the cyanobacteria and drinking of the water should be avoided at all times."
- Manawatu Standard