Blue-green algae turns river toxic

Keep away from the Waimea River - it's infested with a toxic algae which can be fatal to dogs - and people.

The Tasman District Council has now put up warning signs and is advising everyone to avoid contact with the water in the river.

This follows the death of a dog and other cases of dogs becoming ill with vomiting and diarrhoea.

The blue-green algae, or cyanobacteria, forms thick, dark-brown mats in the river.

Cawthron research scientist Susie Wood went to look at the algae in the river up from the Appleby bridge and said: "It's the worst I have seen."

While so far it is dogs that have been affected, she warned it could be just as dangerous to humans, causing paralysis and even death.

She and others have been researching the cyanobacteria for five years to learn more and set up guidelines.

The TDC is now issuing warnings and monitoring other major rivers in the district.

The council said most cyanobacteria would be washed away with the next substantial rainfall event.

"If you are concerned about your animals, you should contact a veterinarian immediately. You or your vet can report any animal illness resulting from contact with the blue-green algae to the council," a council spokesman said.

Dr Wood said the algae grew on rocks on the river bed but on the Waimea River had become detached along the river edge. "That's really dangerous," she said. "There's so much mat there's kilograms of toxins." It had a distinctive musty, earthy smell which dogs were attracted to.

If they ingested the toxins their muscles contracted causing respiratory arrest. Symptoms included paralysis, frothing and vomiting.

"Fortunately we've never had a case of human poisoning but this river poses a high risk."

Even splashes could cause skin irritation, said Dr Wood.

Children playing in the river would be at risk. People should avoid drinking the water, swimming or other recreational activity causing splashing.

The increase in algae had been caused by stable weather and increased nitrogen.

"We've had three to four weeks of low rainfall, the river flow is more stable and allows the algae mats to become well developed. The reason we are getting the algae mats is we have higher nutrients from nitrogen."

Dr Wood declined to say whether the higher nitrogen was from runoff.

"We don't know that, we have never worked on that river," she said. "We would need to do some research on that river."

It would be up to the council to decide if it was necessary, she said.

Nelson City Council put up warning signs to dog owners along parts of the Maitai River a month ago.

Dr Wood said the scientists' research stemmed from dog deaths from the Hutt River, to get a better understanding of the toxic algae formation, how important the river flow and nutrients were, to come up with guidelines on collecting and interpreting samples, and issuing warnings.

Their research was progressing to predict which rivers could be at risk so councils and other organisations could be proactive, she said.

The Nelson Mail