Warning on potentially toxic algae

Low water flows, high nutrient levels and warm weather combine to encourage growth of phormidium.

Low water flows, high nutrient levels and warm weather combine to encourage growth of phormidium.

ALGAE ALERT: Gisborne District Council is asking people to report any sightings of the algae phormidium. Picture supplied

AN ALGAE that can kill dogs and make people sick could be present in some of the district’s waterways, and residents are asked to stay alert.

Over summer, because of lower water flows, higher temperatures and an increased nutrient load, rivers tend to grow more algae, including the naturally occurring phormidium.

The algae is widespread throughout lakes and rivers around New Zealand, including those with very good water quality, said Gisborne District Council’s acting environmental and regulatory group manager Lois Easton.

In this district, it has so far only been detected in areas along the Motu River.

“We found phormidium along the length of the Motu River last year,” Ms Easton said.

They detected it at various sites along the river this year as well, but the rain on Wednesday washed it out. It could return if dry conditions return.

“We are checking for blooms at monitoring sites, particularly in places where people swim and fish or where animals drink, because if it grows in large quantities it can pose a risk to health.”

Phormidium looks like a dark brown leathery mat that forms on rocks in stony riverbeds.

The algae is not always toxic but when it blooms in large quantities it can start to produce cyanotoxins harmful to dogs, livestock and people.

Dogs are attracted to its musty smell and it can be fatal if they eat it. Symptoms include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.

The algae can come loose and wash up on riverbanks and shallow areas where animals are more likely to eat it. Humans can also become ill if they come into contact with it or drink water with high concentrations of the algae.

With low flows in waterways the algae has a long time to grow because there are no flushing flows to scour rocks clear.

“In autumn to spring we will get periods of rain and higher flow (called freshes), which will scour the algae out of the river,” Ms Easton said.

“This happened last summer when we first found it at the Motu. We had several decent freshes, which reduced the cover of the phormidium and other algae.

“We want people to check out waterways, particularly on their own properties in places where people or animals regularly go, and let us know if they find the algae.”

Council staff will continue to monitor routine sites closely for any risk to public health.


AN ALGAE that can kill dogs and make people sick could be present in some of the district’s waterways, and residents are asked to stay alert.

Over summer, because of lower water flows, higher temperatures and an increased nutrient load, rivers tend to grow more algae, including the naturally occurring phormidium.

The algae is widespread throughout lakes and rivers around New Zealand, including those with very good water quality, said Gisborne District Council’s acting environmental and regulatory group manager Lois Easton.

In this district, it has so far only been detected in areas along the Motu River.

“We found phormidium along the length of the Motu River last year,” Ms Easton said.

They detected it at various sites along the river this year as well, but the rain on Wednesday washed it out. It could return if dry conditions return.

“We are checking for blooms at monitoring sites, particularly in places where people swim and fish or where animals drink, because if it grows in large quantities it can pose a risk to health.”

Phormidium looks like a dark brown leathery mat that forms on rocks in stony riverbeds.

The algae is not always toxic but when it blooms in large quantities it can start to produce cyanotoxins harmful to dogs, livestock and people.

Dogs are attracted to its musty smell and it can be fatal if they eat it. Symptoms include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.

The algae can come loose and wash up on riverbanks and shallow areas where animals are more likely to eat it. Humans can also become ill if they come into contact with it or drink water with high concentrations of the algae.

With low flows in waterways the algae has a long time to grow because there are no flushing flows to scour rocks clear.

“In autumn to spring we will get periods of rain and higher flow (called freshes), which will scour the algae out of the river,” Ms Easton said.

“This happened last summer when we first found it at the Motu. We had several decent freshes, which reduced the cover of the phormidium and other algae.

“We want people to check out waterways, particularly on their own properties in places where people or animals regularly go, and let us know if they find the algae.”

Council staff will continue to monitor routine sites closely for any risk to public health.


For phormidium findings contact the council on 0800-653-800 with the location and send a photo to service@gdc.govt.nz

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