Hutt at Heart
Mats of Toxic Algae on rocks in the Hutt River

What is toxic algae and why does it grow in our river?

It’s still unsafe to swim in the Hutt River due to the risk posed by toxic algae. But what is toxic algae and why does it grow in our river?

Toxic algae, also known as cyanobacteria, are an ancient group of bacteria that have been around for over three billion years. They occur in almost every environment and are common in rivers and lakes in New Zealand.

Long periods of warm, dry weather and less ‘flushing flows’ (caused by rain) can cause the algae to bloom. It doesn’t necessarily happen every summer and it doesn’t always create a public health risk – unfortunately this year, we’re just unlucky.

Toxic algae loves the Hutt River. It grows best on gravel beds; it’s not disturbed by the lack of flushing river flows and, especially at the moment, it’s enjoying making nutrients from the beautiful summer weather.

Toxic algae generally forms brown or black mats that grow on rocks in the river bed. When the mats come loose they can wash up on the riverbank or form floating ‘rafts’. If they are exposed to air, the mats dry out, turn a light brown or white colour and produce a strong musty odour. Dogs love the smell and even a small amount, the size of a 50 cent piece, can be fatal to people and dogs if swallowed.

So if you pop down to the Hutt River, stay away from the water’s edge and leave your pups at home! And if you’d like frequent updates, check out our Facebook page: facebook.com/huttcitycouncil

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Subscribe to Hutt at Heart – Te Manawa, your monthly email newsletter where you can find out about fantastic things going on across our city and at Hutt City Council.