Swimmers are being warned to stay out of Lake Rotopotaka, known by many as Coca Cola Lake because of the colour of the water, because an algal bloom that could make them ill, and could be lethal to dogs.
Last week local kaitiaki erected signs at the dune lake's Ramp Rd access points, behind Tokerau Beach, warning people to stay out of the water.
Kaitiaki o Tokerau Nina Raharuhi, who represents Haititaimarangai Marae, said a kaitiaki ranger had first noticed the bloom's distinctive blue-green colour, water tests confirming the presence of cyanobacteria. The group was working with the Northland Regional Council, and would continue testing the water on a weekly basis.
It was not known how long the lake was likely to be off limits. The last bloom, in February 2020, cleared up in time for summer 2020-21.
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In the meantime Raharuhi urged people to stay out of the water and to keep dogs away, even from the shore.
Blooms normally occurred in summer, and it was not clear what had triggered this one, but faeces from Canada geese, which were present in the area in large numbers, could be a factor, as could the lake's current low water level.
She did not think a large fire two summers ago was likely to be a factor. The blaze, which began when a car hit a power pole, wiped out much of the vegetation around the lake.
Raharuhi said the lake was known for its medicinal properties, with its clay in particular used for healing.
Cyanobacteria occur naturally, and are found almost everywhere on Earth. At times, however, they can explode in numbers and become hazardous to people and aquatic life. Blooms are more common in summer in still, shallow water.
They can be triggered by prolonged high temperatures and pollution, such as farm run-off or bird faeces. Cyanobacteria are also called blue-green algae, though they are technically not an algae.
Blooms are also an ongoing problem at Lake Ōmāpere and Kaitaia's Kauri dam, which once provided the town's water.