A popular freshwater swimming spot in the Far North has been closed due to an algal bloom which can make people ill and is lethal to dogs.
Lake Rotopotaka, a dune lake on the Karikari Peninsula in Doubtless Bay, is a busy cooling-off spot in summer and a year-round tourist attraction due to the dark brown colour of the water.
Caused by naturally occurring tannins, the colour gives the lake its unofficial name, Coca Cola Lake.
Earlier this week, however, local kaitiaki (guardians) put up signs at the lake's Ramp Rd access points warning people to stay out of the water.
The reason is a bloom of cyanobacteria, an organism which can make people sick and can be fatal for dogs.
Kaitiaki o Tokerau Nina Raharuhi, who represents Haiti-tai-marangai Marae, said a kaitiaki ranger noticed the bloom's distinctive blue-green colour last week.
Water tests revealed 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.
In the meantime she urged people to stay out of the water and keep dogs away even from the lake shore.
Blooms normally occurred in summer so it was not clear what had triggered it.
Raharuhi said 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, sparked 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, Northland's largest lake, and Kauri Dam, a water reservoir near Kaitaia.