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Lake Clearwater isn't living up to its name, this week health officials have issued a warning for "Te Puna a Taka" in the Ashburton Lakes Basin on Monday, after the discovery of toxic blue-green algae.
The Canterbury DHB says algal blooms are caused by a combination of nutrients in the water, such as nitrogen and phosphorus, and after warm and calm weather.
Canterbury's Medical Officer of Health, Dr Ramon Pink, says the algal bloom can produce toxins harmful to humans and animals.
And people should avoid contact with the water until further notice.
Forest and Bird regional conservation manager Debs Martin says people used to be able to see down to the very bottom of the lake.
She told reporter Anan Zaki that water quality standards in the region aren't being met.