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It's not as simple as blaming farmers for the potentially toxic algal bloom returning to waters in Otago, baffled Otago Regional Council staff say.
The council warned the public last week to avoid contact with, and keep their dogs away from, Lake Waihola and Tomahawk Lagoon after discovering the algal bloom.
Otago Regional Council director of environmental information and science John Threlfall said he had taken samples of the blue-green algal bloom on Monday, but was waiting on the toxicity results.
However, the algal bloom could change quickly from being toxic to non-toxic, or vice-versa.
There was no correlation between the increase in the blue/green algal bloom and the increase in nutrient-use by farmers, Mr Threlfall said.
''It's not that simple."
The native algae had been found in waterways far away from farming before and the blue/green algal bloom was last seen in Lake Waihola in 2002.
The conditions at Lake Waihola had not changed since last year so council staff were unsure why it had returned this year, he said.
''We don't know why it blooms."