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Published: 2011-01-11 00:00:00

A potentially toxic blue-green algal bloom has been detected in Lake Ohakuri, the largest lake in the Waikato River hydro system.

Environment Waikato’s regular testing of the lake’s water quality has revealed samples taken last week contained relatively high levels of blue-green algae cells.

The biovolume of blue-green algae in samples from Lake Ohakuri is 3.2 cubic millilitres per litre of water, above the recreation health guidelines of 1.8 cubic millilitres per litre.

Biovolume refers to both the number of cells and their size, which are taken into account as part of new guidelines for measuring and assessing levels of blue-green algae in waterbodies.

Environment Waikato water scientist Bill Vant said algal blooms occur naturally in hot and sunny weather, such as that experienced over recent weeks.

They also depend on other factors, such as the amount of available plant nutrients, phosphorus and nitrogen within a lake.

“Blue-green algae can be present in high numbers in the water, but still not be visible to the eye,” he said. “They are triggered by fine, warm and calm weather, and have the potential to produce toxins which can affect the health of humans and animals.

“Although blue-green algal blooms are common in the shallow lakes around Hamilton and Huntly, this is the first bloom in the Waikato River for several years.

“We monitor the waterways throughout the year, but the frequency increases to weekly in summer as the potential for an occurrence of the blue-green algae bloom grows,” he said.

Additional information on the blue-green algal bloom can be obtained by visiting or view the public health warning issued by the Bay of Plenty district health board.