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Published: 2003-08-04 00:00:00

A blue green algal bloom is still affecting Lake Ngaroto near Te Awamutu, despite the colder weather.

Environment Waikato’s monitoring of the lake this week revealed 32,000 cells per millilitre of blue green algae (Anabaena planktonica) in the lake, well above the recreation health guideline of 15,000 cells per millilitre. This type of algae can produce harmful toxins.

Several Waikato lakes developed algal blooms over January, leading to health warnings being issued for Lakes Kainui, Hakanoa, Ngaroto, Waahi and Whangape. Cell counts for most of the other lakes have receded to below health guidelines, although Environment Waikato is waiting for a second low count at Lakes Kainui and Hakanoa before the Medical officer of health could clear them. Levels at Lake Kainui were 8300 cells and at Hakanoa 12,000 cells.

Freshwater scientist Grant Barnes said blooms of blue green algae occurred naturally during calm warm periods. They were worsened by other factors, such as the amount of available plant nutrients, phosphorus and nitrogen within a lake. The blooms were typical of many shallow lakes in agricultural catchments in the Waikato.

Most disappeared with colder weather, however some had not as the particular species was tolerant of winter temperatures. The blooms had been reported in the Waikato for at least 20 years, and particularly warm, sunny and calm summer weather was a possible reason for an increased incidencethis year.

There were limited options for treating the blooms. Adding chemicals may not be acceptable in New Zealand, their success could not be guaranteed and their use could actually encourage resistance, he said.

“Restoration work around the margin of Lake Ngaroto is a positive step towards reducing nutrients. Fencing drains, protecting headwater wetlands and seeps and minimising fertiliser use around the lake would also help reduce the incidence of these blooms occurring.”

Environment Waikato was working with Public Health and district and city councils on monitoring and keeping residents in the affected areas informed.