Algal levels reduced at Rotoiti

Algal levels at Okawa Bay on Lake Rotoiti have been reduced significantly following treatment with a product to prevent potentially toxic blue-green algae (cyanobacteria) blooming on the lake.

Tests this week have shown that cyanobacteria levels at the bay are now well below the threshold where effects occur. The Regional Council had been alerted to a risk of the harmful algae forming in routine monitoring, and from its environmental advisors, including Professor David Hamilton of Waikato University.

Regional Council Lakes Operations Manager Andy Bruere said the treatment – using a modified zeolite product called Aqual P – was designed to prevent a bloom from occurring, which had potential effects on people’s use and enjoyment of the lake. The presence of blue-green algae in high concentrations can close the lake to swimmers and boaties,

“The Regional Council has experience with the application of Aqual P on Lake Ōkaro, and tests have shown it is safe to use in the natural environment. It was applied to that lake in 2007 and 2009 to deal with eutrophication problems (where nutrients enter the lake and deplete oxygen). The product will rapidly settle and flocculate algae, and lock up phosphorus in the water column.”

The treatment was trialled at Okawa Bay because the algae was potentially toxic, and the location was a good spot to undertake the technique, he said.

A slurry was applied to Okawa Bay from a helicopter to lock up phosphorous and drop the algae to the bottom of the lake. Okawa Bay residents were given information about the process.

A comprehensive monitoring programme has been designed to determine the success of the application and ensure any effects are identified.