Local Focus: Curtains of air bubbles trialled to combat green algae in Lake Tūtira

Green slime which has plagued a once popular Hawke's Bay lake has retreated as winter approaches and there are hopes to make the water pristine year-round.

Lake Tūtira — 40km north of Napier — used to be a much-loved swimming and holiday spot before the water quality deteriorated several years ago.

Jeremy Crichton has been fishing the lake for 45 years and said "it gets pretty nasty – a lot of algae" in summer.

"There is no real movement in the lake," he said.

"Most lakes have a few streams coming into them and an outlet. This one doesn't."

In neighbouring Lake Waikopiro, a curtain of air was seen as a solution, as it has been found to oxygenate and destratify the water.

"We put an air curtain in Waikopiro to test a theory that keeping the lake well mixed will help us keep down algal blooms and fish kills," Hawke's Bay Regional Council environmental scientist Dr Andy Hicks said.

"You pump air down to the bottom of the lake, where you have a pipe with a lot of small holes in it.

"Those small holes let out bubbles, the bubbles rise as they get bigger and drag water up.

"So you are generating a circulation current. That means the surface and the bottom are mixed and well-oxygenated the whole time."

Pumping air into the bottom of the lake to try to combat the slime had been tried before at Tūtira but Hicks said it was under-powered and vertically orientated.

A line of bubbles was more effective at mixing the whole lake than a previous trial that had several columns of bubbles.

"If it is successful at Waikopiro, then we will consider moving it intto Tūtira,'' Hicks said.
''But it is the ambulance at the bottom of the cliff.

"The long-term strategy for both lakes and for all water-quality problems in Hawke's Bay and New Zealand is really to stop the problem at its source, which is nutrients and sediments coming off the land and getting into these areas where we don't want it to be."

Hicks said there was clear evidence the lake was once in good health year-round.

The search for a solution was a joint effort between the regional council and the Maungaharuru-Tangitū Trust, which said Lake Tūtira was one of its most important taonga.

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