Air pump urged to help dying lakes

By JO-MARIE BROWN

Engineers who have designed an aeration system to improve the water quality of a South Island lake believe the technology could be used to save Lake Rotoiti.

From tomorrow, thousands of bubbles will be blown into the bottom waters of Lake Opuha, 15km north of Fairlie, to restore oxygen levels. This will limit the amount of nutrients available for algal growth.

The bottom waters of several lakes in the central North Island, including Rotoiti, are also starved of oxygen in summer.

In those conditions, phosphorus is released from the sediments, allowing toxic blue-green algal blooms to flourish.

Harrison Grierson's director of engineering, Colin Cranfield, said the aeration system was designed to mix Lake Opuha up so that its cooler waters - which normally stratify in summer and remain on the bottom - reach the surface and absorb oxygen from the atmosphere.

"The air bubbles will force their way up to the surface and in doing so they induce currents in the water body itself," Mr Cranfield said.

"It's slow and it takes a while for the momentum to develop but once it's under way it doesn't require much energy at all to keep it going."

Lake Opuha was created in 1998 when a dam and power station were built for electricity and farm irrigation.

Mr Cranfield said the lakebed was rich in nutrients and the water quality had begun to deteriorate - although not to the extent seen in Rotorua's lakes.

"I think the important thing to note from Rotoiti's perspective is that if the decision was made [to install an aeration system], it can be put together in a matter of a few months."

But Professor David Hamilton, who is leading scientific research into the state of Rotorua's lakes, said artificial aeration was unlikely to work on a lake the size of Rotoiti.

Aside from high running costs, there was a risk that more algal blooms could occur if nutrients were stirred up from the sediments and rose to the surface with the oxygen-depleted water.

"I wouldn't discount it as an option at this stage but I would say it's highly unlikely to be feasible."

An alternative for Rotoiti would involve injecting liquid oxygen into the lake's bottom waters to restore oxygen levels without mixing the lake, Professor Hamilton said.

"The idea is that the liquid oxygen will hopefully dissolve completely in the water without the formation of bubbles."

Environment Bay of Plenty's Paul Dell, who is co-ordinating efforts to save Rotorua's lakes, said both techniques had been used here and around the world.