Device To Prevent Lake Stagnation

Press, Volume CIV, Issue 30892, 27 October 1965, Page 24

Device To Prevent Lake Stagnation

"The Press" Special Service ROTORUA, Oct. 26.

The Marine Department is considering a series of trials with an unusual new machine that could solve the problem of enrichment in New Zealand lakes.

Developed overseas, the machine is a type of aerohydraulic cylinder that, when

installed on the lakebed, keeps water circulating. It is powered by a compressor that forces air into the bottom of the cylinder and, like an engine piston, this carries up and out of the cylinder, drawing water with it.

The water of the deeper levels of the lake is churned

over and prevented from settling and stagnating. This process involves the simple principle that air bubbles rise to the surface and for this reason the machine is commonly called the bubble gun.

It has been employed with success on hydro lakes in Britain. One lake in Lanca-

shire had enrichment as a problem, with chemical nutrients and both plant and animal plankton. The lake, 27 acres in area, was fitted with five bubble guns, each 17ft long and 12in in diameter. They churned over the deep water at the rate of 10 million gallons a day and successfully restored oxygen to the low-lying levels of the lake. In other lakes, some thousands of acres in extent, far bigger guns have achieved the same result. Local biologists believe similar success could be expected in New Zealand. A report on the gun is being considered in Wellington by the Marine Department and this could lead to a research programme probably on some of the Rotorua lakes.

Dr. G. R. Fish, Marine Department biologist in Rotorua, said the bubble gun could certainly solve the problem of lack of oxygen in deep lake levels.

Lack of oxygen was thought to be killing most plant and fish life in these levels, while the lack of circulation was causing decomposition of material and consequent stagnation.

“It would probably take several years to get the guns working effectively,” said Dr. Fish, “but we hope that they then would be able to bring the lakes back to a reasonable condition. Some now are as bad as pea soup.”

The increased oxygen would mean an upsurge in not only fish life but also plant life. Lakeweed and plankton would probably thrive, even though lake enrichment had been halted.

“This is why we need some trials,” he said. “There are factors for and against the bubble gun and the only way we can sort them out is by practical experiment.”