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SIMON.HENDERSON

@thenews.co.nz

Waging war on Lake Wanaka weeds is Don Robertson, of Hawea.

As chairman of the Guardians of Lake Wanaka and a trustee of the Upper Clutha Lakes Trust, he has been involved in finding ways to manage the growth of lake weed (lagarosiphon) for many years.

Lagarosiphon was first observed in Lake Wanaka in 1972 and was now widespread in the south end of the lake, in particular Roys Bay, Glendhu Bay, Parkins Bay and Paddock Bay, Mr Robertson said.

The current programme was “control, not eradication”.

Management of lagarosiphon in Lake Wanaka was about trying to “move the line” south.

“There is a line defining areas that you do different things in, and that line has been pushed south.”

The weed was prolific, and regenerated after being removed.

“It takes a fairly small piece to regenerate a new plant, and the new plant will soon spread.”

Several techniques were used to manage the spread of the weed in Lake Wanaka.

In the top parts of the lake where there were fewer weeds, diving excursions took place one or two times a year.

“The entire shoreline is visited by divers who hand weed any stray plants.”

The upper part of the lake was “pretty good” with less weed, Mr Robertson said.

Growth of lagarosiphon in Paddock Bay, Parkins Bay and Glendhu Bay was treated by suction dredging – “an underwater vacuum cleaner that slurps everything out and fires it into a barge”.

Spraying of the herbicide diquat dibromide from helicopters was occasionally undertaken in the bays, but that required very good conditions, “very good weather, and there has to be low or no sediment on the leaves of the plants, a bit of sediment will make the stuff ineffective”.

In the last two years a “more promising, more environmentally friendly” option had been using hessian matting.

The matting was laid on the lake bed in Paddock Bay and Parkins Bay with “quite rewarding” results.

It killed lagarosiphon but native plants were able to grow up through the hessian “and start to proliferate”.

About $410,000 was being spent each year on management in Lake Wanaka, Mr Robertson said.

Most of the funds came from Land Information New Zealand (Linz), some from the Otago Regional Council (ORC), some from Queenstown Lakes District Council (QLDC) and the rest was from “a very generous and anonymous benefactor, who put up a good chunk for a five-year span”.

Push-back of lagarosiphon had been successful for about 65% of the area of the lake where it occurred, in that it had either gone or was under control.

“There seems to be a kind of acceptance amongst all the players that what’s happening now is not too bad, pretty good value for money, and biologically reflects the kind of difficulty and constraints of dealing with that kind of organism.”

Management of lagarosiphon was undertaken by environmental planning and design consultancy Boffa Miskell.

Project manager Marcus Girvan said the co-ordinated work taking place in Lake Wanaka had been very successful.

“This is most evident in the shifting of the containment line, which was brought down the lake a few years ago.”

Use of hessian matting had proved to be an effective tool for killing off lake weed and preventing it from spreading across Lake Wanaka.

“The results of the matting are often more enduring than other weed control methods as it totally shades out the lake weed,” he said.

The matting was biodegradable and meant less herbicide could be used.

“Although it’s relatively expensive at around $60,000 per hectare, once it is laid very little follow-up control is required, which will reduce our reliance on herbicides in the long run,” Mr Girvan said.

Linz biosecurity and biodiversity director Dave Mole said working together with regional councils and local community groups, they were not only stopping the spread of lagarosiphon but were removing it from the lake as well.

“Through the use of various techniques, we’re now seeing native plant species thriving in areas previously overrun with lagarosiphon,” Mr Mole said.