Weeding Lake Wanaka

1 October 2008

For the first time in decades many Lake Wanaka bays are now virtually clear of the invasive weed lagarosiphon. LINZ's Landscan editor joins NIWA staff and the Guardians of Lake Wanaka on a midwinter inspection of the lake.

It’s the dead of winter, there’s a deep frost on the ground and the morning exodus to Wanaka’s skifields has begun. Up on the slopes there is at least the prospect of sun above the grey cloud hanging over Lake Wanaka. That’s cold comfort for NIWA’s John Clayton, who is heading in the opposite direction – that is, he’s about to launch himself overboard into Lake Wanaka.

Contracted to monitor LINZ’s Lake Wanaka lagarosiphon control project, John’s apparent enthusiasm for this midwinter weed inspection draws shakes of the head from those of us shivering topside.

John, an aquatic plants specialist, is checking how well the autumn’s efforts to knock back the weed have worked. With drysuit sealed, and snorkel and mask adjusted, he plunges in and grasps a length of cord to start the first of a series of tows off headlands and bays at the southern end of the lake.

Lagarosiphon has been congesting Lake Wanaka’s shallows for decades, affecting boating, fishing and swimming. It can grow to the surface from depths of four metres, and because it regenerates from fragments, regrows rapidly and is easily spread by wind or if caught in boat or fishing gear.

A 10 year lagarosiphon management plan produced by LINZ in 2005, after public consultation, marked the start of a concerted effort to bring the weed under control.  The highest priorities were to remove the weed everywhere north of Roys Peninsula and from a number of problem sites south of the peninsula. Elsewhere at the lake’s southern end the goal was containment while priority areas were treated.

Three years on, inspections in April and June this year confirmed that lagarosiphon is now well under control at the northern sites. For example West Wanaka, Fishermans and Minaret bays, where substantial infestations were once present, are now virtually clear of the weed and require only hand weeding to check regrowth.

David Morgan, LINZ’s biosecurity portfolio manager, says suction dredging and hand weeding, the main tools used in the operation, have proved very effective, while the herbicide Diquat applied by helicopter has been used to tackle particularly dense infestations.

David says the success in the north means more resources are now being channelled into problem sites at the southern end of the lake. Here the problem has been more visible, with the weed close to shore at popular beaches and in bays regularly used by boaties and anglers.

“At this end of the lake boat ramps and jetties have been kept clear to minimise the risk of the weed getting caught up in boats and spreading both within the lake and to other lakes.”

Herbicide was applied in Glendhu Bay for the first time in May, and the bare sandy lakebed visible as the boat floats overhead is testimony to the effectiveness of this first pass.

“We’ve had an excellent result in Glendhu Bay and over in Stevensons Arm, but more work will be required to keep on top of the regrowth.”

Paddock Bay in the southwest corner of the lake is the best it’s been in many years. David quips that before the control “you could almost walk on the weed it was so dense.”John’s inspection of Paddock Bay found “a few scattered shoots of lagarosiphon, and large areas of bare sand and sediment. Native vegetation is now beginning to recolonise the bare sediment.”

Varying results from aerially applied herbicide do occur. “We’re pretty good at nailing significant weed beds, but some areas are hard to get.”

This is due to a range of factors, including the steepness of the slope the weed is on; how much dirt is on the weed (Diquat is a contact herbicide and is rapidly deactivated by sediment); and the condition of the weed – older gnarly stems are harder to treat.

This has led to some work considering other techniques, including direct injection of the herbicide into weed beds by a scuba diver using a 2m wand attached to a spray rig and pump on a boat.

“There is considerable potential for using the diver injection technique to enhance the effectiveness of weed control along shorelines such as Bishops Bay that have traditionally proven difficult to control.”

John’s technical advice informs decision making by the Lake Wanaka Lagarosiphon Managers Committee which comprises representatives from the Guardians of Lake Wanaka, Queenstown Lakes District Council, Otago Regional Council, LINZ and Department of Conservation.

This group meets twice yearly to review progress and prioritise control efforts. Work takes place in April/May and August/September each year to make the most of the optimal water clarity and weather conditions, but more importantly the two control periods ensure that any new growth is quickly dealt with.

Russell McGeorge of the Guardians says the group are very pleased with the results achieved to date. “The methodical and science-based approach to the control of the lagarosiphon infestation in the lake has resulted in steady progress containing the infestation to a limited region at the south eastern end of the Lake.”

“Much of the lake is now essentially free of the weed and the containment zone is being reduced in size.”

Back on the water the sun has finally emerged along with a snowy vista beyond Paddock Bay. John’s clutching a hot cuppa with chilled fingers and enthusing about the great result in the bay with David Morgan and Russell McGeorge.

The boat operator chips in that fishing guides are reporting good sport in Paddock Bay – perhaps as good a sign as any of how things are going!