The health of Lake Dunstan and the Clutha River is being debated as poison drops carried out earlier this month concern locals. 

A brief protest took place at the opening of the Lake Dunstan Trail last weekend, with a sign stating “stunning bike track, stinking lake”, which touches on the growth of aquatic weed and subsequent poisoning aimed at killing it off. 

Diquat was controversially used earlier this month to spray Lake Dunstan, with some locals arguing not enough warning was given. 

One anonymous writer to The Central App says the aerial spraying took place with very little warning. 

Toitū Te Whenua Land Information New Zealand (LINZ) Biosecurity and Biodiversity Group Manager Megan Reid says invasive aquatic weeds in Lake Dunstan need to be eradicated so as to not cause irreversible environmental damage. 

“Left uncontrolled, invasive aquatic weeds such as lagarosiphon can choke waterways, smother native aquatic plants, interfere with electricity generation and create havoc for recreational users of our lakes and rivers,” Megan says. 

“Toitū Te Whenua LINZ has responsibility for almost 2 million hectares of Crown lands and waters across Aotearoa New Zealand including some of Otago’s lakes and rivers, such as Lake Dunstan.  

“To retain the use of lakes and rivers and restore native plants, fish and birds we use a range of well tested methods to control weed growth in line with consent requirements.”

Lagarosiphon is widespread in Lake Dunstan. 

“On this lake (Lake Dunstan) we focus our efforts on stopping the spread within and to other lakes in the region, through targeting key amenity areas and areas frequently used for recreation,” Megan says. 

“As part of our regular control work Toitū Te Whenua LINZ carries out weed control in Lake Dunstan to stop the spread of lagarosiphon and retain recreational use of the lake.

“We use a range of methods including scuba divers hand weeding to remove invasive plants and the application of very low concentrations of the herbicide Diquat to weed infested areas.

“We focus our work on areas most used for recreation including boat ramps, inlets, and camping and swimming areas.

“In addition to this work LINZ has also allocated nearly $1m over the next four years to the Lake Dunstan Charitable Trust through the Government’s Jobs for Nature programme.

“The funding is expected to deliver 10 jobs to the region and will go toward landscape restoration, community outreach and the development of a community vision for the lake.

“This project will complement the significant contribution LINZ makes annually to control lakeweed and maintain the Lake Dunstan shoreline and surrounds.

“All lake and river users also have a role to play in stopping the spread of water weeds. If you’re moving between waterways, you must clean all your gear using the 'Check, Clean, Dry' method. This stops plant fragments moving between waterways, which can be all it takes for a new infestation to begin.

Diquat has been banned for use in a number of countries worldwide, however it is commonly used in New Zealand. 

“Diquat in very low concentrations has been used safely and effectively to control invasive aquatic weeds in New Zealand lakes for 50 years,” Megan says.  

“It’s used particularly to control large, dense weed beds that pose a risk to lake use.

“Despite the low risk associated with applying very low concentrations of Diquat, we take a careful approach to its use. 

“As a precaution, we advise lake users not to swim or take drinking water from the vicinity of the treatment areas for 24 hours after each application.

You can find more information about Diquat here, including what it is, how it works, why it is used and how it is used safely.

Many people have said there weren't enough warning of the spraying and only noticed when it was being done.

Megan says LINZ contacted key groups and organisations before the spraying. 

“We use public notices, signs and our website – linz.govt.nz - to keep the public informed. We also contact key groups before any operation.

As for Lake Dunstan itself, there has been talk about letting Lake Dunstan go back to a more river-like landscape rather than a lake to avoid some issues. 

Megans says issues are routinely discussed between various stakeholders and community groups to come out with the best outcome achievable. 

She did not say if discussion had taken place in regards to letting the lake revert to a more river-like state through Cromwell. 

“LINZ is committed to ongoing lakeweed control in Lake Dunstan and has made a significant investment through our Jobs for Nature project with the Lake Dunstan Community Trust, which supports community aspirations for the lake.

“LINZ, Contact Energy and Otago Regional Council are working together to address lake issues according to their responsibilities, and to ensure their work is coordinated and efficient.”