Hornwort control for Lake Karapiro

27 March 2007

An aquatic weed management programme aimed at controlling the introduced invasive weed hornwort in Lake Karapiro is due to begin later this year.

A working group comprised of Waipa District Council, Land Information New Zealand (LINZ) and Environment Waikato said today the weed control is part of a coordinated effort underway to manage Lake Karapiro and its facilities and supporting infrastructure.

LINZ was yesterday granted resource consent to apply the herbicide Diquat to control the weed, which infests about 200 hectares of the lake.

LINZ spokesman Paul Jackson said the work this year would target a 50 hectare area of the lake above the rowing course. The work is programmed for May and September/October, and is aimed at preventing weed from floating into the most actively used area of the lake.

Rowing New Zealand will continue to use a mechanical harvester to control hornwort on the course itself.

"With the World Rowing Championships coming up in 2010, the control work will help ensure the rowing course is up to international standards," Mr Jackson said.

"Significant local, regional and central government funding has been committed to ensuring the world champs are a success, and this weed control work will help protect this investment," Mr Jackson said.

Waipa District Council Chief Executive John Inglis said Waipa supported the control work because of the community benefits that it would bring.

"Lake Karapiro is a significant local asset worth the effort to preserve. Users know the problems presented by hornwort, as it detracts from the amenity value of the lake, and will welcome these control measures.

"Council is focused on the long-term development of Lake Karapiro and surrounding areas. This includes the Rowing World Championships in 2010, but care for the lake goes beyond this to encompass the community's aspirations for this important Waipa icon," Mr Inglis said.

Mr Jackson said Diquat has been used safely and effectively to control aquatic weeds in lakes throughout New Zealand for more than 40 years. Diquat was ideal because it rapidly deactivates in water and leaves no toxic residue.

Waipa District Council says the qualities of Diquat, the massive dilution effect, and using the activated carbon process also employed to remove blue-green algae from the water during treatment means this weed management programme will have no effect on drinking water.

LINZ consulted with a range of parties including the Waikato District Health Board, iwi and Rowing New Zealand before the resource consent was granted.

Public notices advising when the work is scheduled would be placed in newspapers, radio and on the LINZ website, and notices would be erected around the lake. One to two days would be required to complete each application. Two applications are anticipated this year; in May and August/September.

Water quality monitoring will be carried out as a condition of the resource consent.

MEDIA RELEASE ENDS

Lake Karapiro weed control factsheet

About hornwort

Hornwort is an invasive introduced aquatic weed present in Lake Karapiro. It is widely established in the North Island lakes and rivers. Hornwort grows rapidly, forming dense weed beds or rafts (up to 10 m deep) that choke waterways and are very difficult to eradicate. It shades out other aquatic plants, affecting aquatic invertebrates and fish. Hornwort also affects fishing, boating and other recreational activities, and hydro-electric intakes. For more information on hornwort, see: http://www.biosecurity.govt.nz/files/pests-diseases/plants/hornwort/hornwort-factsheet.pdf - (pdf 88KB).

Diquat

Diquat has been used to safely and effectively control invasive aquatic weeds such as hornwort and lagarosiphon in New Zealand lakes for more than 45 years. Diquat is a contact herbicide which kills the area of plant that it touches.

New Zealand's regulatory authorities and scientific agencies such as the National Institute of Water and Atmospheric Research approve its use, and have almost 50 years' experience and testing to back this position. Diquat is a substance approved by the Environmental Risk Management Authority (ERMA).

National and international studies of Diquat use continue to show no evidence of risk to humans, animals, fish life and most native aquatic plants when used appropriately.

Diquat is used in many countries around the world to control a variety of diseases (eg on fish farms) and as an effective control of invasive aquatic weeds. It is also used as a safe and effective pre-harvest desiccant on a range of grain crops and potatoes.

Diquat in water

Scientific research has shown Diquat is rapidly deactivated (rendered inert) usually within only a few hours after it is applied.

There is no toxic residue from Diquat and weeds affected by the herbicide fall to the lakebed and decompose.

Diquat is also deactivated by ultraviolet light and suspended particles so that it is quickly removed from water, with most ending up in an inert form in sediments on the lake bottom.

How is Diquat applied?

At Lake Karapiro, the herbicide will be applied by helicopter or boat; gel is added to the herbicide so that the herbicide sinks rapidly in water. The control of aquatic weeds using a herbicide requires calm water and weather conditions.

Where will Diquat be applied?

Hornwort infests approximately 200 hectares of Lake Karapiro. A 50 hectare area south of the rowing course will be targeted for control each season.

When will the weed control be carried out?

Two applications will be required in 2007. These are programmed for May and September/October. Each application requires 1-2 days to complete.

How will the public be informed?

Land Information New Zealand will advise the public about the weed control operations via local newspapers and radio stations, signs around Lake Karapiro and the LINZ website.

Can Diquat enter the water supply?

Diquat can easily be removed from the water through the drinking water treatment process, thereby ensuring zero exposure to any trace of Diquat in drinking water supplies. The process Waipa District Council uses to remove blue-green algae from the drinking water will also remove any residual Diquat. This process will be employed during the week of all applications as a precaution.

This would be a precautionary measure only. In the low concentrations of diquat to be used in Lake Karapiro, there would be no predicted deviation from the national drinking water standards at any public water supply intake. This is due in part to the massive dilution factor, the fact that Diquat will be applied a safe distance from the intake pipes, and because Diquat is inert after such a short time.

Mechanical harvesting of weed

Rowing New Zealand will continue to manage hornwort on the rowing course by using a mechanical harvester around the rowing lanes and placing a chain boom immediately above the lanes prior to and during regattas to prevent weed from floating down onto the course.

Mechanical harvesting is impractical for controlling weed over a larger area because it is an extremely expensive, labour-intensive and logistically complex way to manage and control large masses of aquatic weeds.

LINZ experience has shown that the cost of mechanical harvesting of aquatic weed can be 20 times more expensive than herbicide and can have a greater impact on the aquatic life. The weed is collected from the lake and temporarily stored on the boat or barge which then has to be offloaded to temporary storage sites, often on the shore. The weed is then removed from the site and transported away from the lake. In addition, mechanical harvesting not only takes out the weed but also invertebrates living on the weed that fish feed on.

Contact for further information:

Dave Chowdhury
Senior Communications Advisor
Land Information New Zealand
Ph 04 496 5402; 027 222 9566
Email: dchowdhury@linz.govt.nz

John Inglis
Chief Executive
Waipa District Council
Ph 0800 WAIPADC or 07 872 0030

Waipa District Council, Environment Waikato and Land Information New Zealand each have statutory responsibilities for Lake Karapiro and the Domain. With the World Rowing Championships coming up, they have been working together to coordinate preparations and lake management through to 2010 and beyond.

A memorandum of understanding (setting out the commitment and intent of the parties to work together) for formal coordinated lake management is underway. Other key stakeholders such as Te Mana Taiao and the Raukawa Trust Board are involved in that process, recognising the role they play in overseeing and advising on cultural and community issues.