Waitaki weed surveillance plan


The issue

As part of its commitment to the environment and ongoing stewardship of the upper and mid Waitaki waterways, Meridian Energy, in partnership with LINZ, routinely spray invasive aquatic weed in the Ohau canals and Lake Benmore. These weeds reduce native species habitat and adversely impacts Meridian's hydro generation operation.

A management strategy forms a critical element of the overall weed control programme. NIWA and Meridian are developing a management strategy on LINZ crown owned lakes for pest aquatic plants – weeds, the alga Didymosphenia geminata (Didymo) and filamentous green algae (both native and introduced) - in the Waitaki Catchment. This Plan will be of benefit to managers (LINZ, Environment Canterbury), operators (Meridian, Genesis) and lake users. 

The Catchment is home to the Waitaki hydro scheme, a series of interconnected lakes and canals which are used by Meridian and Genesis to generate electricity for hundreds of thousands of New Zealand's homes.

Learn more about the Waitaki hydro scheme 

However, these waterways are also home to numerous aquatic weeds, as well as Didymo and filamentous green algae. Meridian wants to ensure these plants don't interfere with the Scheme's operation.

Some of the problems these plants can cause are:

  • the risk of blocked cooling water filters due to increased algal growth.
  • the potential for material to drift and block intake screens, due to an increase in spread of elodea and lagarosiphon.
  • invasion by hornwort and egeria, which could also cause issues with blockages.

The solution

NIWA was brought in to develop a weed surveillance plan which would help to better identify, monitor and manage risks from unwanted aquatic plants.

Working with its partners this Plan will support Meridian's key goals, which are:

Avoid invasion by new unwanted aquatic plants

  • The key to avoiding problems with these pest species is to prevent their invasion in the first place. This requires surveillance, to make sure they are detected early,followed by their containment and removal. Education is also key, as substantial numbers of people use the Catchment for recreational purposes; it is particularly important during events such as the Maadi Cup, National Rowing Championships, and other rowing events.

Contain and control Lagarosiphon major (lagarosiphon)

  • This requires active surveillance inside and outside of the containment area, as well as the education of lake users in the area, especially for Lake Benmore and Lake Ruataniwha.

Minimise the impact from unwanted aquatic plants blocking intakes and screens, such as elodea which grows in the canals or lagarosiphon which has the potential to spread further through the catchment and add to the impact from other weed drift.

  • Containment of lagarosiphon followed by active removal from all known sites is considered the best and most effective option. As with above, active surveillance is key for monitoring the extent and location of existing pest species which may become problematic.

The approach

In an ideal world, it would be possible to inspect every site which is able to support these unwanted aquatic plants. However, the Waitaki Scheme presents a huge potential habitat for these plants, making it too expensive – in both time and money – to do so.

As a result, a surveillance plan has been designed based on ranking and prioritising sites according to the current status of aquatic vegetation, their risk of invasion by new weed species and the risk posed to Meridian's operations. It focuses on providing a practical, detailed user manual to guide surveillance activities in terms of locations, priorities and which methods to use. The surveillance information has been condensed into an Excel spread sheet so that all of the essential information is on one page, for ease of planning and prioritising. Comment boxes are used throughout the spreadsheet to provide additional clarification and guidance.

Maps of each discrete area (e.g., areas within individual lakes and canals) provide an overview of each location, along with numbered sites (each with its own individual map) showing the specific areas which require surveillance. Each individual map marks the area which needs surveillance, including information on which surveillance method should be used.

This is the first time anyone has tried to develop a surveillance strategy based on historical surveillance knowledge, while providing a structured site specific survey process appropriate for each lake and identified risk species.

This surveillance plan is intended to be a living document – it will be reviewed annually, as well as after any new invasions, and updated accordingly. This is important because site ranking and priorities will change with the spread of current weeds, and the introduction of new ones.

New surveillance sites will also need to be added where new invasions or introductions happen.

Page last updated: 
26 February 2019
Underwater surveillance - a NIWA diver detects a recently established plant of lagarosiphon. [John Clayton]
Surface-reaching weed beds of lagarosiphon in Lake Benmore. [John Clayton]
Weed suveillance underway in the Auhuriri Arm of Lake Benmore. [John Clayton]