Study

Assessment of the December 2004 Lagarosiphon major control programme in Lake Wanaka

  • Published source details Clayton J. & Franklyn G. (2005) Assessment of the December 2004 Lagarosiphon major control programme in Lake Wanaka. National Institute of Water and Atmospheric Research report, HAM2005-046.

Summary

Study 1

Curly water-thyme Lagarosiphon major, a waterweed native to South Africa, is now present in many ponds and lakes throughout New Zealand where it is considered invasive. Since becoming established in Lake Wanaka (New Zealand's fourth largest lake) 30 years ago, its accelerated spread around the lake has become the cause of concern both for amenity value (the lake is a popular tourist destination), and native flora and fauna alike.

A Lagarosiphon management control programme was therefore prepared by the Lake Wanaka Managers' Committee, and two methods of control were decided upon, application of diquat herbicide (described here) and suction dredging (see Case 262).

Study area: In response to invasion by curly water-thyme Lagarosiphon major, a management control programme was undertaken in Lake Wanaka (South Island), New Zealand in December 2004.

Baseline surveys: Several baseline sites were selected in November 2004, a month prior to commencement of Lagarosiphon control measures. This was done to provide an objective description of representative sites selected for treatment by diquat (or suction dredging - see Case 262). These sites remained confidential to the National Institute of Water & Atmospheric Research (the agency undertaking the assessment of the control success) so that the contractors could not focus their efforts on these areas. An assessment of the Lagarosiphon control effectivness was carried out in the third week of April 2005 by scuba diving the fixed profiles recorded prior to treatment, using an underwater video camera.

Herbicide application: Application of the 'aquatic' herbicide diquat (formulated with Aquagel® at manufacturers recommended rates) was undertaken by helicopter (7 December 2004) and boat (14 December 2004). Favourable weather conditions prevailed at the time of treatment. Areas treated by helicopter were Paddock Bay (approx. 20 ha), Parkins Bay (8.5 ha), Quartz Creek (2 ha), Stevensons Island (0.03 ha), the head of Stevensons Arm (1 ha) and West Wanaka Bay (0.7 ha). Areas treated from boat were Bishop's Bay Bluff to Paddock Bay narrows (0.15 ha), Quartz Creek to the neck of Stevensons Arm (excluding Isthmus) (0.5 ha) and Colquhouns to West Wanaka Bay (0.1 ha).

Effects of diquat treatment on Lagarosiphon: In the third week of April 2005, an assessment of the effectiveness of the attempted control was carried out at the baseline sites. Results were variable.

The most effective outcome was within Paddock and Parkins Bays where the most expansive areas of Lagarosiphon were present and which contained around 80% of the total waterweed sprayed in the lake. Diquat worked well in these relatively enclosed bays, most of the weed having collapsed onto the lake bed. Much of the upper stem sections and leaves had decomposed leaving only more resistant 'woody' stem material. However, at the time of the inspection (four months after treatment), plant recovery was widespread with many new shoots emerging from the defoliated stems lying on the sediment. Recovery was variable with new shoot growth commonly between 0.2 and 0.5 m long. Without further treatment full recovery was predicted within one year.

Although the diquat treatment did not kill the Lagarosiphon, without it the waterweed beds would have reached the water surface (as in previous years), resulting in fragmentation due to boating activities, wildlife and wave action, leading to further dispersal of fragments out of these bays.

In contrast to Paddock and Parkins Bays, all other diquat treated areas had minimal to no discernable effect on Lagarosiphon. This was put down to inadequate contact time, a common problem when treating less extensive areas of waterweed, especially when lying alongside large areas of open water, where gradients are steep or where colonies are at an early stage of invasion. Other factors may have also contributed to lack of success, e.g. detritus accumulation on leaf surfaces preventing the diquat from being absorbed by the plant. However, even in patches where Lagarosiphon shoots were clean and had no detritus or algae coating the leaf surfaces, there was little or no damage.

The viscosity of Aquagel® used and application procedures therefore need to be evaluated to maximise contact time of diquat in target treatment areas, especially smaller ones.

Effects of diquat on other water plants: The diquat treatment in the two bays was very effective in control of Canadian pondweed Elodea canadensis (another introduced waterweed but less invasive than Lagarosiphon). In Lake Wanaka, Lagarosiphon out-competes Elodea, but Elodea commonly grows in a band below Lagarosiphon (which will not grow in water deeper than about 6.5 m). Elodea is much more susceptible to diquat than Lagarosiphon. The affected Elodea beds (previously 1 - 1.5 m tall at depths from c. 5 - 7 m) were very much reduced but the first stages of recovery were apparent four months after treatment and, as with Lagarosiphon, full recovery was expected within a year.

Both Paddock and Parkins Bays support substantial native plant communities growing in water too deep for Lagarosiphon and Elodea to persist. These native plants consist mostly of stoneworts or charophytes (several species of Chara and Nitella). These charophyte beds (ecologically important in clear water lakes such as Wanaka) were not impacted by diquat, even when directly exposed within treatment areas.

Follow-up treatment: A re-spray with diquat four months after the intial treatment (while recovering shoot growth is young, soft, clean, green and particularly susceptible to diquat) was recommended in order to kill vegetative buds that were re-growing from the collapsed woody stems.

 

Study 2

 Curly water-thyme Lagarosiphon major, a waterweed native to South Africa, is now present in many ponds and lakes throughout New Zealand where it is considered invasive. Since becoming established in Lake Wanaka (New Zealand's fourth largest lake) 30 years ago, its accelerated spread around the lake has become the cause of concern both for amenity value (the lake is a popular tourist destination), and native flora and fauna alike.

A Lagarosiphon management control programme was therefore prepared by the Lake Wanaka Managers' Committee and two methods of control were decided upon, suction dredging (described here) and application of diquat herbicide (see Case 261).

Study area: In response to invasion by curly water-thyme Lagarosiphon major, a management control programme was undertaken in Lake Wanaka (South Island), New Zealand in December 2004.

Baseline surveys: Several baseline sites were selected in November 2004, a month prior to commencement of Lagarosiphon control measures. This was done to provide an objective description of representative sites selected for suction dredging (or treatment with diquat - see Case 261). These sites remained confidential to the National Institute of Water & Atmospheric Research (the agency undertaking the assessment of the control success) so that the contractors could not focus their efforts on these areas. An assessment of the Lagarosiphon control effectivness was carried out in the third week of April 2005 by scuba diving the fixed profiles recorded prior to control, using an underwater video camera.

Suction dredging: From 21 November to 7 December 2004, diver operated suction dredging was carried out around Mou Waho and Mou Tapu Islands, a small area around the boat ramp on Ruby Island and also 10 m either side of the three boat ramps adjacent to the Wanaka marina and the nearby yacht club facilities. Underwater scooters were used by divers to locate isolated Lagarosiphon colonies in outlying islands and these were marked using GPS coordinates for subsequent removal by suction dredge. Diquat control was not attempted around boat ramps and the marina due to concerns by the public over its safety.

In April 2005, the areas that had been dredged were assessed, results were variable.

Dredging of outlying colonies: For targeted outlying areas, complete removal was the primary objective but this was not acheived. On Mou Waho and Tapu Islands inspections found that several of the largest colonies had been successfully removed, but a further six colonies were found with the largest being around 0.5 x 0.5 m in area. These were mostly in shallow water, some had been identified prior to dredging but had still been missed.

All plant material must be removed to prevent regrowth, avoiding loss or escape of collected fragments - Lagarosiphon cannot re-grow from root material but re-growth will occur from stem fragments, including any left buried within the sediment. It was recommended that within 3 – 4 months, all suction dredged areas should be revisited to remove any re-growth. Follow up removal may be critical in terms of securing a long-term successful outcome.

Suction dredging in high density Lagarosiphon beds: Suction dredging in high density beds of Lagarosiphon (around the boat ramps and marina) was primarily aimed at removing the bulk of the weed (rather than complete removal) in order to reduce the risk of boats and trailers picking up fragments and dispersing them to other lakes. Lagarosiphon around the boat ramp and marina has been dense for many years, and during periods of low water level, surface-reaching weed extended from the shore into the lake for several metres. Suction dredge operators focused on clearing a strip 10 m either side of the boat ramps, and clearing beneath the boats in the marina. The weed was cleared to a depth of approximately 2 m with 80-90% removal of biomass. Lagarosiphon recovery was apparent in both areas with plant shoots re-growing from remaining stem fragments.

If suction dredging continues to be the method of control around the boat ramps and marina, then this will have to be repeated at regular intervals (dependent on regrowth rate) to maintain open water. Suction dredging can potentially be substantially improved, both in terms of the machinery used and techniques e.g. at least half the cost was attributable to weed removal, with holding chambers or collection bags needing to be towed back to shore and safely disposed. One option that could be investigated is that of deepwater disposal of dredged plant matter.


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