Study

Suction dredging to control curly water-thyme Lagarosiphon major, an invasive waterweed, in Lake Wanaka, Otago, New Zealand

  • Published source details Clayton J. & Franklyn G. (2005) Assessment of the 2004 Lagarosiphon major control programme in Lake Wanaka. NIWA Client Report: HAM2005-046 report.

Summary

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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