Project partners involved in the Lake Ngatu lagarosiphon control back in 2020 included NgāiTakoto, AquaAg, Department of Conservation and Northland Regional Council. Photo / NRC
Project partners involved in the Lake Ngatu lagarosiphon control back in 2020 included NgāiTakoto, AquaAg, Department of Conservation and Northland Regional Council. Photo / NRC
A Far North lake considered a taonga has become rid of a pest weed five years after it was treated with herbicide.
Lagarosiphon or South African oxygen weed moved into Lake Ngatu, northwest of Awanui, in 1988 after it first established in the wild in New Zealand about 1950.
Northland Regional Council chief executive Jonathan Gibbard said the weed was introduced as an aquarium plant but became a problem in the country’s water bodies.
He said lagarosiphon, if left unchecked, could have filled up Lake Ngatu’s water column. The result would have a detrimental effect on the lake ecology, water quality, and its recreational values.
Attempts were made to control the weed in Lake Ngatu using a suction method and handpicking but failed.
In 2019 the Northland Regional Council partnered with NgāiTakoto, Bushlands Trust and the Department of Conservation as part of the Freshwater Improvement Fund Dune Lake Project – co-funded by the Ministry for the Environment – to find a new solution.
After a public meeting with lake users, an agreement was reached to try the herbicide Aquathol K which is a specific herbicide that is safe once in the water.
Gibbard said a successful application to the Environmental Protection Authority was made to undertake a full lake treatment rather than the quarter lake treatments normally required.
In September 2020, the herbicide was applied to the 56ha lake using an airboat, the $270,000 cost jointly covered by Environment Ministry and the council.
Gibbard said the lagarosiphon beds had died down within weeks and were being replaced by healthy native plants.
The council surveyed the lake every year since 2020 to search for lagarosiphon.
“No lagarosiphon has been seen since 2020 and NRC is now able to declare with confidence that lagarosiphon has been eradicated from Lake Ngatu,” Gibbard said.
Lagarosiphon or South African oxygen weed.
NgāiTakoto spokesperson Kaio Hooper said the outcome was a testament to past mahi and partnerships formed to enhance and preserve the taonga, Lake Ngatu.
“Over the years, significant mahi has been carried out, such as removing and controlling wilding pines, restricting power boats, undertaking pest plant control, upgrading lake drainage, installing physical barriers and signage, and planting thousands of locally sourced native trees.”
Hooper said many whānau initially had questions and reservations about the new control method.
“However, through working alongside NRC throughout this process; including testing, collaborating with experts, sharing knowledge, and being part of both the planning and implementation, we gained confidence in this approach and strengthened our team’s capacity as a result.”
Hooper said the lagarosiphon-free outcome sets a precedent for the importance and significance of these lakes.
“And reinforces the continued mahi that is still required, especially as we face future challenges from developments, subdivisions, land use practices, and climate change.”
Gibbard said pest plants, fish and invasive freshwater clam could all reach Lake Ngatu if gear used at other waterbodies was not checked, cleaned and dried between each waterway.
“Pests like these would ruin a treasured lake like Ngatu if introduced and not controlled.”
Gibbard said the pest freshwater clam has never been eradicated from a lake it has infested, so anyone visiting the Waikato must be certain they do not bring it back to Northland.
