South African oxygen weed, or lagarosiphon, could have completely choked Lake Ngatu if left unchecked. Photo: Supplied / NRC / Trevor James
An invasive aquarium plant that threatened to choke a Far North lake has been officially eradicated, five years after the entire lake was treated with herbicide.
Lake Ngatu, about 15km northwest of Kaitāia, is a popular destination for swimming and waka ama, but was infested with oxygen weed.
The 56-hectare water body is also a rare and ecologically valuable example of a dune lake.
Northland Regional Council chief executive Jonathan Gibbard said earlier attempts to get rid of the weed by suction and hand-picking had failed.
South African oxygen weed, or lagarosiphon, was introduced to New Zealand as an aquarium plant, he said.
It started appearing in the wild around 1950 and was first found in Lake Ngatu in 1988.
Left unchecked the weed could have filled the entire lake, with serious consequences not just for ecology and water quality, but also recreation.
A number of waka ama groups used the lake for training and competition, and it was a popular swimming spot in summer.
Project partners involved in the Lake Ngatu oxygen weed project in 2020 included NgāiTakoto, AquaAg, Department of Conservation and Northland Regional Council. Photo: Supplied / NRC
Gibbard said the council started working with local hapū NgāiTakoto, the Bushlands Trust and the Department of Conservation in 2019, as part of the Freshwater Improvement Fund Dune Lake Project co-funded by the Ministry for the Environment.
Following a public meeting with lake users, the project group decided to try the herbicide Aquathol K, which was safe once in the water but highly effective against oxygen weed.
Gibbard said the Environmental Protection Agency, which oversees herbicide use, allowed the entire lake to be treated at once rather than the quarter-lake treatments normally required.
The herbicide was applied using an airboat In September 2020, with the $270,000 cost split between the ministry and the council.
Within weeks the weed beds were dying off and being replaced by healthy native plants.
Gibbard said the council had surveyed the lake every year since then to check for oxygen weed.
With no lagarosiphon seen since 2020, the council was now able to declare with confidence that it had been eradicated.
South African oxygen weed, or lagarosiphon. Photo: Supplied / NRC / Trevor James
NgāiTakoto spokesman Kaio Hooper said the eradication was a testament to past mahi and partnerships aiming to enhance and preserve the taonga that was Lake Ngatu.
"It's been a long journey for our whānau to reinstate our obligations as kaitiaki of this lake," he said.
"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 at the time many whānau had questions and reservations about the use of herbicide.
However, the hapū gained confidence and strengthened its capacity as it worked alongside the council during a process that included testing, collaborating with experts, sharing knowledge, and being part of planning and implementation.
"It 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," Hooper said.
Meanwhile, Gibbard said lake users had to remain vigilant, despite the weed's eradication.
Pest plants, fish and invasive freshwater clams could all reach Lake Ngatu if gear used elsewhere was not checked, cleaned and dried between each waterway.
"Pests like these would ruin a treasured lake like Ngatu if introduced and not controlled," he said.
The pest freshwater clam in particular had never been eradicated from any lake it had infested, so anyone visiting the Waikato had to be certain they did not bring it back to Northland.
* Go to [www.nrc.govt.nz/pestcontrolhub Northland Regional Council's pest control website] for more information about lagarosiphon/oxygen weed and how to control it.
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