Source: Northland Regional Council
Lagarosiphon or South African oxygen weed has officially been declared eradicated from the Far North’s Lake Ngatu five years after the entire lake was treated with herbicide to control it.
Northland Regional Council CEO Jonathan Gibbard says the weed – introduced to New Zealand as an aquarium plant – moved into the wild in about 1950 and has subsequently become a problem in water bodies throughout the country. It was first found in 56-hectare Lake Ngatu, north-west of Awanui, in 1988.
“Attempts were made to control the weed using a suction method and hand picking, but this didn’t work.”
He says if left unchecked, lagarosiphon could have filled up the lake water column and had a detrimental effect on not only the lake ecology and water quality, but on the recreational values of the lake, which is very popular for swimming and waka ama.
In 2019 the council had started working with project partners 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 (MfE) to find a new solution.
After a public meeting with lake users, it was agreed to try the herbicide Aquathol K, a specific herbicide that is safe once in the water and very effective against lagarosiphon.
“A successful application to the Environmental Protection Agency, which manages the rules for use of herbicides, was made to undertake a full lake treatment rather than the quarter lake treatments normally required, bypassing the need for multiple and very costly treatments.”
In September 2020, the herbicide was applied to the whole lake using an airboat, the $270,000 cost jointly covered by MfE and the council.
Mr Gibbard says within weeks the lagarosiphon beds had died down and were being replaced by healthy native plants.
The council had surveyed the lake every year since 2020 to search for lagarosiphon to ensure all plants were killed by the herbicide in 2020.
“No lagarosiphon has been seen since 2020 and NRC is now able to declare with confidence that lagarosiphon has been eradicated from Lake Ngatu.”
NgāiTakoto spokesperson Kaio Hooper says the lagarosiphon-free outcome is a testament to the past mahi and partnerships formed towards the enhancement and preservation of the taonga that is Lake Ngatu.
“It has been a long journey for our whānau to reinstate our obligations as kaitiaki of this lake.”
“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.”
Mr Hooper says at the time the programme was introduced, NgāiTakoto understood that it was a new control method “and many of our whānau had questions and reservations”.
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.”
He says NgāiTakoto are grateful and energised by the lagarosiphon-free outcome. “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.”
Meanwhile, Mr Gibbard says despite the weed’s eradication, lake users must remain vigilant, as pest plants, fish and now invasive freshwater clams, could all reach Lake Ngatu, or any lake, if gear being used at other waterbodies is not checked, cleaned and dried between each waterway.
“Pests like these would ruin a treasured lake like Ngatu if introduced and not controlled.”
He says in the case of the pest freshwater clam, it 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.
