Northland Regional Council is trialling an innovative method to suppress an invasive oxygen weed found in Lake Rotokawau on the Poutō Peninsula.
The Council’s Biodiversity, Biosecurity Marine, and Maritime teams collaborated towards the end of 2025 to lay wool matting on 412m square metres of the lake floor with support from local commercial divers.
The New Zealand grown wool, which resembles a carpet underlay, offers a non-toxic and cost-effective solution to limit the growth of the Egeria densa weed.
Council Biodiversity Manager, Lisa Forester says that until now the lake had been hand-weeded by divers – a process which is painstakingly slow, labour-intensive and costly.
“Using a natural fibre such as wool matting will allow native plants to easily recolonise while acting as a weed suppressant, preventing the oxygen weed from pushing through and hopefully eradicating the pest plant over time,” said Ms Forester.
Kaipara constituency Councillor, John Blackwell says ecological monitoring including weed surveillance on dune lakes’ is part of the Council Biodiversity team’s everyday mahi.
“We are looking forward to the results of this experiment as Rotokawau is one of the 12 lakes ranked as ‘outstanding’ in Te Taitokerau. This particular lake supports threatened underwater plant species and kākahi (freshwater mussels), as well as 14 other threatened species, but is also impacted by the invasive oxygen weed, Egeria densa,” he said.
“If we don’t act now the oxygen weed will eventually fill the lake, smothering native plants in the process and degrading the water quality.”
Council says the Biodiversity team will revisit the lake in late summer to check on the wool matting and continue weed checks twice a year.
