The unusual contraption cutting weeds from Lake Horowhenua is best described by its skipper as something out of Waterworld — the infamous cinematic flop.
It’s part boat, part tractor, with wheels like a paddle steamer, and teeth which mow aquatic weeds at the bottom of the lake. It chugs along at a walking pace cutting and scooping up about ten tonnes of weed a day.
“It’s not a Ferrari,” skipper Julian Everth said.
“It’s a weed harvester essentially. A very slow-moving barge is probably more accurate.”
The NZ$300,000 machine, made in the US, is one of just two in the country.
The teeth are lowered down so they’re 30 centimetres above the lake floor and start chomping away at the exotic weeds.
There’s two main types of weed being harvested – curled pond weed and an oxygen weed, commonly used in fish tanks.
Mowing Lake Horowhenua also known as Punahau has been done since 2021.
The project is a partnership between Horizons Regional Council and the local iwi, Muaūpoko.
“We’re harvesting the same as you do with your lawn. And the reason for that is when these plants grow like crazy, they actually change the PH in the water column,” said Horizons Regional Council freshwater manager Logan Brown.
Punahau, a shallow dune lake near Levin, has a notorious history for pollution.
It has previously been ranked as one of New Zealand’s most polluted lakes and water quality there is still rated very poor.
Runoff from intensive farming and 25 years of sewage discharge from Levin has contributed to its poor health, while two of the streams running into the lake have the second and fourth highest nitrogen levels in the country.
“if you’ve got a very nutrient-rich system which Lake Horowhenua is, the same as your lawn, if you give it lots and lots of fertiliser it will grow really fast, it’s the same with the lake system,” said Brown.
He said the volume of weed after four years of mowing is not necessarily decreasing but that the water chemistry of the lake is improving overall.
“The algal blooms this year is a little bit worse than it has been for the past two to three years, but it’s not at the volumes that we used to see ten years ago when you could have it easily a centimetre thick and bright green.”
For Muaūpoko people, the lake is a taonga.
“The state of the Punahau has a significant impact on the spiritual wellbeing of our people. Our lake has been the price paid to create wealth and support growth in this region for over a century,” said Dean Wilson, deputy chair of the Lake Horowhenua Trust, which represents owners of the lake.
The Muaūpoko Tribal Authority said it can see the mauri of the lake continue to improve.
Deanna Hanita-Paki, a Lake Horowhenua Trust trustee, is helping sort through the weeds for any bycatch.
She’s noticed the lake improve with the return of grey mullets and native plant life.
“We’ve got the native weed and that’s come back, so we’re pleased with the native weed coming back, it’s been missing for a long time,” she said.
It takes about seven weeks to mow the lake.
Last year a record 400 tonnes plus of weeds were harvested and the council expects that record to be broken again this summer.
The weeds are transported to a green waste centre in Feilding and reused as compost.