Lake Rotoehu in New Zealand is undergoing an unusual treatment for invasive waterweeds: periodic mowing, according to the Bay of Plenty Regional Council.
Like many beautiful natural sites worldwide, Lake Rotoehu has deteriorated in recent years thanks to the introduction of human-caused pollution and invasive species. Invasive species are plants, animals, and other forms of life that have been transplanted from their native habitats to new regions where they perform surprisingly well. Without natural predators and competitors, they multiply and spread, using up resources, crowding out native species, and sometimes even preying on native plants and animals and driving them toward extinction.
In the case of Lake Rotoehu, the problem is hornwort and other invasive water weeds. Hornwort can grow up to 10 meters (about 30 feet) deep — deep enough to block sunlight from reaching native species. It also soaks up nutrients and can contribute to algal blooms in the once-pristine lake.
So the Toi Moana Bay of Plenty Regional Council invested in a weed harvester. When the level of the lake is low enough — as it was this year and in 2022 — this purpose-built craft cuts and hauls up ton after ton of invasive hornwort from the lake.
"We started harvesting lakeweed in 2006 and in 2012 we purchased our own harvester and have been 'mowing' the lake ever since," said Regional Council Lakes Operations Officer Justine Randell, per the Bay of Plenty Regional Council. "Due to exceptionally high lake levels, the last year that Lake Rotoehu was able to be harvested was in 2022. This year's harvest, which ran for three weeks from the end of March, resulted in 330 tonnes [363 tons] of dewatered weed being processed."
Besides clearing up the lake for native plants, this benefits the environment in other ways. The harvested hornwort is processed into a natural fertilizer for local dairy farms, replacing synthetic fertilizer that would pollute local waterways. The plant that does the processing collects the fumes for use as biogas in local homes.
This is one step in a larger program to restore the lake to its natural condition. "While invasive weed removal is essential for reducing nutrient loads and improving water quality, it must be combined with ongoing research and broader catchment efforts to create a more sustainable future for the lake," said University of Waikato Te Aka Mātuatua Senior Research Officer Whitney Woelmer, per the Bay of Plenty Regional Council. "Ultimately tackling the immediate lake quality issues, while investing in innovative, long-term solutions, is the strategy we are all working towards to help protect the lake for generations to come."
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