Our Changing World: Restoring native plants in our lakes and rivers

Source: Radio New Zealand

Mānuka, rimu, ponga, kōwhai, harakeke, tōtara – we can probably all name a few of the commonly found New Zealand terrestrial trees and plants. But what about those that are out of sight and out of mind – on the bottom of our lakes and slow-moving rivers?

There are more than 75 species of native freshwater plants in Aotearoa, but they are disappearing.

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How do we know? Well, 19th century botanist accounts are one piece of evidence, says Mary de Winton.

“Back in the 1800s they had, you know, a huge raft of plant species growing right across the bed of the lake.” Today some of the same lakes are completely bare.

Mary is a freshwater ecologist with Earth Sciences New Zealand in Kirikiriroa Hamilton. She’s often diving in freshwater environments for science, and across her 40-year career she’s seen changes herself.

“We’ve seen a lot of lakes that have lost their vegetation over the last four decades. I can think of five in the Waikato off the top of my head that I’ve actually seen both in their vegetated state and in a de-vegetated state several years later.”

Pest fish, invasive weeds, high sedimentation rates, reductions in water quality – the challenges facing our freshwater ecosystems are numerous. For plants, lack of light can be a big issue, says Mary. “The water itself gets very murky, either from algal blooms or from suspended sediment. And when you get those kinds of environments, there’s very little light that’s getting down to the bottom where the plants are growing. And if you lose those plants, it’s incredibly hard to get them back again.”

That’s why a team at Earth Sciences New Zealand, in collaboration with local iwi and international researchers, began a project to investigate how native plants might be given a ‘helping hand’.

Inspired by roll-out turf, the stuff you might use on a lawn in a new build, the team investigated ways to grow plants in a type of matrix that could then be rolled out on the lakebed and left there.

For the ‘RotoTurf’ project to work, they needed their matting to be biodegradable, but also robust enough to stay together long enough for the plants to grow to a decent size. After trying a range of materials, they decided that wool and muka (the fibres from harakeke/flax) matting were the two most promising. Alongside the matrix trials, they also investigated which plants would be best suited to grow in the matting and be transplanted into a partially degraded lake. Milfoil and native pondweed came out on top as the most likely candidates.

To test their RotoTurf design the team ran trials at Lake Ohinawai. The turf was unrolled on the lakebed in December and left there until August to see how the plants went through the seasons. They were also surrounded by a plastic mesh cage, to keep pest fish, like koi carp, out.

After eight months the plants were still there, growing right to the surface, says Mary. The milfoil seemed to be doing particularly well.

A three-year funded project, RotoTurf has since wrapped up. Now there is some interest in the design from iwi groups and other agencies who are keen to return plants to their lakes.

But for proper restoration RotoTurf alone will not be enough, says Mary, without also addressing the lakes’ underlying water quality issues.

“The best candidates… are those that have already had some works undertaken in the catchment…. some intervention for the nutrient and sediment loads, that have had some riparian planting around the edge of the lake… that gives any kind of in-lake restoration the best chance of success.”

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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand