NIWA is using biodegradable “life rafts” to reintroduce native plants to degraded lakes.
NIWA biogeochemist Ben Woodward says they work in similar way to when you roll out a lawn onto soil without having to wait for plants to grow from seed.
“We use mats called ‘roto turf’ – roto is Te Reo for lake and turf refers to dense aquatic vegetation. They act like a carpet with plants growing on top, which are then rolled out onto the lake sediment. They are a quick way to reintroduce plants into a body of freshwater that doesn’t have a native community anymore,” said Ben.
Roto turf is a NIWA led Smart Ideas project funded by the Ministry of Business, Innovation and Employment (MBIE). The project infused scientific research with Mātauranga Māori to ensure the best outcomes from the research.
Tawera Nikau from Matahuru Marae has been working with NIWA on the project.
“I swam in this lake about 30 years ago, but I wouldn’t want my kids swimming in it now. There are 14 lakes in north Waikato that are all degraded, so it’s really important for us to change that,” said Tawera.
Many of Aotearoa’s shallow lakes are in poor condition, with permanently murky water meaning aquatic plants don’t have sufficient light to grow. Without them, lake-bed sediment gets resuspended, and a feedback loop is created that results in poorer water quality.
Project lead Deborah Hofstra says that they are testing a range of plants, including milfoil, pondweed, and charophyte.
“Many of these native aquatic plants are long-lost from our degraded lakes, and our seed stores may not be healthy or plentiful enough to easily re-establish them, which is why we came up with this project.
“When putting vegetation into a lake with poor water quality, you need to think about the photic zone - the top layer of a body of water where sunlight can penetrate, allowing for photosynthesis and plant growth. We put light sensors in with the plants during roto turf development so we could learn the optimal levels of light that they could grow at,” said Deborah
To make the mats, the team trialled different types of products, including those made from hessian, coconut fibre, muka (flax), and wool. In the end, they chose wool and muka fibre.
“Both materials were stable enough to remain intact during months of cultivation and can break down once used to transfer plants into the lake. We used different plants, plant densities, and lake locations to see what species thrived in a variety of conditions.
“Our dream would be to grow enough plants and have a mass planting event that reaches a threshold where the plants are self-perpetuating, the water becomes clearer and the ecosystem is in much better health,” said Deborah.
Initial trials have yielded great data and clearly show it’s possible for milfoil and pondweed to survive in a degraded lake after being cultivated elsewhere and transplanted. Tawera Nikau says a lot has been learnt.
“We’ve built a really great relationship with NIWA and are delighted with the results so far. We’re confident this technique works, so hopefully with more time and funding we can expand to larger areas and our kids can eventually swim in these lakes again.”
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