How a Waikato farm wetland is improving water quality – and welcoming wildlife.
Constructed wetlands are typically built to improve water quality by filtering contaminants from farm run-off. But a recent study by the National Institute of Water and Atmospheric Research (NIWA) has revealed that they can also kickstart remarkable biodiversity gains by providing habitat for native plants and animals.
At Owl Farm in the Waikato, a purpose-built wetland designed to clean up agricultural drainage has become a thriving ecosystem. It’s one of five constructed wetlands in the region studied by Dr Brandon Goeller, NIWA Riparian and Wetland Scientist, who specialises in aquatic ecosystem health.
‘We found the constructed wetlands were useful for putting shallow vegetated and deeper, open-water pond habitats back in the landscape,’ says Goeller. ‘They provided habitat for invertebrates and fish, and places for birds to hide, nest, and feed.’
Native wetland plant species – such as raupō, kuta, pukio, umbrella sedge and kuawa – have taken hold and flourished at the Owl Farm site.
Across all five wetlands studied, the team identified an impressive array of biodiversity: 113 plant species, 20 bird species, five mammal species, 85 terrestrial invertebrates, 47 aquatic invertebrates, six fish species and two amphibian species.
Native species made up 96 per cent of the aquatic invertebrates found, including dragonflies and caddisflies, although most of the aquatic invertebrates were zooplankton, worms, and beetles.
‘Species that disperse quickly were some of the first to appear. We observed pūkeko, paradise shelducks, mallards and tūī,’ Goeller says.
A stand-out find was the presence of fernbirds – a native species dependent on wetlands – at Owl Farm. Just 100 metres from the Waikato River, the wetland created a pocket of suitable habitat that may not have existed otherwise; however, the return of native species also came with challenges.
‘We found predators like rats, feral cats and possums using the wetlands, too,’ Goeller says. ‘If we want to maximise the benefits for biodiversity, pest management will be essential.’
The study concluded that these constructed wetlands are doing far more than filtering farm run-off; they’re acting as ecological stepping stones, helping connect fragmented habitats and supporting the re-establishment of native species across agricultural landscapes.
‘Besides improving water quality, these wetlands help restore natural ecological networks,’ Goeller says. ‘Designing them for a dual purpose – water quality and biodiversity – has real environmental pay-off.’
Constructed wetlands can also reduce nitrate contamination, lower flood risk and provide vital spaces for native species. NIWA has developed guidance for designing and constructing new wetlands for treating agricultural runoff, and this will help maximise other co-benefits.
For more information, including NIWA’s guidance on how to design or construct a new wetland, visit https://niwa.co.nz. To read the full scientific paper by Dr Goeller et al, visit www.mdpi.com/2073-4441/15/14/2526. This work was part of a broader research project looking at the ability of constructed wetlands to remove nitrates (Hoang et al, 2023, ‘Owl Farm nitrate removal study’, www.mdpi.com/2073-4441/15/9/1689).