The report was commissioned to provide information on the nutrient reductions required to achieve water quality targets set by the community in the Land and Water Regional Plan (LWRP). The conclusions from the Cawthron Institute found that significant reductions in nutrient loads will be required to preserve water quality in the high-country lakes studied.
The lakes are sensitive to nutrient inputs from several sources, including land use, recreation amenity and birdlife.
‘Sensitive’ lakes have ecological and cultural importance
Several of Canterbury’s high-country lakes are recognised as being ‘sensitive’, with specific provisions in the LWRP included to protect them. In addition to having high ecological values these lakes are recognised in statute as culturally significant historical food gathering and mahinga kai sites.
In response to concerns at the state of these lakes, three rūnanga – Te Runanga o Arowhenua, Te Ngāi Tūāhuriri Rūnanga and Te Taumutu Rūnanga – initiated a working group in 2019 of statutory agencies (Environment Canterbury, DOC, Toitū Te Whenua Land Information New Zealand, Ashburton District Council and Fish & Game) along with local farmers and other resource users to address this issue.
Actions discussed by working group
The newly-released report from the Cawthron Institute was discussed in a meeting of the cross-sector working group on Monday 26 July. All members of the working group reiterated a commitment to the development of solutions to the deteriorating lake water quality.
Several new actions were discussed, including:
- taking catchment level information to develop farm-scale options
- new requirements for sewage systems at Lake Clearwater village, including monitoring the effectiveness of new systems, and
- further work on quantifying the impacts of birdlife on different lakes.
Reduction in nutrients needed to make improvements
Environment Canterbury director of science Dr Tim Davie said: “We know how special the Ashburton Lakes Ō Tū Wharekai are to the people of Canterbury and it’s clear from the report that the water quality of these lakes is in a delicate state.
“Most of the lakes studied require a significant reduction in nutrients to meet the Land and Water Regional Plan limits. Some of the lakes are at risk of ‘flipping’, which means entering a long-term turbid, algal-rich, degraded state, from which lakes usually do not recover easily.
“Lakes are often accumulators of nutrients and legacy issues can persist for years after nutrient loads are reduced. Climate change is likely to further increase this vulnerability.
“This means that to make any positive improvements there will need to be substantial and urgent action in the catchment. We’ve already made a start with setting up the working group and – now that we have clear scientific data – we need to focus on actions to protect these precious lakes.”
Dr Davie added that the science also shows there is not one single cause of degradation.
“While farming land use is a contributor, so are the impacts of amenity users and birdlife. The situation is further complicated by differences between each of the lakes. There is no single solution that will address these issues.”
The lakes studied at Ō Tū Wharekai were Heron, Emily, Clearwater, Camp, Emma, Denny and two of the Māori Lakes.
Diverse ecosystems sensitive to nutrient enrichment
DOC aquatic director Elizabeth Heeg said DOC is concerned about the impact of declining water quality on the ecological values of the lakes and streams of Ō Tū Wharekai.
“This area contains a diverse range of ecosystems that provide nationally and regionally important freshwater habitats for threatened and at risk native birds, fish, and invertebrates. Many of these freshwater habitats and species are sensitive to nutrient enrichment,” Heeg said.
“We are committed to working closely with Environment Canterbury, mana whenua and landowners to protect the ecological values of these high-country lakes and streams, and implement a much-needed catchment-scale response, to halt and reverse declining water quality.”
The Cawthron Institute’s report also studied five lakes in the Selwyn district – Lyndon, Grasmere, Pearson, Sarah and Hawdon – finding that these lakes also require moderate or large nutrient load reductions to meet plan objectives.
Lake nutrient load reductions are therefore not unique to the Ashburton lakes/Ō Tū Wharekai and may be needed in many high-country lakes across Canterbury.