Over the last five years, Te Roto o Wairewa has shown signs of improvement, with improved water quality, which has enabled macrophytes (aquatic plants) to flower for the first time in many years. The species that has flowered in the lake is Myriophyllum triphyllum, which is known to have high aesthetic, wildlife, and fisheries value. Tim Davie, Environment Canterbury Surface Water Science Manager, said the growth is a sign of a stable lake environment with improving water quality. “Macrophytes grow in lakes along the margins where their roots can reach the lake bottom but in shallow lakes such as Wairewa they can grow almost anywhere.
“They are generally seen as an indicator of a healthy environment because they require good light penetration in the water column and a stable lake level.
“It is a positive sign that they are flowering because they provide shade and habitat for fish, they soak up nutrients, offer food for wildlife, and bring oxygen to the bottom of the lake which helps maintain a stable chemistry. The fact that macrophytes have re-established in the lake is a reflection of the active management of the lake by Wairewa Rūnanga in conjunction with Christchurch City Council and assisted by the Banks Peninsula Zone Committee.
Over the years changing sea levels, the growth of Kaitōrete Spit and changing land-use in the catchment have all contributed to alterations in the lake form.
The changes in the lake have increased the nutrient content in the sediment and water, namely nitrogen and phosphorus. This increase in nutrients along with a shallower lake has made the lake highly eutrophic, degraded the water quality, and caused algal outbreaks in summer which produce toxic bi-products.
Through the use of controlled openings and closings of the lake to the sea (at Birdlings Flat) the lake has been able to be kept at a higher, more stable level over the last three summers, with less salt water incursions, thus the salinity level has decreased as well.
Richard Simpson, chair of the Banks Peninsula Zone Committee, said the committee supports Wairewa Rūnanga in its aspirations for a flourishing lake ecosystem in Wairewa; the largest freshwater body on Banks Peninsula. “The work of local rūnanga with the Christchurch City Council has undoubtedly contributed to the improved lake environment and the re-emergence of macrophytes in the lake. The Banks Peninsula Zone Committee actively supports the aspirations of Wairewa Rūnanga through our Zone Implementation Programme, which was developed with the community to deliver water management goals for local fresh water.
“We are also working towards developing a sub-regional chapter to set water quality limits for the lake, which will feed in to Environment Canterbury’s proposed Land and Water Regional Plan and provide the statutory framework to help deliver Wairewa Rūnanga’s aspirations for the lake. The committee also supports local landowners across all of Banks Peninsula to reduce the environmental impacts of landuse on water quality and improve biodiversity values through Immediate Steps Biodiversity funding,” he said.
Wairewa Rūnanga continue to work with the local community, CCC, ECan and Banks Peninsula Zone Committee on the lake and the issues around the lake. We are still working with CCC on the joint consent for the opening of the lake, along with the issues that CCC have advised us of with regards their legal position not to reinstate the causeway across the canal.