Lake Benmore studied in depth

The water in Lake Benmore is going under the microscope in a study that will help set parameters for future irrigation and land development in the Omarama and Mackenzie basins.

The water quality study, the first of its kind to be carried out in the Canterbury region by Environment Canterbury (ECan), has already started and results are due out in June.

The study is being conducted by the National Institute of Water and Atmospheric Research (Niwa) and the results will produce a water quality planning model for the lake which may be applied to others in the catchment.

It will identify the existing water quality and provide a guide to the level of nutrients, particularly nitrogen and phosphorous, the lake can take without affecting its values, including recreational, aesthetic and ecosystems.

A study last year identified Lake Benmore as the second most-fished lake for trout and salmon in New Zealand behind Lake Taupo and it is also a popular holiday destination.

The planning model will be used as part of assessing future applications in the catchment for irrigation and land intensification, including resource consent applications ECan will consider later this year.

"This investigation will provide critical information that will be of benefit for the long-term management of water quality in the Waitaki lakes," ECan director of investigation and monitoring Ken Taylor said.

It was the first time ECan had undertaken a project to model and assess the degree of acceptable nutrient loading on a lake.

The project involves gathering water quality data for use in running the models.

Niwa had accessed historical data and had been monitoring water quality at three sites in Lake Benmore and one site each in Lakes Aviemore and Waitaki each fortnight from the beginning of December to the end of March.

A second element, to take place shortly, is setting up and monitoring large enclosed plastic bags of water moored in the lake to determine the effects of nutrients on algal growth.

The model will help determine whether the lakes will grow additional algae and, as a result, change colour or become more turbid.

It will also look at whether water deep in the lake could become oxygen-depleted as a result of additional algae growth decaying on the bottom.

The results will determine acceptable levels of nutrients in the lakes.

Data gathering is expected to be completed by May. Preliminary results will be discussed with interested groups in June.

Lake Benmore is the only lake where the full study is being conducted, with Lakes Aviemore and Waitaki being monitored.

Mr Taylor said Lakes Tekapo, Pukaki, Ohau and Ruataniwha were not being monitored because it was considered they were not at risk of being exposed to the effects of significant land intensification.

- david.bruce@odt.co.nz