Lake Hood will remain closed to water sports as cyanobacteria levels remain too high, despite earlier hopes of reopening the popular site.

Environment Canterbury’s latest water test recorded 2mm³/L of total cyanobacteria biovolume. According to Health New Zealand guidelines, two consecutive weekly results below 0.5mm³/L are required before a public health warning can be lifted.

Ashburton District Council Group Manager of Infrastructure and Open Spaces Neil McCann said the result was disappointing.

“The previous week’s test was under 0.5, and with the colder weather recently, we were really hoping for another low reading,” he said.

The lake was closed in April following a public health warning and two reported cases of illness linked to contact with the lake water.

McCann said significant work had taken place over the past six months to improve water circulation, especially in the ski lane and canals.

“It was disappointing we had to close the lake, but the primary concern of Health New Zealand, Environment Canterbury and Council has to be the health of people on or in the water.

“We are already looking ahead to what we can do to manage or mitigate algae levels in coming summers.”

Council is working with a Lincoln University scientist with extensive experience in cyanobacteria and is being supported by NIWA to better understand the lake’s dynamics.

A hydrodynamic model is also being developed to simulate water circulation, which McCann said is key to understanding how nutrients, sediment and algae accumulate.

Internationally recognised cyanobacteria expert Professor Susanna Wood is reviewing proposed treatment options and will trial some solutions in the lake’s canal system.

“There is a lot to learn about cyanobacteria and how it behaves in this particular lake,” McCann said. “It will be a matter of finding a way to manage algal growth in the warmer months, using all the various operational tools we have.”