The latest round of water quality testing at Lake Hood in Canterbury showed a higher algae level than when the health warning was put in place last week.

Environment Canterbury testing recorded a total cyanobacteria biovolume of 10mm3/L at the water ski lane on January 8.

That result triggered a health warning for the presence of potentially toxic harmful blue-green algae (planktonic cyanobacteria), but the lake has remained open.

Testing on January 15 had the level double to 20mm3/L.

Ashburton District Council infrastructure group manager Neil McCann said while ECan testing focuses on the worst site, all 10 test sites have shown an increase in cyanobacteria biovolume.

The type of cyanobacteria present in high concentrations is Dolichospermum (formerly called Anabaena), which can be present as cloudy, discoloured water and/or green globules floating in the water column or form surface scums or sheens.

It’s the fourth straight summer that algal blooms have triggered a health warning.

Ashburton District Council, which took over management of the lake in June 2024, is investigating potential solutions to the algal problems.

The council has called in the leading cyanobacteria scientist in New Zealand, Lincoln University-based Professor Susie Wood, to lead the investigative work towards a solution.

That work started in December.

Six shipping containers have been placed in the canals for a pilot trial of treatment options to stave off algae blooms.

The scientists returned to Lake Hood this week to set up the site for a full scientific study of the products, which will start in February.

McCann said the pilot trial “allowed us to get to know the testing environment” and the second phase will go for around three months.

“We’ll also be installing sonic devices in other spots to see how they can mitigate the growth of cyanobacteria.”

The sonic equipment will float in the canals and be marked by buoys.

McCann said the council is grateful to the scientific team and the work it was doing, which was not being funded by the council.

“We are supporting with operational assistance, like putting the containers in the canal, but the costs associated with the scientific study, like testing water and sediment samples, is being covered by the Canterbury and Lincoln universities and the Our Lakes, Our Future project.”

He said the council had spent around $12,000 on the trials, which included purchasing the containers.

The scientific study is part of a national collaboration by many research organisations to improve the health of New Zealand lakes, and is part of the Our Lakes, Our Future project.

“While they’re doing on-the-groundwork over the next three months, we’ll be allowing water to flow into and out of the lake as usual, the weed harvester will be operating, and we’ll be continuing our regular water quality updates.”

The health warning on the lake will not impact the trials, he said.

The lake was closed last year because the toxic cyanobacteria present was Microcystis and is known to be harmful, and the two substantiated cases of people falling ill from exposure to the lake water, he said.

“These same conditions are not currently present, so the lake remains open, however we’ll review that status as required,” he said.

By Jonathan Leask