No quick fix for 'unsightly' but non-toxic Rotokawau Virginia Lake algal bloom

Whanganui District Council says testing of an "unsightly" algal bloom at Rotokawau / Virginia has shown it to be non-toxic. Photo/ Bevan Conley

An "unsightly" dark green algal bloom covering Rotokawau Virginia Lake has sparked concerns about the lake's health and the impression it leaves with visitors to Whanganui.

But Whanganui District Council is hoping it will fix itself naturally as winter approaches.

The thick film which has settled at the southern end has prompted calls to the council and discussion on social media.

A letter in Tuesday's Chronicle said "as a Whanganui icon and a place visited by tourists the health of the lake is disgraceful and a poor reflection on our city".

"Something needs to be done immediately."

Testing has revealed the algae is Microcystis wesenbergii, which the council says is non-toxic and likely to "resolve naturally as we move into the winter season".

"Algal blooms are increasingly common throughout New Zealand, especially during the warmer seasons," council chief executive Kym Fell said.

"We expect the algae that has accumulated at Rotokawau Virginia Lake this year is likely to disappear with cooler weather and lower light levels along with more rain which will help to flush out the lake area."

Ultrasonic units which the council installed under water to help break down algae were not working properly and would soon be replaced, Fell said.

Residents says a thick green algal bloom on Rotokawau/Virginia Lake is a bad look. Photo/ Bevan Conley.

"We are doing everything we can, but ultimately we are dealing with a natural process, which is exacerbated by warmer temperatures.

"It happens every year at Rotokawau Virginia Lake and throughout the country."

Fell said this particular type of algae easily formed thick surface scum in calm conditions which looked unpleasant and could smell bad near the lake edge.

In the past, the potentially toxic Cyanobacteria, or blue-green algae, had been found the lake.

"Fortunately this particular type of algae is non-toxic."

The Chronicle has asked council if any other mitigation is planned or if it has done research into water quality and other factors which may be causing algal growth.