Cause of algal bloom sought

The company in charge of the Opuha Dam is trying to find out why a potentially toxic algal bloom continues to plague the Opihi River.

People and animals have been advised to avoid the river near the Saleyards bridge at Pleasant Point, where a concerning level of the bloom was detected last week.

Environment Canterbury issued similar warnings in December, but Opuha Water chief executive Tony McCormick admitted the situation was disappointing.

"It's not something that is unique to the Opihi. There have been issues in the Waimakariri and Ashburton this summer. However, we would dearly love to understand what triggers it."

Mr McCormick said the company had commissioned the National Institute of Water and Atmospheric Research to do some modelling on the flushing flows required to mitigate the situation, as well as looking into the possible cause of the bloom.

"There's a lot of science that we need to obtain. We have more ability to influence the stretch above the confluence with the Opihi than we have on the area near the Saleyards bridge, but we have to start somewhere," he said.

The algal bloom, also known as phormidium, appears as dark brown-black mats that become attached to rocks along the riverbed.

Contact by skin or swallowing can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset, and other effects in humans. Dogs have also been known to become very ill or die as a result of coming into contact with it.

Temuka pharmacist Allan Campbell said people appeared to be more aware of the ill-effects of swimming in rivers where phormidium was present.

"In previous summers, we have had local children suffering from sores, rashes and mucky eyes. These days, I don't see many people swimming in the river. The local population are more aware of the risks, and tend to go to the Temuka River," he said.

However, Mr Campbell, who is a member of the Opihi Catchment and Environment Protection Society, said it was disappointing that the river was in this state.

"The river didn't used to have these problems; it was a lovely swimming and picnicking spot."

Mr McCormick said the Niwa study should take about six weeks to complete.

In the meantime, the company was pumping air into Lake Opuha in a bid to improve water quality while flows remain slightly lower than normal. It had also spoken to members of the protection society about its plans. "We all want to see an improvement," Mr McCormick said.

The Timaru Herald