Algae problem perplexes

17:53, Dec 19 2013

Opuha Water's boss is perplexed by an outbreak of algae near an irrigation source but suspects it is due to silt.

The company manages the Opuha Dam, which provides water to 239 irrigators over a 16,000-hectare area.

Chief executive Tony McCormick said the algal outbreak affected "about a dozen" of its suppliers by blocking up the irrigation screens.

Although it might have happened in the past, Mr McCormick said it appeared to be a "new manifestation". "We're at a loss as to why it's occurred."

Mr McCormick said Opuha had been working with National Institute of Water and Atmospheric Research (Niwa) scientists to determine the most likely cause and solution.

"We had a couple of major floods in the last year, the most recent one was in July ... this has led to a buildup of silt in some parts of the system," he said.


"I think the warmer summer weather might have exacerbated it ... this is something we will need to address."

Mr McCormick said the main area of concern was near the Levels Plain irrigation scheme. Since the summer water quality monitoring season began last month, health warnings have been issued for recreational spots on the Temuka River near State Highway 1, Waihi River at Winchester and Waihao River below Bradshaws Bridge.

Mr McCormick said the algae discovered in the Opuha system did not seem to be toxic. "It's not didymo or phormidium [toxic black algae]."

Earlier this year, the company released a "one-off" flushing flow of about 40 cubic metres per second into the Opihi River in order to rid the area of a buildup of didymo just below the dam.

Opuha Water will soon file consents for a $2 million weir system which could allow for better flushing flows.

The Timaru Herald