Heavy rain hoped to lower algae levels

Toxic algae has been detected in multiple Southland rivers, including the Oreti river pictured...
Toxic algae has been detected in multiple Southland rivers, including the Oreti river pictured here. PHOTO: ODT FILES
There is some good in heavy rain after all.

Abundant rain during the past couple of days should give some benefits for Southland rivers affected by toxic algae.

Environment Southland’s surface water quality senior scientist Katie Blakemore hoped the high river levels would remove some of the algae present.

However, how much would not be known until flows receded, she said.

"High flows are effective in physically removing algae from rivers and streams.

"The amount of flow that is required to dislodge and transport algae varies by site due to differences in in-stream characteristics and hydrology.

"Any algae removed could be transported by the river flow, and so it may be transported downstream or deposited as the water level recedes but would not spread to other water bodies."

Algal growth was driven by several factors including nutrients, light and temperature.

Therefore, the response time for algae to re-establish following high flow events varied, she said.

"Generally, algae growth is greatest over summer when temperatures are highest, daylight is longest and flows are often at their lowest."

This year the bacteria had been detected in the Waihopai and Oreti rivers, Mataura River at Gore and Waiau River at Tuatapere.

The cyanobacteria’s toxic algae grows in the bottom of riverbeds and looks like thick dark brown or black mats and is potentially fatal to dogs and animals — taking between 15 minutes to an hour from exposure to kill the animal.

While the bacteria had been found close to Southland townships, water supplies have not been affected as supplies are drawn from bores.

Land Air Water Aotearoa information describes how the bacteria is present in rivers most of the time but at a low level.

Blooms become more common during low-flow periods, but the conditions in which they can become toxic are not well understood.

Laboratory testing was necessary to determine toxicity levels.

Ms Blakemore said stable flow was the most significant factor in controlling toxic algae growth.

Despite the high rainfall over the last 48 hours there has been limited flooding and rivers were not particularly impacted.