The high levels of toxic algal bloom in some South Canterbury rivers are likely to remain as long as the dry weather continues.

Public health warnings are in place for the Opihi River at State Highway 1 and just downstream at Waipopo, the Pareora River near Pareora Huts and The Black Hole, a popular swimming spot on the Waihao River.

Some warnings have been in place for at least a month.

However, Environment Canterbury water quality scientist Dr David Kelly said until flows increased, the situation was unlikely to change.

"We've had a very long period of dry, warm weather, with low flows. We've had a few blips, but not enough. Until there is more rain, I can't see it moving too much," he said.

Dr Kelly said ECan was co- funding a doctorate study with the Cawthron Institute and Victoria University on the effects and causes of toxic algal growth in rivers.

"The research has suggested a number of factors. Lack of flows is one, but increased nutrient levels could be another. It's very difficult to disentangle the effects," he said.

"Obviously we should be concerned about any individual warnings, because the algal blooms could have consequences for people's health."

Canterbury medical officer of health, Dr Daniel Williams, has said exposure to toxic algae could cause skin rashes, nausea, stomach cramps and tingling and numbness around the mouth and fingertips.

People and animals, particularly dogs, should avoid all affected areas until the warnings have been lifted. No-one should drink the water from the rivers at any time.

Cawthron Institute scientist Dr Susie Wood recently said that land-use intensification was a contributing factor in the spread of toxic algal blooms.

Dr Kelly said there has been increased focus on toxic algal growths over the last few years, as it was only recently that the Environment Ministry issued guidelines on it.

Last week, the Opuha Dam company released about 40 cumecs that flushed out some of the algae, but the flow's force would not have been enough to have a noticeable effect on the Opihi River.

- © Fairfax NZ News

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