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A spate of toxic algae appearances in  has prompted warnings at  across the region.

On Thursday, Environment Southland released an alert saying harmful algae had been found at Whitestone River, near Te Anau.

It followed an alert the day before for a section of the Waihōpai River in Invercargill.

Katie Blakemore, Environment Southland senior scientist surface water quality, said algae was a natural part of a river’s ecosystem, but some species were capable of producing toxins.

Problems were more common in summer, when there hadn’t been much rainfall and river flows were stable.

“We can’t answer how long it will be around for,” she said.

“Largely a bloom could continue until we get a rainfall event that changes the flow conditions in the river.

“Flow is probably the most critical factor.”

In the meantime, Blakemore was encouraging people to check rivers before swimming or letting their dogs off the leash.

The council issued warnings for monitored locations, but conditions could change between visits and not all sites were checked.

Environment Southland senior scientist Katie Blakemore says dark green or black algae are the ones to watch out for. Photo / Environment Southland

Dark green or black algae spelled bad news.

Blakemore said the council asked the public to stay informed and check Environment Southland’s website for details of what to look out for.

There are currently three warnings in place for Southland — Whitestone River downstream of Manapouri-Hillside Rd (Fiordland), Waiau River near Excelsior Creek inflow (Fiordland), and Waihōpai River upstream of Queens Drive (Invercargill).

Last week, the council also revealed it was investigating reports of an algal bloom in Doubtful Sound, and whether it was connected to an unusual number of fish deaths.

• LDR is local body journalism co-funded by RNZ and NZ On Air