A green sludge has emerged at South Australia's Lake Bonney after a levee was built to cut it from the swollen River Murray, leading health authorities to warn against swimming in the popular Riverland lake.
Locals who live near Lake Bonney in Barmera flagged on social media that the lake, which was popular for water sports, had started to turn green and stink.
SA Health has confirmed it is blue-green algae.
It comes after the decision was made by the Berri Barmera Council and SA State Emergency Service to cut off the lake from the River Murray in an effort to prevent properties becoming inundated and to protect wastewater infrastructure.
Lake Bonney joins the Murray via Chambers Creek and through Nappers inlet, but it does not have an outlet.
In a statement, SA Health said it was made aware of "very low numbers of green or blue-green algae" last week, but evidence provided on Monday indicated the presence of "higher concentrations".
The department said that "unless large volumes are ingested, the most likely health impact is skin irritation" and has advised people "not to swim in parts of the Lake with coloured surface films or scums".
Barmera local Tonni said shutting off Lake Bonney from the River Murray was the "wrong decision."
"It's starting to smell and people are staying away from it," she said.
"I wouldn't let my kids swim in it just for safety reasons."
The mother said she understood the lake was cut off amid the flood situation, but was worried about potential long-lasting environmental consequences.
"It's going to make it black water," she said.
"It's happened before and it's taken a long time to get it back to normal."
Blue-green algae is often caused by a lack of mixing water and ongoing warm weather conditions.
Government research agency CSIRO stated that if the algae contained species that release toxins, it may take "more than three months before they degrade".
SA Health said it was currently awaiting test results from samples taken at the lake to determine the species and concentration of the blue-green algae identified.
SA State Emergency Service chief executive Chris Beattie said the plan was to reopen Lake Bonney to the river "after the peak flow goes through".
"We are aware that the isolation of that lake from the main river flow will increase some environmental concerns," Mr Beattie said.
"We've got strategies in place to open up that temporary levee as soon as we practically can."
But Mr Beattie said that the Lake Bonney levee had proved beneficial.
"The measures taken by the department have shown that flooding that it's had into that lake already has massively reduced the amount of solidity."
The Berri Barmera Council has been contacted for comment.
