A risk advisory was issued on Tuesday at Lake Micmac in Dartmouth; however, officials aren't certain the water is toxic yet.

Signs cautioning those hoping to swim at the lake went up shortly after potentially-hazardous algae were spotted.

"Right now, what we've seen is something in the lake that looks like blue-green algae," says HRM environmental performance officer, Cameron Deacoff. "We don't know if it is for sure or not. It's impossible to assess that correctly just with your own eyes."

Testing is ongoing, with first results expected by Friday morning. Until then, officials are urging everyone – pets included – to stay out of the water.

While the risk advisory might come as a surprise to those who enjoy the lake, experts say algae is to be expected when temperatures are warmer.  

"Once you hit over 20 degrees, then you're always out weekly doing your monitoring and checking for visual signs of algae," says natural resources program coordinator for the City of Moncton, Heather Fraser.

And the lake's algae situation isn't a rare occurrence in the Maritimes. Recently, three popular lakes in Moncton were closed after blue-green algae was confirmed. Additionally, the organisms are also being blamed for the death of a dog in Fredericton.

Experts say there are four known species of blue-green algae that can poison the water. They understand the mechanics of the process, but not all the biology.

"Sometimes they go toxic, sometimes not," says retired marine ecologist, Herb Vandermeulen. "When the blooms have peaked, or just started to peak, and they're starting to crash; that seems to be when they release it, but the reasons why they release it is not really clear."

Meanwhile, HRM reminds the public that Lake Micmac isn't closed to anyone wanting to visit – they have only put an advisory in place. If their tests do not find toxins in the water, the advisory will be lifted; however, if toxins are present, the advisory will stay in place until test results find no presence of toxins in their samples.

With files from CTV Atlantic's Bruce Frisko