Moncton's Tower Road reservoir is playing host to some unwelcome guests.

Daily water sampling has revealed blue-green algae in the Greater Moncton area's secondary drinking water source – a potential result of low, stagnant water and high, early fall heat.

“It's very, very important for people to understand the water is safe to drink because it does not come from the Tower Road reservoir. Our drinking water supply today comes from Turtle Creek,” says Isabelle LeBlanc of the City of Moncton.

But some residents are worried about what could happen if the secondary source of water is needed.

“It should have been one of the priorities to make sure the backup reservoir was going to be useable,” says resident Donna Hunt. “I'm a little concerned with the fact that it's blue-green algae. It's something we depend upon.”

Experts say the blooms are formed by excessive nutrients in the water, caused by things like agricultural or storm water runoff.

Microbiology professor Doug Campbell says algae blooms are becoming more prevalent in North America, and they're much easier to prevent than get rid of.

“Once these toxins are produced, they're fairly persistent in the water,” Campbell says. “They aren't necessarily destroyed by boiling, they aren't necessarily removed by water treatment. They can be, but ordinary water treatment wouldn't always get rid of them.”

While blue-green algae is present in all bodies of water, elevated levels can be toxic to humans and animals. Water samples from earlier in September show the algae levels are still safe by Health Canada's standards.

The provincial Department of Environment ran thermal and aerial imaging on both of Moncton's reservoir as it addresses the situation.

In the meantime, the water supply in Moncton, Riverview and Dieppe is safe for all use.

With files from CTV Atlantic’s Cami Kepke.