The reservoir in the Irishtown Nature Park in Moncton is closed while floating algae is being tested.
The algae is potentially toxic and people should not drink, play or boat in the water until it is verified. As well, they should not allow their pets near the water.
The Irishtown Reservoir is the oldest in the city but is only used now for recreation, and has popular walking trails around it.
For the past few years, crews have been battling a problem known as algae bloom, and had to close the reservoir early last summer because of it.
The City of Moncton's Heather Hawker said the substance in the water is now changing colour.
"This week is starting to do, it's called a flip where it changes from a pea-green colour to a blue, and it'll look almost like jelly in the water and there's an awful smell … like a musty smell so that means the algae is starting to die off," said Hawker.
"So we close it now until we know what species of algae we're dealing with just as a precautionary measure."
Hawker said results should be back within a week.