The lake at Irishtown Nature Park is closed to recreational use due to blue-green algae.
The City of Moncton says the bloom is not visible but testing has revealed a high concentration of toxins in the water.
Kayaking, canoeing and other recreational activities are suspended until further notice.
Pet owners are asked to keep their dogs away from the water and not let them drink or swim in the water until further notice.
Signage has been posted throughout the park.
The City says updates will be provided as conditions change.
