Swimmers are being asked to avoid Dartmouth's popular Lake Banook due to a possible blue-green algae bloom. A release from the Halifax Regional Municipality Sunday afternoon announced a risk advisory was in effect for Birch Cove Beach, and people were asked to avoid swimming there until further notice. City staff are currently investigating whether or not it is a toxin-producing algae, the release said. Blue-green algae naturally occurs in freshwater environments and can multiply rapidly during the summer, leading to extensive growth called a bloom. Some types of this algae produce toxins during blooms and those toxins may leak into the water when the bloom fades, the release said.