Blue-green algae blooms in parts of Lake of the Woods can change the appearance of the water and these blooms have the potential to cause some harm to pets or people who choose to take a drink or a swim. Blue-green algae blooms are common in parts of the lake, which straddles Ontario, Manitoba and Minnesota. However, they are generally not seen late in the year or in some of the locations they are now being found. As well, the blooms can be toxic.