Algae are simple photosynthetic plants that live either in water or damp areas. Algae are the primary producers of organic matter at the base of the food chain and they also provide oxygen for other aquatic life. Shoal Lake, Winnipeg’s water source, always contains various types of algae. When conditions are favourable, most often in July and August, the numbers of algae can increase dramatically creating a condition called a “bloom”. Algae blooms can produce an unpleasant taste and odour in tap water.

Dr. Margaret Fast, Medical Officer of Health with the Winnipeg Regional Health Authority says, "These tastes and odours may be distasteful, but algae normally found in Winnipeg’s water supply are not known to cause adverse health effects. Residents can continue to use the water, and there is no need to take special precautions."

Only a few dozen of the many thousands of species of algae produce a toxin that may be of health concern at high levels. Winnipeg water is routinely tested during the summer season for microcystin-LR, which is one of the toxins of concern. All the results of the samples taken during 2000 were below the detection level for the toxin. The detection level is well below the recommended Canadian Drinking Water Guideline. This year, monitoring for microcystin-LR started on May 4, and the results are still below the detection level. The monitoring program will continue until late fall.

Increased algae growth can also cause water treatment devices and water tap strainers and aerators to plug up more frequently. A build-up of sediment may also occur in hot water tanks. Residents are encouraged to perform regular maintenance on these appliances to ensure they continue to function satisfactorily.