Blue-green Algae

Blue-green algae are naturally present in lakes and streams. They are usually present in low numbers but can rapidly increase in warm, shallow, undisturbed surface water that gets a lot of sun. (Simcoe Muskoka District Health Unit)


One key contributing factor to the growth of blue-green algae is the amount of available nutrients such as phosphorus and nitrogen. Blooms can be caused by agricultural and storm-water runoff as well as leaching from septic systems. (Government of Ontario)


Your health may be impacted when surface scum or water containing high levels of blue-green algal toxins are swallowed, come into contact with the skin, or when airborne drops containing toxins are inhaled while swimming, bathing, or showering. (Simcoe Muskoka District Health Unit)


If you suspect a blue-green algal bloom (refer to Algae Quick Guide, below), it can be reported by calling 1.866.MOETIPS (1.866.663.8477)


How can I reduce the risk of blue-green algae?

  • Ensure that your septic system, or your neighbours’ system, is not leaching into the lake. If you are concerned about a particular septic system, please contact the By-law Officer, Township of Lake of Bays. 705.635.2272 ext 246 or
  • Do not feed waterfowl. Ducks and geese are contributing factors to an increase of phosphorus and nitrogen in the lake.
  • If you have a lawn, do not use fertilizers to enhance it. This also leads to increased phosphorus and nitrogen levels.
  • Do not add sand to the shoreline. It may contain amounts of phosphorus along with other negative effects (refer to the Healthy Waterfronts page, under the Environmental tab).
  • Remove grass clippings and leaves to prevent decomposition in and around the shore.
  • Do not allow pets to defecate in or near the lake. Pick-up and properly dispose of waste material.

Algae Identification Guides

Adobe Acrobat Document 1.4 MB
FOCA Blue-green Algae Fact Sheet.pdf
Adobe Acrobat Document 3.6 MB

Other Resources