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Eutrophication is synonymous with natural aging of lakes. Lakes have a life expectancy of several thousand years. They are born with clear water and silt up progressively. Over time, they transform to become ponds, bogs and eventually disappear as grasslands.

At present, the aging of lakes has greatly accelerated due to human activities. So much so that, instead of aging over a period of thousands of years, we will now be able to witness the changes over the course of one lifetime.

Figure 1 represents the natural eutrophication process versus the accelerated process caused by human activities.

Figure 1: Eutrophication of lakes

Eutrophisation des lacs

Depending on their degree of eutrophication, lakes are classified in one of the following trophic levels:

An oligotrophic lake is said to be young and low in nutrients. It is rather deep and its water is clear, with good transparency in summer. There are low levels of phosphorus. Vegetation grows at different depths, not only on the surface. Its water is of good quality, with few aquatic weeds and no microscopic algae problems. In summer, an oligotrophic lake has high concentrations of dissolved oxygen in the deep, where water remains cold. These are favourable conditions for the presence of trout, whitefish and other species popular with anglers.

Mesotrophic lakes find themselves in the middle, between the two extremes that are oligotrophic and eutrophic lakes. Their levels of transparency, concentrations of nutrients and dissolved oxygen are between those of oligotrophic and eutrophic lakes. Perch is a characteristic species of mesotrophic lakes.

Eutrophic lakes are made up of warm, shallow waters that are turbid and murky.  The concentration of dissolved oxygen decreases in depth, rendering the environment inhospitable for fish like trout. The dominant species found here are warm-water species such as bass, carp and catfish, which are more tolerant of warmer temperatures and low levels of oxygen. The eutrophic lake’s capacity to produce algae and aquatic plants, combined with the lack of oxygen in the deep, assures that the organic matter produced in abundance, far exceeds the assimilative capacity of the lake. Its inability to assimilate or recycle the biodegradable matters results in an accumulation of debris, which further reduces its oxygen reserves.

To find out more about eutrophication, please visit the following website :

Blue-green algae (cyanobacteria)
Blue-green algae (cyanobacteria) are aquatic micro-organisms that may, under certain conditions, proliferate excessively. Generally, they multiply in summer, in waters that are rich in nutrients. When there is an abundance of them, they form ‘’algal blooms’’ or even foam, that are visible on part or all of the body of water. The water might take on the appearance of green paint or pea soup.

To find out more about Blue-green algae, please visit the following websites :

Actions to slow the process of eutrophication and decrease the occurrences of cyanobacteria
To slow down the process of lake eutrophication and decrease the occurrences of cyanobacteria, we must first reduce our use of phosphorus and limit its infiltration into our waterways. This job requires the collaboration of all, given that phosphorus originates from point and non-point sources throughout the entire watershed basin.

The following are some practices that will help reduce the levels of phosphorus in our lakes.

Citizens, merchants and businesses

  • Restore all lake and watercourse shorelines to their natural state.  In order to achieve this, you must stop cutting the grass on the shore (over a width of 10 to 15 metres, depending on the slope) so as to allow shrubs to grow there naturally.  You must also minimize the removal of vegetation by developing only one access to the water per property.
  • Do not spread fertilizer on the shoreline
  • Stabilize any eroded shores by planting shrubs or by consulting experts who can recommend environmental engineering techniques.
  • Keep your septic system in good working order and address any deficiencies thereof, to ensure that it does not become a source of surface water pollution.

Citizens engaging in agricultural activities and/or owning farm animals 

  • Restore to a natural state the shorelines of waterways that flow into lakes
  • Do not spread fertilizer on the shoreline
  • Prohibit livestock access to watercourses
  • Stabilize any area of the shore or ditches suffering from erosion
  • Continue to adopt sound agro-environmental practices


  • Inform and encourage citizens to protect lakes and watercourses (through articles, briefing papers, meetings, lectures, road signs, etc.).
  • Plan interventions aimed at protecting shorelines (e.g. naturalization of municipal shorelines) and improving the quality of waters that flow into lakes (through filtration and water retention works).
  • Enforce bylaws that serve to protect the shorelines and littoral zones of lakes and watercourses.

Document for download :
Living at the water’s edge