Lake Water Trophic Status
(Updated April 2013)
Trophic status of a lake is the degree of biological production within a lake - a key component of water quality. Trophic status is usually based on the total mass of algae in a lake, which is represented by the concentration of photosynthetic pigment (chlorophyll-a) in water samples.
Lakes with low levels of biological production are often clean and clear, supporting small fish populations. As biological productivity increases, more algae grows and the water can become murky; the density of large aquatic plants also increases with increased productivity. Decomposition of algae and plants can use up oxygen and stress the resident fish.
In lakes where biological productivity is extremely high, water quality can be impaired to the point where some recreational activities - including swimming and water-skiing - may not be advisable. Additionally, treating drinking water from these lakes is difficult and costly.
The map below illustrates the trophic status of Alberta lakes based on the average summer chlorophyll-a concentrations.
Trophic Status Categories
Oligotrophic lakes: contain low levels of phosphorus, which acts to limit biological production, meaning a lower algal biomass. This results in lakes containing low concentrations of chlorophyll-a. These lakes tend to have clear water and sufficient oxygen throughout the year to support fish and other aquatic organisms.
Mesotrophic lakes: contain moderate levels of phosphorus, which acts to support greater biological production. Greater concentrations of chlorophyll-a are therefore present due to increased algal biomass. Water clarity in mesotrophic lakes is moderate; there is an increased probability of oxygen depletion in the deepest areas.
Eutrophic lakes: contain even greater concentrations of phosphorus, resulting in greater concentrations of chlorophyll-a. These lakes tend to experience high densities of large aquatic plants (macrophytes) and algae, and support productive fisheries. Surface accumulations or 'blooms' of algae may occur during the warmest months, which can significantly reduce water transparency to the point of discouraging recreational activities such as swimming or waterskiing. Oxygen depletion in deep waters may occur throughout the year as a result of excessive microbial decomposition of macrophytes and algae.
Hypereutrophic lakes: contain very high concentrations of phosphorus and chlorophyll-a. Algal blooms are common in these lakes, often lasting throughout the summer and well into autumn. Oxygen depletion can occur throughout the year and may extend to the surface, resulting in fish kills.
What does the indicator show?
Some Alberta lakes have small populations of algae and large aquatic plants, while others have large populations. Water quality is impaired by increased biological growth in many lakes which limits their suitability for fish, recreation and drinking water sources.
Algae and aquatic plants require nutrients such as nitrogen and phosphorus for growth. To a large degree, concentrations of phosphorus are naturally elevated in many Alberta lakes. This is due to the nutrient-rich surrounding soils and underlying glacial deposits both a result of weathered sedimentary bedrock. However, phosphorus can also increase as a result of urbanization, industry and agricultural practices. Once in a lake, nutrients can remain available for algae and plants growth for many years.
Trend: No clear trend
What actions are being taken?
The Alberta government has monitored lake water quality since the 1940's. Today, several programs specifically monitor water quality in Alberta's lakes. Additionally, the government supports and works with the Alberta Lake Management Society, a non-profit organization promoting understanding and good management of Alberta's lakes, reservoirs and their watersheds.