Ministère de l’Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs
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Quebec Volunteer Lake-Monitoring Program


What is eutrophication?

Lakes age naturally and this evolution normally occurs over hundreds or thousands of years. This phenomenon called eutrophication is the gradual process of nutrient enrichment of a lake, as it changes from an oligotrophic state (nutrient-poor) to a eutrophic state (nutrient-rich). This enrichment enhances biological productivity, resulting in increased abundance of microscopic algae (phytoplankton) and aquatic plants. This increased productivity is associated with a change in lake characteristics such as a greater accumulation of sediments and organic matter, a reduction in dissolved oxygen, and the replacement of living organisms by species better adapted to the new conditions. Natural eutrophication can be accelerated by shoreline development and human activities in the watershed. These activities contribute to increased nutrient inputs in lakes. Premature aging is one of the main problems affecting recreational lakes and lakes located in agricultural and urbanized areas.

The process of lake eutrophication

The process of lake eutrophication

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How to monitor eutrophication?

Lake aging assessment is carried out by measuring nutrient concentrations in lake water, and by monitoring changes in water quality and biological communities over time. The most commonly used parameters are:

  • Total phosphorus, is the nutrient naturally scarce in lakes, thereby limiting the growth of aquatic plants and algae. When phosphorus becomes too abundant as a result of human activities, it stimulates the growth of plants and algae. There is a direct relationship between phosphorus concentration, lake productivity, and trophic level. Eutrophic lakes have high phosphorus concentrations.

  • Chlorophyll a, is an indicator of the biomass (amount) of microscopic algae present in a lake. Chlorophyll a concentration increases with nutrient concentrations. There is a relationship between increased chlorophyll a and the trophic level of a lake. Eutrophic lakes produce large quantities of algae.

  • Water transparency, indicates the amount of light penetration into a lake. It is measured using a Secchi disk. Transparency diminishes with increased lake algal biomass. There is a relationship between water transparency and trophic level. Eutrophic lakes are characterized by low water transparency.

    Water transparency may be significantly influenced by lake water colour. Dissolved organic carbon (DOC) gives a general description of the organic material dissolved in water as a result of decomposing plant or animal material. It is used to evaluate the presence of humic substances that cause the yellow or brownish colour of lake water. These humic substances are common in wetlands such as bogs, swamps and marshes. Water transparency, and hence Secchi disk readings decrease with an increase in dissolved organic carbon concentration. Thus, by measuring DOC on a regular basis, the influence of water colour on transparency results can be assessed. Suspended solids such as clay or organic particles may also reduce water transparency, especially in shallow lakes.

  • Dissolved oxygen concentration in the deep waters (hypolimnion) of a thermally stratified lake. Low dissolved oxygen concentrations are often related to large amounts of decomposing organic matter originating from dead organisms such as algae and aquatic plants. Therefore, eutrophic lakes often lack oxygen in the hypolimnion.

  • Abundance of aquatic plants in the littoral zone – the shallow transition zone between dry land and open water of a lake where sunlight reaches the bottom. The shallow water, abundant light and nutrient-rich sediments provide ideal conditions for aquatic plant growth. Aquatic plant abundance and areal cover serve as an indicator of a lake’s trophic level. Eutrophic lakes are generally abundant in aquatic plants relative to oligotrophic lakes.

  • Abundance of periphyton on rocks in the littoral of a lake. Periphyton refers to microscopic algae attached to submerged rocks, branches, wharf pilings and the surface of the sediment layer. The presence and abundance of periphyton increases with nutrient enrichment of lakes.

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How can eutrophication be assessed?

Evaluation of the tropic state of a lake is done in two ways. The first approach compares monitoring results with reference values or guidelines used to interpret the data. The second approach monitors the evolution of these measurements over time to detect signs of lake ageing.

Water quality parameters measured as part of the Quebec Volunteer Lake-Monitoring Program include total phosphorus, chlorophyll a, and water transparency. Dissolved organic carbon is also recorded in order to take into account the effect of water colour on transparency measurement.

Trophic levels are used to classify lakes according to their degree of biological productivity; their state may vary from highly oligotrophic to highly eutrophic. The evolution of a lake along the range of trophic level does not occur suddenly. Rather, it is a gradual ageing process, where changes are observed with increasing eutrophication. In order to determine the lake trophic level, water quality results are positioned on a trophic scale. This classification is made using reference values (see the following table and classification diagram) for phosphorus and chlorophyll a concentrations, as well as water transparency measurements. The reference values adopted for the broad trophic classes (ultraoligotrophic, oligotrophic, mesotrophic, eutrophic, and hypereutrophic) correspond to the best-known and used limits.

Lake trophic classes with corresponding values of total phosphorus, chlorophyll a, and water transparency1

Trophic Class Total Phosphorus (µg/l) Chlorophyll a
Main class Secondary class (transitional)  Average Average Average
Ultraoligotrophic   < 4  < 1 > 12
Oligotrophic   4 - 10 1 - 3 12 - 5
  Oligomesotrophic 7 - 13  2.5 - 3.5 6 - 4
Mesotrophic   10 - 30 3 - 8 5 - 2.5
  Mesoeutrophic 20 - 35 6.5 - 10 3 - 2
Eutrophic   30 - 100 8 - 25 2.5 - 1
Hypereutrophic   > 100 > 25 < 1

1 Summer average values, which correspond to the period during which thermal stratification occurs between upper and lower water layers in deep lakes.

Changes observed in the parameters used to classify lakes vary from one lake to another, mainly because of differences in physical and morphological characteristics. Signs of eutrophication also differ among lakes. The trophic status of a lake must therefore be interpreted with caution, especially when a lake falls into the transitional categories, oligomesotrophic or mesoeutrophic.

These two categories illustrate how eutrophication is a gradual process. Eventually, with the monitoring of several Quebec lakes, we should be able to adapt the current trophic classification diagram to better reflect the scale of variation in water quality parameters in our lakes.

Lake Trophic Classification Diagram

Lake Trophic Classification Diagram

To assess the trophic level of a lake, the Quebec Volunteer Lake-Monitoring Program considers three water quality parameters. However, since measurements are solely taken in the pelagic zone, i.e. the deepest part of the lake, signs of eutrophication in the littoral zone are not assessed. Thus, besides phosphorus, chlorophyll a, and transparency measurements, aquatic plant abundance in the littoral zone must also be taken into account.

Results from the first sampling season will enable a first assessment of the lake’s trophic status. However, since phosphorus concentrations, algal biomass and water transparency vary both in time (on a seasonal and an annual basis) and in space (sampling sites), the RSVL recommends sampling over a period of two to three consecutive years. This will allow the gathering of sufficient data to confirm or refine the lake’s trophic status.

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How do we measure parameters?

Water transparency is measured with a standard size Secchi disk (20 cm in diameter). Transparency is the depth at which the disk disappears from sight when lowered and reappears when slowly raised.

A 500-ml water sample is collected from the surface layer between 0 and 1 metre, using a decontaminated bottle. The collected water sample is then used to fill individual bottles provided for each parameter. Samples are kept cool until delivered to the laboratory, according to the conditions outlined in the water sampling protocol.

Both transparency measurements and water samples are taken at the sampling station(s) located over the deepest part of the lake.

Analyses of trace level total phosphorus, chlorophyll a (value corrected for pheophytin a interference), and dissolved organic carbon are carried out at the Centre d’expertise en analyse environnementale du Québec. The analytical methods summarized in the following table the can be found on the CEAEQ website.

Summary of analytical methods

Parameters Analytical method  Detection limit Sample preservation method Time required for analysis
Trace level total phosphorus (trace level TP) Mineralisation by persulfate method and determination by automated colorimetry adapted for trace state content in lakes
MA. 303 - P-T-Lake
Edition: 2018-04-10
0.6 µg/l Glass bottle (60ml) without preservatives 105 days
Chlorophyll a
(Chl a)
Fluorometry method
MA. 800 - Chlor. 2.0
Edition: 2022-01-05
0.05 µg/l Opaque polypropylene bottle (250ml) 48 hours
(≤ 10°C)
72 hours
Dissolved organic carbon
Infrared detection method
MA. 300 – C 1.0
Edition: 2016-02-26
 0.2 mg/l Plastic bottle (125ml) with a few drops of hydrochloric acid 28 days

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