Quebec Volunteer Lake-Monitoring Program
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
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:
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.
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.
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
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 VLMP 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.
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.