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Glossary of terms


Water quality parameters


The quality of the water in the Lake Memphremagog Watershed has been the subject of several different research programmes, and has been described in numerous publications.

There are many ways by which the quality of the water in the Memphremagog ecosystem can be described, but the information obtained through numerous studies shows a very disquieting picture: Lake Memphremagog is undergoing accelerated eutrophication, or aging. Sediments and phosphorous are accumulating, chiefly from diffuse sources in the watershed from both sides of the Canada-U.S. Border.
A healthy aquatic ecosystem is important because of the many benefits it provides. Lake Memphremagog's ultimate service is as a reservoir of quality potable water for 160,000 users. It is also a prized location for bathing, boating and fishing.

What contributes to a healthy aquatic ecosystem?

First, a healthy water ecosystem is in equilibrium - its components interact to favour balance. To determine if the system retains its balance, one must gauge the quantity of each of its components, using several parameters which are easily measurable, but whose measurement is not influenced by the observer. Some observations indicate the presence or absence of a certain component; others measure its importance. These latter parameters are qualitative. However, to judge equilibrium more exactly, the best gauges are quantitative.

What are the qualitative physical and chemical parameters that are most often employed?

Qualitative parameters such as conductivity, transparency and UV spectrum are frequently used. The tools to measure these parameters permit a rapid gauge of the chemical and physical balance to determine if the water is normal or not, at a relatively low cost.


Conductivity is the capacity of water to allow the passage of an electric current. Pure water is a poor conductor – it is the dissolved elements and suspended particles which give it conductivity. Therefore, if conductivity is high, there is a high level of suspended and dissolved matter in the water.

Water transparency

The water molecule itself diminishes light that passes through it, but again it is the suspended and dissolved matter that most reduce its passage. Transparency is very easy to measure with the use of a Secchi Disk. However, this measurement is only useful as a measure of the ecosystem over time, to monitor changes in transparency.

RAPPEL 2005. Technical document in French

UV Spectrum

By measuring the form and amplitude of ultra-violet light passing through a water sample, one can determine the content of dissolved and suspended matter. This technique can also determine the concentration of materials such as phosphorous, oxygen and pesticide residues.

RAPPEL 2005. Technical document in French

What are the quantitative physical and chemical parameters most often employed?

Quantitative physical and chemical parameters such as the concentrations of phosphorous, oxygen, hydrogen ions (H+) and chlorophyll a, as well as suspended matter, are most often used to evaluate ecosystem health. Their concentrations are measured in water samples from the ecosystem under study. When in doubt, other tests could include concentrations of pesticide residues or hormone disrupters. To determine the total load of any substance in a body of water, the concentration measured must be multiplied by the flow volume.

Concentration of phosphorous

The element phosphorous is present in all aquatic ecosystems and is important to its health. However, when its concentration is too high it can unbalance the ecosystem and promote the growth of aquatic plants, algae and cyanobacteria. An abnormal level of phosphorous is a symptom of accelerated eutrophication, or aging of the water body. In northern fresh water ecosystems such as Lake Memphremagog, phosphorous is the element which controls the growth of its vegetation. Phosphorous can only be used by plants in its organic form, such as phosphate. As phosphates are usually absorbed quickly and their concentration difficult to measure, preferably all forms of phosphorous are measured to obtain a better idea of the phosphorous available for aquatic plant growth.

RAPPEL 2005. Technical document in French

Concentration of oxygen

All living organisms need oxygen. Fish, insects and microorganisms living in the water use the oxygen dissolved in water. The more organic matter digested by aquatic microorganisms, the more oxygen they require and this can lead to a reduction in the dissolved oxygen available in the water – to the point that there is not enough for fish to survive.

RAPPEL 2005 Technical document in French


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