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Dissolved Oxygen: Key Indicator of Aquatic Health

Late Spring, mid-summer and early fall each year, Environment Council volunteers visit specific deep-water sites on Ston(e)y and Clear Lakes and lower the probe of a Dissolved Oxygen (DO) meter into calm water. The probe measures temperature and DO levels at one-metre depths in deep water to a depth of 0.5 meters from the bottom of the lake. Collected data are forwarded to our consultant at the Ministry of the Environment, Conservation and Parks (previously the Ministry of Environment and Climate Change).

Dissolved oxygen is the amount of oxygen that is present in water. Water bodies receive oxygen from the atmosphere and from aquatic plants. Running water, such as a swift moving stream or strong lake current, contains more oxygen than the still water of a pond or calm lake. All aquatic animals need DO to sustain life/to breathe. When oxygen levels are low or completely diminished aquatic life is threatened; e.g. fish may move to other habitat, suffer in poor health or die. DO is a significant indicator of our lakes’ ability to support aquatic life.**

* Other factors such as phosphorus, clarity and E.coli bacteria are important indicators. The Kawartha Lake Stewards program monitors these factors on a yearly basis.


Monitoring DO: How data are collected and used 

Certain amounts of DO are lost when microorganisms in the water use DO in the process of decomposing organic matter such as large algae blooms. This is most apt to occur during the summer season. DO levels fluctuate between spring, summer and early fall seasons, and may also change on a daily basis dependent upon the ecology of the water body.

Generally, DO levels below 3 milligrams per liter (mg/L) are of concern and waters with levels below 1 mg/L are considered to have very little or no oxygen, and are usually devoid of life. (It’s important to remember that each aquatic organism has its own oxygen requirement.)


Monitoring DO requires sampling during each of our three seasons, optimally over a period of years. Water quality data are reported to the Ministry of the Environment, Conservation and Parks. The data provide important trends over time reflecting changes in the lakes’ temperature and capacity to support life. These measurements are particularly important as we now see fluctuating weather and temperature throughout our seasons. Information is used to project hot spots – lakes in need of additional assessment or intervention (Ryerson University is currently conducting research on this topic).

The data are reviewed by biologists with focus on changes in our lakes’ oxygen content which may signal threats to fisheries and other organisms. These observations are very useful in supporting our attempts to curb inappropriate development and promote best management practices and municipal due diligence in preventing increased nutrient run-off to the lake ecosystem.

Data are also used for developing educational materials and for informing the planning process with respect to lake capacity assessment issues.

We have been advised that the Ministry is grateful for volunteer DO sampling as their staff numbers do not allow for researching all of our Ontario lakes.

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