Water Quality

Water Quality – 2020

Water quality testing is an important diagnostic tool to help residents of Otter Lake monitor the health of the lake. The ecological and trophic status of a lake is generally determined by the levels of nutrients it contains and these are what we (OLLA) and RVCA measure at different sites around the lake at least three or four times a year. OLLA and RVCA’s test sites are shown on the map on the right. The sites have been chosen to be representative of the whole lake. Sites 05A, O5B and 06 represent the three deepest water sites (more than 90ft). Sites 04, 07, 08, 11 and 18 are in areas where there are known inflows from streams and wetlands into the lake. Other sites are in shallow bays where there is an increased tendency for weed and algae growth.

An overview of Factors that Influencing Water Quality

Recreational water quality can often be expressed in terms of how clear the water appears. Water clarity is influenced by the amount of soil sediment and phytoplankton, or microscopic algae, present in the water. Clarity is measured by a simple visual test using a Secchi Disk, a 20 centimetre black and white disk attached to a measured line. The disk is then lowered into the lake until it is no longer visible and the depth recorded. Additional information on water quality is gained through analysis of samples for nutrients, specifically phosphorus and nitrogen, which gives an indication of how much nutrient and energy is available for the growth of algae and aquatic plants.

Nutrients & Bacteria

Nitrogen in various forms is an abundant and essential nutrient in aquatic ecosystems. In addition to fertilizers, agricultural waste and wastewater contribute nitrogen into lakes. In large amounts, ammonia and nitrates can be toxic to aquatic organisms. Total Kjeldahl Nitrogen (TKN) which is what we and RVCA measure, determines the concentration of all forms of nitrogen in the lake. While there are no precise guidelines for acceptable levels of TKN, according to RVCA, TKN in water bodies not influenced by excessive organic inputs such as Otter Lake typically range from 100 to 500 µg/L.

Phosphorous is generally recognized as the limiting nutrient in freshwater ecosystems and the major nutrient contributing to eutrophication in lakes. Since phosphorous is the principal source of energy for all living organisms the amount of phosphorous in the environment will determine how fast an organism grows and proliferates. Phosphorus is therefore the principal limiting factor in the growth of algae, meaning that algae growth will occur in greater amounts as more phosphorus is added to a lake. Most of the phosphorous that enters a septic system from detergents containing phosphorous will emerge intact, enter the water table and eventually the lake. Phosphorus levels below 5 µg/L are typical of oligotrophic lakes that generally are clear and deep with few nutrients. Such lakes are typically found in the northern regions of Ontario. Phosphorous levels above 20 µg/L are typical of eutrophic lakes that are laden with nutrients which lead to excessive algae and plant growth. Mesotrophic lakes have phosphorous concentrations between these two extremes and are typical of the lakes found in our region of Ontario.

Bacteria are present in all lakes, they will be found in the feces of the wildlife (fish, waterfowl, beavers, etc.) that inhabit the lake. Coliforms are bacteria found in the large intestine of humans and other mammals and are usually present in soil. While some strains of coliforms do produce toxins, most are not harmful to humans. Some such as Escherichia coli (E. Coli) do produce pathogenic toxins. Therefore, levels of E. Coli are often used as indicators of possible contamination by fecal matter. Thus high E. Coli levels in lakes or rivers can be an indication of septic pollution. The recommended safety level of E. Coli in a lake for recreational activity is not more than 100 colony-forming units (cfu) per 100ml of water. E.coli at any level is unacceptable for drinking water, therefore some form of treatment and purification is necessary for anyone who draws water from the lake for drinking purposes. 

OLLA’s water quality testing regimen has indicated occasional higher than usual coliform and E. Coli counts in certain areas of the lake. This article contains some useful information on E. Coli.  For more information on this please follow this link:  Demystifying Escherichia Coli

Results for 2020

Testing in 2020 was a challenge for both OLLA and RVCA because of the COVID pandemic. Safe distancing in a small boat was virtually impossible to achieve and restrictions were implemented by Caduceon Labs that performs testing of surface water samples. Hence, OLLA believed water sampling could not be safely performed and did not sample in 2020. RVCA was able to achieve reduced sampling and testing of surface water in some lakes in the Rideau Valley watershed including Otter Lake. The results of water testing in 2020 are shown in the table below.

The results indicate that the trend that has been seen for the past five years remains the same. Low levels of E. coli were detected at some sites tested. These low values do NOT present a health hazard for swimming since in Ontario, public beaches are closed only when E. coli levels exceed 100 cfu/100ml.
 Total Kjeldahl Nitrogen levels were in the acceptable range of between 200 – 500 µg/L, very similar to previous years. As for phosphorous, none of the sites tested had values greater than 10 µg/L and the average for the year was less than 7 µg/L, the lowest ever recorded for Otter Lake. Secchi depth readings were again between 5 and 6 meters indicating that the lake remains very clear and there were few algae blooms last Summer despite the hot weather, probably because of the low phosphorous levels. As part of its Watershed Watch Program RVCA has put together a map of water quality (based on phosphorous levels) of all the lakes in the Rideau valley catchment area that are surveyed annually. This map is shown on the right. Lakes with phosphorous levels of less than 10 µg/L are shown in light blue, 10 – 15 µg/L in light green, 15 – 20 µg/L in dark green and over 20 µg/L in red. As can be seen there are few lakes in light blue and Otter Lake is one of them. Click on the map to view in more detail.
For more details on water quality see the Lake Steward’s Reports.