Surface Water Quality in Big Rideau Lake

Prepared by: Sarah MacLeod-Neilson                                                       Surface Water Quality Co-ordinator,                                           Rideau Valley Conservation Authority

Surface water quality conditions in Big Rideau Lake are monitored by the Rideau Valley Conservation Authority’s (RVCA) Watershed Watch program.  Watershed Watch samples several watershed lakes to assess nutrient concentrations, water clarity, dissolved oxygen availability, bacterial counts (Escherichia coli) and pH.  The collected data is summarized using the Canadian Council of Minister’s of the Environment’s Water Quality Index (CCME WQI).  Each parameter[1] is evaluated against established guidelines to determine water quality conditions. Data from four deep point sites has been used to calculate the WQI rating for Big Rideau Lake, which was determined to be “FAIR” (2008-2013).  The lake is monitored four times each year from May-October.

Relatively few nutrient exceedances, good oxygen conditions for fish habitat, clear water and rarely elevated pH levels or bacterial counts characterize the water quality in Big Rideau Lake. The following will discuss how each of the above parameters factors into the lake’s water quality.

For details on water quality monitoring locations and results please refer to the Rideau Lakes Subwatershed report (


Total phosphorus (TP) is used as a primary indicator of excessive nutrient loading to the lake; when present in high concentrations it may contribute to abundant aquatic vegetation growth and depleted dissolved oxygen levels. Total phosphorus is an essential nutrient for aquatic plant and algae growth, it is usually found in limited supply in surface water. The Provincial Water Quality Objective (PWQO) is used as the TP Guideline and states that in lakes concentrations greater than 0.020 mg/l indicate an excessive amount of TP within the water column, and may lead to abundant algae and/or aquatic plant growth. TP is found naturally in aquatic sediments and accumulates as plants die off and decompose; releasing nutrients back to the environment. However, it may become abundant through contributions from both point and non-point sources including: runoff from fertilizers, failing septic systems, manure, phosphate detergents, storm water outfalls, carried in eroding sediments, atmospheric deposition and water treatment works.  

 Total Kjeldahl nitrogen (TKN) is also used as an indicator of nutrients found in the aquatic environment and includes two forms of nitrogen; ammonia and organic nitrogen. These forms of nitrogen are not readily available for plant uptake. However, when a natural process of oxidation occurs they are converted to a form readily available for use by plants. RVCA uses a guideline of 0.500 mg/l to assess TKN[2] within surface waters.  Nitrogen enters surface water through many of the same sources as TP.

Within Big Rideau Lake nutrient concentrations generally meet guidelines, apart from some near shore areas.  For more information on water quality results and monitored sites please refer to the Catchment Reports available here:

Sites with particularly high results should be further investigated to determine if sources of nutrient inputs can be reduced; such as the diversion of runoff (overland and storm water) and enhanced shoreline buffers.  Areas where high concentrations of nutrients are persistent may observe excessive aquatic plant growth, algae blooms and depleted oxygen concentrations.  Nutrient exceedances may be partially attributed to the natural aging of a lake, but can be slowed with the help of all lake residents by reducing nutrient inputs through practices such as proper maintenance of septic systems, keeping shorelines natural and using phosphate free soaps and detergents. 

Water Clarity

Water clarity is measured using a Secchi disk; a 30 cm disk marked into black and white quadrants and lowered on a line into the water column until it can no longer be seen (Figure 1). These measurements are taken at the deep points.  It should be noted that Secchi depths in many waterbodies have been influenced by the colonization of zebra mussels resulting in clearer waters than may have been seen prior to the introduction of this species.

Figure 1. Secchi disk and aqua scope used to measure water clarity.

Figure 1. Secchi disk and aqua scope used to measure water clarity.

Collected data indicates that waters are very clear and sufficient sunlight can penetrate the water column to support aquatic life (photosynthesis) and sufficient visibility for safe recreational use (i.e. boating, swimming).


Two other factors, dissolved oxygen/temperature and pH are also assessed to provide an overall sense of the health of Big Rideau Lake from a fish habitat perspective.

Dissolved Oxygen and Temperature

Dissolved oxygen and temperature measurements are taken at 1m-2m increments at monitored deep points from surface to bottom to examine habitat conditions for water and cool water fish species.  Suitable conditions for warm water fish species are determined by temperatures less than 25°C and dissolved oxygen greater than 4mg/l.  Suitable oxygen/temperature conditions over depths ranging 18-33 m.

Optimal conditions for lake trout habitat are determined by temperature less than 10°C and dissolved oxygen greater than 7 mg/l.   Spring and early summer typically have good conditions for lake trout habitat but as temperatures warm throughout the summer available habitat becomes more limited, particularly in the upper portions of the water column. 


pH is a basic water quality parameter used to assess the acidity of water, an important factor for aquatic life. The Provincial Water Quality Objective (PWQO) advices that the pH of surface waters should be between 6.5 and 8.5 to protect aquatic life and prevent irritation for anyone using the waters for recreational purposes.

In some areas of the Rideau Lakes Watershed, surface waters tend to be a bit more alkaline (higher pH) which can generally be attributed to geology rather than anthropogenic activities; biological activities such as photosynthesis may also affect pH.

Overall the water chemistry data at the deep points describes good habitat conditions for warm and cold water fish species.  There is some evidence that the warming of the water column in the late summer/early fall may limit the amount of habitat for sensitive cold water species such as lake trout.  pH conditions are typically within the range recommended for the protection of aquatic life, indicating a healthy environment for aquatic species.

Escherichia coli

Escherichia coli (E. coli) counts are used as an indicator of bacterial contamination by fecal material which can contain harmful pathogens; results may be used to assess the suitability of surface water for recreational use.  E. coli is sampled at monitored shoreline sites twice each sampling season. 

The results from collected data provide evidence that bacterial contamination is not a significant concern in Big Rideau Lake and the water should be safe for recreational use such as swimming and boating activities.  


Overall, the water quality conditions in Big Rideau Lake generally meet established guidelines.  In some areas, nutrient concentrations are a continued concern.  It is important that nutrient loading is minimized wherever possible to protect from deterioration in the lake environment as well as the wildlife and human populations it supports. Big Rideau Lake is an important part of the aquatic and terrestrial ecosystem for a variety of organisms; it is also an important feature within the community, providing recreational opportunities, beauty and an economic resource. There are a variety of ways that property owners and lake users can help protect and enhance Big Rideau Lake.  Please see the Rideau Valley Conservation Authority’s Rideau Lake’s Subwatershed and associated Catchment reports for more information and to learn what you can do to help protect the lake environment.(


[1] Please note that E. coli results have not been included in the calculations of the CCME WQI due to differences in sampling frequency and location, E. coli is monitored twice during the sampling season and only at nearshore sites.

[2] No Ontario guideline for TKN is presently available however waters not influenced by excessive organic inputs typically range from 0.100 to 0.500 mg/l, Environment Canada (1979) Water Quality Sourcebook, A Guide to Water Quality Parameters, Inland Waters Directorate, Water Quality Branch, Ottawa, Canada