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Water Quality

Lake Mahon water quality is monitored by measuring transparency (Secchi disk depth) and collecting water samples for lab analysis.

Regular testing for bacteria began about 1979. The Association is concerned about swimming conditions (not potable water), and results have generally been quite acceptable. For current and past results, see

In 1992, extensive water testing led to a report on Lake Mahon Water Quality that was done in response to abnormal (but temporary) coliform counts, but also to report first-time phosphorous testing, which showed phosphate concentrations over twice a recommended level. In 2002, a bulletin was issued on Steps to Improve the Quality of Lake Mahon, and in 2003, two lake samples had 0.02 and 0.03 mg of phosphorous per litre.

Beaver

Lake Mahon has four inlet creeks, with one from a beaver pond. A technician from the Rideau Valley Conservation Authority advised in 2002 that beaver ponds can have elevated concentrations of bacteria, but bacteria do not live very long outside the digestive tracts of mammals, so routine inflow from the beaver pond probably wouldn't pose more of a risk than the presence of roosting birds or other creatures living in and around the lake. See Options for controlling beaver (attached).

RSVL (Réseau de surveillance volontaire des lacs)

In 2005, the Association began participation in a Quebec provincial RSVL program. Water transparency is measured regularly, and testing is done at least every five years for phosphates, chlorophyll, and organic carbon.

RSVL has concluded that eutrophication process is at an intermediate stage in Lake Mahon, and measures to limit inputs of nutrients from human activities must be implemented quickly to slow this process and preserve the state of the lake and the uses it allows.

See the RSVL subpage for reports. 

For more recent results (independent of RSVL), see Water quality 2019 and Nutrient testing (in Posts).

Water Clarity

Water clarity is measured as the depth where a “Secchi disk” disappears from view, one scale is 0 to 3 metres, 3 to 5 metres, and over 5 metres. Secchi depth for Lake Mahon is typically 3 to 4 metres. 

Nutrients

Phosphorous is the principle nutrient causing the “aging” (eutrophication) of lakes, with more algae and weeds, and less clear water.

RSVL uses a scale of less than 0.01 mg/l (oligotrophic), 0.01 to 0.03 mg/l (mesotrophic), and greater than 0.03 mg/l (eutrophic).

The Rideau Valley Conservation Authority (RVCA) uses total phosphorus (TP) is as a primary indicator of excessive nutrient loading, contributing to abundant aquatic vegetation growth and depleted dissolved oxygen levels. The Ontario Provincial Water Quality Objective is used as the TP Guideline: concentrations in lakes greater than 0.020 mg/l indicate an excessive amount of TP within the water column, while concentrations below 0.010 mg/l are generally considered to be minimal and unlikely to have problems associated with nutrient loading. 

The RVCA uses Total Kjeldahl nitrogen (TKN) as a secondary indicator of nutrient loading, with a guideline of 0.500 mg/l used to assess TKN within surface waters. No Ontario guideline for TKN is available; however, waters not influenced by excessive organic inputs typically range from 0.100 to 0.500 mg/l.

Nitrogen is reportedly the limiting nutrient for Eurasian milfoil, not phosphorus, i.e. Eurasian milfoil responds to nitrogen enrichment but not phosphorus enrichment (see Eurasian milfoil).

Nitrogen and phosphorous both come from the natural decay of organic matter in lakes, but can also come from decay of human sewage and farm animal waste. Most fertilizers contain nitrate, which is subject to run-off, and phosphate, which soil particles can hold on to (or fix) so that it does not wash away unless there is soil erosion. Phosphorous can enter the water system from detergents with phosphate (detergents are limited to 0.5% phosphorous in Canada).

Algae

Algae are microscopic single-celled plants that form the base of the aquatic food chain. They can live suspended in water (planktonic), form bottom or surface mats, attach to rocks or plants, or look like plants. Concerns from excessive growth include blooms mixed in the water (pea soup), low water clarity and a green colour, and a disagreeable smell as algae breaks down. Most mats are filamentous green algae (thread-like), and are non-toxic. Filamentous green algae can also "cloud" around plants.

Algae blooms are typically occur in late spring or early summer, and do not release toxins, whereas blue-green algae bloms occur in mid to late summer.

See Algae ID Card, Algae in the Severn Sound Area (YouTube).

Blue-Green Algae

See Blue-Green Algae (in News). Blue-green algae are bacteria (cyanobacteria), not algae.

Overall Water Quality

Various water-quality parameters are blended together by the RVCA, allowing water quality to be represented by a single rating (Very Poor, Poor, Fair, Good, Very Good), based on the Canadian Council of Ministers of the Environment Water Quality Index. The parameters for lakes are nutrients (phosphorous, Kjeldahl nitrogen), dissolved oxygen, pH, and Secchi depth. Water quality is one indicator of watershed health, the other indicators are forest cover, wetland cover, and riparian cover.

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