Lake Mahon water quality is monitored by measuring transparency (Secchi disk depth) and collecting water samples for lab analysis. In 2019 we again tested phosphorous, e-coli, and nitrogen (Eurasian milfoil growth apparently reacts to nitrogen but not phosphorous).

• The average Secchi disk depth was 3.8 metres in 2019, compared with a long-term average of 3.5 metres. So far, the average for 2020 is over 4 metres. With greater transparency, sunlight can reach greater depths to stimulate plant growth.

• E-coli results were 0-0-0-0 in 2019, and 2-3-3-15-2 in 2018. Under Quebec standards, 0 to 20 is considered excellent (for swimming).

• Reported phosphorous levels for 2019 were not credible, despite requests for the lab to clarify. Phosphorous levels were very low in 2018, averaging 0.004 mg/l (below 0.01 mg/l is considered minimal).

• The nitrogen measure TKN averaged 0.3 mg/l in 2019, compared with 0.4 mg/l in 2018. The Rideau Valley Conservation Authority uses 0.5 mg/l as an upper TKN guideline (waters not influenced by excessive organic inputs typically range from 0.1 to 0.5 mg/l).

• Nitrogen and phosphorous 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 phosphate and nitrate, but soil particles can hold on to phosphate so that it does not wash away unless there is soil erosion, whereas nitrogen is subject to run-off. Phosphorous can enter the water system from detergents with phosphate (detergents in Canada are limited to 0.5% phosphorous).