Calcium concentrations in our lakes: What do they mean?

2018 Muskoka Watershed Report Card Logo


The 2018 Muskoka Watershed Report Card features eight different indicators of watershed health. Calcium concentration is one of these indicators, categorized as a health indicator rather than a threat indicator.


Calcium is an essential nutrient for all living organisms and plays an important role in buffering against acid rain. Between 1960 and 1970, acid rain intensified in the Muskoka region, causing calcium to leach from watershed soils into lakes at a faster rate than it could be replenished through weathering. This event, coupled with land clearing and logging, depleted the calcium bank. As a result, the pool of calcium within soils slowly depleted, as did the pool of calcium within our lakes.


Today, we are still seeing the effects of low calcium concentrations in the majority of Muskoka’s lakes, as well as the ecological effects which are both widespread and pronounced. Daphnia, a tiny zooplankton species, use the calcium in water to harden their carapaces. Daphnia are a keystone herbivore within lake food webs, making them an invaluable species. They provide food to many different fish species and eat algae, which keep the water in our lakes clear. Therefore, calcium concentrations are important to monitor because if they drop below the required levels for species such as Daphnia to thrive, these species become stressed, which will in turn have multiple negative impacts upon other species and watershed health in general.


In the 2018 Muskoka Watershed Report Card, each of the 19 subwatersheds within the Muskoka Watershed will receive a grade based upon the average calcium concentrations within their lakes. A total of 187 lakes are included in this assessment. Red indicates a stressed subwatershed, yellow indicates a vulnerable subwatershed, and green indicates a subwatershed that is not stressed.


Check out how your watershed ranks for calcium concentration, and what you can do to help by visiting


2018 Report Card - Calcium Indicator