Spatial patterns, trends, and the potential long-term impacts of tree harvesting on lake calcium levels in the Muskoka River Watershed, Ontario, Canada

Abstract

The issue of calcium (Ca) decline in surface waters of eastern Canada is an emerging concern that may be made worse by timber harvesting. In the Muskoka River Watershed (MRW) in Ontario, the mean lake Ca concentration in 104 lakes decreased by 30% since the 1980s, with the rate of decrease slowing over time consistent with changes in lake sulfate (SO4) as the region recovers from acid deposition. Recent data suggested that smaller lakes, at higher elevation, in smaller catchments with higher runoff that are minimally impacted by the influence of roads and agriculture are associated with lower Ca concentrations and thus are the lakes most at risk of amplified Ca depletion. Using proposed annual allowable harvest cuts from 10-year forest management plans, 38% of 364 lakes assessed in the MRW will fall below a reported critical 1 mg·L–1 Ca threshold compared with just 8% in the absence of future harvesting. It is concluded that Ca decline poses a serious threat to aquatic ecosystems and should be taken into consideration in future forest management plans.