Long-term trends in catchment export and lake concentrations of base cations in the Dorset study area, central Ontario

Decreases in exchangeable base cation reservoirs, base cation export, and lake concentrations have been reported for acid- sensitive regions in Europe and eastern North America. These decreases have produced smaller than anticipated increases in alkalinity in surface waters in response to reductions in sulphate emissions and may have ecological consequences. This study presents annual export rates of Ca, Mg, K, and Na from 20 forested catchments between 1978 and 1998 and concentrations in seven downstream lakes between 1975 and 2005 in an acid- sensitive region of central Ontario, Canada. In contrast to monovalent cation export, decreases in divalent cation export continued after an extended dry period ended. Lake concentration trends showed three distinct periods. The first period ( 19751976 to 1982- 1983) coincided with high runoff and was characterized by relatively high Ca, Mg, and K concentrations. This was followed by a 10- year period of fluctuations around the long- term mean. Concentrations then dropped below the long- term means and remained 5%- 20% below the means until 2004- 2005. The latter trend in lake concentrations suggests that export likely remained low but stable after May 1998 ( the last month for which export data are available). Na increased between 250% and 350% in Dickie and Harp lakes as a result of road salt contamination.