Calcium declines in northeastern Ontario lakes

Thousands of lakes in northeastern Ontario, Canada, have been acidified by sulphur deposition associated with emissions from the Sudbury area metal sm elters. However, water quality improvements including increased pH and redu ced sulphate concentrations have followed large reductions in Sudbury emiss ions that were implemented, beginning in the 1970s. Substantial decreases i n Ca concentrations accompanied these other changes in lakewater chemistry. Monitoring of 38 lakes 20-128 km from Sudbury showed declines in Ca concen trations, averaging 2.7 mu eq.L-1.year(-1), over the period 1981-1999. Decl ines were particularly apparent during the 1990s, averaging 3.8 mu eq.L-1.y ear(-1). Paleolimnological reconstructions of the long-term Ca patterns in six lakes suggest that general lakewater Ca declines occurred through much of the 20th century. Comparison of recent measured Ca concentrations in 16 lakes with diatom-inferred pre-industrial Ca concentrations indicates that overall decreases in Ca have been large, averaging 74.6 mu eq.L-1 or 46%. L ong-term Ca patterns may reflect a combination of factors including climati c changes, forest harvesting activities, and leaching by acid deposition, t he effects of which we can not separate. Calcium declines have biological i mplications that will need to be considered in the development of appropria te targets as these lakes continue to recover from acidification.