Chemical trends and status of small lakes near Sudbury, Ontario, 1983-1995: evidence of continued chemical recovery

Abstract

We monitored 23 chemical parameters in 161 lakes northeast of Sudbury, Ontario, in most years between 1983 and 1995 to determine whether lake chemistries were responding to reduced local SO2 emissions. Lakes were typically small (median 4.0 ha, 4.5 m deep), rapid flushing, and acid stressed (median pH 5.58, acid-neutalizing capacity (ANC) 7.1 µequiv. ·L-1). Forty percent of the lakes declined significantly in SO4, base cations, and Al levels from 1983 to 1995, but only 12 and 16% increased in ANC and pH, respectively. Chemical trends were influenced by local climate; pronounced improvements occurred in response to drought conditions (1986 and 1987), but rapid deterioration followed the wet year in 1988. With more typical precipitation levels since 1989, a pattern of gradual improvement returned. Connected, rapid-flushing lakes, those on low or moderately sensitive bedrock, and peatlands showed the greatest improvements in pH and ANC whereas glacial headwaters and lakes on highly sensitive bedrock showed the least improvement. These results confirm that some recovery continues to occur following local emission reductions. However, many small Sudbury area lakes remain very degraded and unsuitable for acid-sensitive biota. Further emission reductions, including long-range sources, may be necessary to restore chemical conditions to critical levels sufficient to support further biological recovery.