Regional trends in aquatic recovery from acidification in North America and Europe

Rates of acidic deposition from the atmosphere ('acid rain') have decreased throughout the 1980s and 1990s across large portions of North America and Europe(1,2). Many recent studies have attributed observed reversals in surf ace-water acidification at national(3) and regional(4) scales to the declin ing deposition. To test whether emissions regulations have led to widesprea d recovery in surface-water chemistry, we analysed regional trends between 1980 and 1995 in indicators of acidification (sulphate, nitrate and base-ca tion concentrations, and measured (Gran) alkalinity) for 205 lakes and stre ams in eight regions of North America and Europe. Dramatic differences in t rend direction and strength for the two decades are apparent. In concordanc e with general temporal trends in acidic deposition, lake and stream sulpha te concentrations decreased in all regions with the exception of Great Brit ain; all but one of these regions exhibited stronger downward trends in the 1990s than in the 1980s. In contrast, regional declines in lake and stream nitrate concentrations were rare and, when detected, were very small. Reco very in alkalinity, expected wherever strong regional declines in sulphate concentrations have occurred, was observed in all regions of Europe, especi ally in the 1990s, but in only one region (of five) in North America. We at tribute the lack of recovery in three regions (south/central Ontario, the A dirondack/Catskill mountains and midwestern North America) to strong region al declines in base-cation concentrations that exceed the decreases in sulp hate concentrations.