Surface Water
Lake ecosystems have improved in northern Europe and North America, where emission reductions occurred. In general, SO4 concentrations have decreased following the emission trends, but nitrogen concentrations have not shown changes. Data from 98 sites were tested for trends in concentrations over the 10-year period 1989-1998. The (grouped) sites clearly showed significant decreases in SO4 concentrations. Nitrate, however, showed no regional patterns of change, except possibly for central Europe: Decreasing trends occurred in the Black Triangle. Concentrations of base cations declined in most regions. All regions showed tendencies toward increasing dissolved organic carbon. Recovery from acidification reflected by an increase in surface water acid neutralizing capacity (ANC) and pH was significant in the Nordic countries/United Kingdom region. In central Europe, there was a regional tendency toward increasing ANC, but large spatial differences were found with the low ANC sites showing the largest recovery. Nonforested sites showed clear and consistent signals of recovery in ANC and pH and appropriate (relative to SO4
trends) rates of base cation declines. Hence, it was concluded that the observed recovery was associated with declining SO4.
In the most acidic sites in central Europe, improvements in water quality have not yet resulted in improvements in biology. Biological improvements of these sites require considerable improvements in water quality with respect to acidification.
In many lakes in Scandinavia, there is evidence of a small but significant recovery and many species that died because of acidification are returning. The positive signs are mainly observed in lakes and streams with limited acidification. For the most acidified waters, the signs of recovery are still small and unclear.
In a study by Stoddard et al., data for 205 sites in eight regions in North America and Europe between 1980 and 1995 were used to test trends. The data they used were primarily from the International Cooperative Program (ICP) Waters study. They found trends of decreasing SO4 concentrations in all regions except the United Kingdom and no or very small changes in NO3. SO4 levels declined from 0 to —4 meq/ liter/year in the 1980s to —1 to —8 meq/liter/year in the 1990s. Recovery of alkalinity was associated with the decrease in SO4, especially in the 1990s.
