Interpretation of trends in acidic deposition and surface water chemistry in Scotland during the past three decades

Trends in major ionic components of bulk precipitation were analysed for two sites, Faskally and Loch Ard forest in Scotland, for the period 1972-2000. The pattern of change was not linear. Large reductions in sulphur deposition occurred in the early 1980s and, to a lesser extent, during 1995-2000, with a period of relative stability between 1988-95. pH increased significantly at both sites but nitrate and ammonia only increased significantly at Loch Ard forest. Longterm chemical data from a total of 37 streams and lochs in four selected regions of Scotland were analysed over three time periods (all available data (mostly 1978-2000), 1988-98 and 1995-2000) to match the deposition patterns. For the whole study period a significant decline in non-marine sulphate was found at all sites while the most consistent increases in pH and alkalinity were recorded at all the high elevation loch sites in the Galloway area. Significant reductions in toxic forms of aluminium were also recorded, mostly at sites where pH had increased. Nitrate trends were equivocal except for catchments with clear-felling operations. For these sites, negative trends were found where felling occurred in the 1980s, while positive trends were found at sites with felling in the 1990s. With the exception of one site, dissolved organic carbon concentrations increased significantly with moorland sites showing smaller increases than forested sites. Associated with this change was a significant increase in complexed forms of aluminium. Trends for the 198898 period were much smaller than those for the whole study period and in many cases were insignificant. This contrasts with the 1995-2000 period when large and significant reductions in sulphate and nitrate were recorded along with increases in marine salts, probably as a result of climatically related events. Qualitative, experimental and monitoring data from lochs in the Galloway area revealed evidence of recovery of fish populations. Interpretation of chemical and biological trends was clearly influenced by the choice of the time series, especially in relation to deposition and climatic changes. Nevertheless, all the fresh waters included in this study are currently in the best ecological condition since the 1970s in the context of recovery from acidification.