Recovery of acidified surface waters from acidification in the United Kingdom after twenty years of chemical and biological monitoring (1988-2008)

In this special issue we present papers based on data from the UK's Acid Waters Monitoring Network (UK AWMN) and other UK acid waters. The AWMN was set up in 1988. It was designed to monitor the chemical and biological response of acidified surface waters in the UK to the planned reduction in the emission of acidic sulphur and nitrogen gases as required by the UNECE Convention on Long Range Transboundary Air Pollution. Most papers in the volume are concerned with the changes that have taken place at the 22 AWMN sites during 20 years of monitoring from 1988 to 2008. They show that significant changes in deposition chemistry, in water chemistry and, to a lesser extent, in biology have taken place, consistent with a recovery from acidification. However, when compared with pre-acidification conditions inferred from lake sediment records, the extent of biological recovery so far is shown to be quite limited. The volume also contains papers on other aspects of surface water acidification in the UK. They include evidence for persistent highly acidic conditions of streams in the North York Moors, data from Scotland showing how afforestation is modifying recovery from acidification and the results of chemical speciation modelling in explaining the relationship between acidification and macroinvertebrate species richness at AWMN and other sites in the UK. The final papers are concerned with projections for the future and the extent to which acidified sites will continue to improve. They conclude that recovery will continue albeit slowly during this century but that other pressures principally from climate and land-use change are likely to alter the recovery pathways towards novel ecological endpoints potentially quite different from past baselines.