The United Kingdom Acid Waters Monitoring Network: a review of the first 15 years and introduction to the special issue.

The United Kingdom Acid Waters Monitoring Network (AWMN) was established in 1988 to determine the ecological impact of acidic emissions control policy on acid-sensitive lakes and streams. AWMN data have been used to explore a range of causal linkages necessary to connect changes in emissions to chemical and, ultimately, biological recovery. Regional scale reductions in sulphur (S) deposition have been found to have had an immediate influence on surface water chemistry, including increases in acid neutralising capacity, pH and alkalinity and declines in aluminium toxicity. These in turn can be linked to changes in the aquatic biota which are consistent with "recovery" responses. A continuation of the current programme is essential in order to better understand apparent non-linearity between nitrogen (N) in deposition and runoff, the substantial rise in organic acid concentrations, and the likely impacts of forecast climate change and other potential constraints on further biological improvement.