Palaeolimnological evidence for recent chemical and biological changes in U.K. Acid Waters Monitoring Network sites

Abstract

1. Palaeolimnological evidence is presented for the long‐term (post‐1850) and recent (post‐1970) trends in acidity of eleven sites in the U.K. Acid Waters Monitoring Network.2. Sites are located throughout the U.K. in areas sensitive to acidification, and results show that all have been acidified since pre‐industrial times. Although there is considerable variation in the timing and magnitude of these changes the results are consistent with other evidence of the widespread and severe acidification of sensitive U.K. freshwaters as a result of acidic deposition.3. The most severely acidified sites generally have the highest critical load exceedance, although there is a only a poor relationship between exceedance and post‐1850 pH change (r = 0.58, P = 0.06) or diatom floristic change (r = 0.52, P = 0.1). These results highlight the difficulty of inferring biological change or ‘damage’ in freshwater ecosystems from current national maps of critical load exceedances.4. Evidence of chemical and biological response to the post‐1970 reduction in U.K. S emissions is variable: seven lakes show continued acidification in the 1970s and early 1980s while four appear to have been in steady state. One afforested site shows continued acidification until at least 1990, the year of coring, suggesting that at this site increased scavenging of acid anions following canopy closure and/or increased nitrate leaching have offset the benefits of reduced S deposition.5. Five sites appear to have been in steady state since the early to mid‐1980s, and two show unambiguous evidence for a recent increase in pH and a reversal in the diatom assemblages to that of earlier levels. The results support and extend the findings of other studies and show that biological recovery is occurring in at least two chronically acidified areas of the U.K. (Galloway, SW Scotland, and north Wales), and that natural recovery can lead to the re‐establishment of biota previously present at the site.