A consideration of potential confounding factors limiting chemical and biological recovery at Lochnagar, a remote mountain loch in Scotland

Lochnagar, a remote loch in the Grampian Mountains of Scotland is one of the most studied freshwater bodies in the UK. It represents a key site in a number of monitoring programmes and has become the UK’s 'flag-ship' mountain lake in various EU funded projects over the last 15 years. Palaeolimnological studies have revealed the extent and diverse provenance of atmospherically deposited pollution at the site and show that the loch began to acidify in the mid-19th century. However, despite abatement strategies dramatically reducing the emission and deposition of non-marine sulphate and trace metals since the 1970s, the loch pH shows little sign of recovery and full basin fluxes of, for example, Pb and Hg show no decline or even a continued increase. It is suggested that the lack of recovery from acidification over the last 15 years of monitoring results from the balancing of the decline in sulphate by increased nitrate, and that this increase is related to winter duration and severity. The lack of response by the sediment record to declines in metal deposition is thought to be due to a continuing input of previously deposited metals from the catchment. Hypotheses for these enhanced catchment inputs involve responses to a changing climate. Site specific climate reconstructions and predictions for the 21st century suggest an accelerated increase in temperature rise and increased winter precipitation and storminess. These predicted changes are likely to exacerbate the input of metals (and other stored pollutants) from the catchment but higher temperatures may also help to promote recovery from acidification.