Reversibility of stream acidification in the Cairngorm region of Scotland

Since 1984, stream chemistry and macroinvertebrate assemblages have been monitored in 17 headwater streams which drain the Cairngorm region of Scotland. Three streams were considered to be acidified (mean pH < 6.0 and mean acid neutralizing capacity (ANC) < 25 mu eq l(-1)), 10 were acid-sensitive (pH < 6.8 and ANC < 150 mu eq l(-1)) and a further four were well-buffered (pH > 6.8, ANC > 150 mu eq l(-1)). The acidified streams had impoverished macroinvertebrate faunas. All of the streams have exhibited a decline in non-marine sulphate concentrations over the last decade in response to reduced sulphur deposition. In the more acidic and acid-sensitive systems this has generally been matched by a corresponding increase in ANC, There is also evidence of an increase in the pH of the most acidic streams, though no increased diversity or abundance of acid mayfly macroinvertebrate taxa species have been observed. Further reductions in deposition and longer time for a reversal of soil acidification appear to be necessary before greater biotic recovery can occur.