The consequences of liming a highly acidified catchment in central Scotland

Research during the mid 1980s identified acidified forested catchments in central Scotland whose hydrochemistry was not capable of supporting native fish populations. Calcium concentrations were around 20 mu eq 1(-1), less than the suggested critical value of 50 mu eq 1(-1), with hydrogen concentrations around 70 mu eq 1(-1), greater than the critical value of about 30 mu eq 1(-1). Limestone was applied by aerial application to the source areas of selected streams in 1990 with around 5% (15 ha) of the total catchment area of 270 ha treated at 10 tonnes ha(-1). Stream monitoring, carried out over the period 1989-1995, showed an immediate response to liming followed by a progressive decline. Calcium values were elevated to > 150 mu eq 1(-1) and hydrogen concentrations reduced to 20 mu eq 1(-1), reverting in time towards pre-liming values. Although salmonid survival was improved during low flow conditions in summer, only a few fry survived to the autumn as acid episodes increased and these were subsequently lost from the system during the winter period Budget calculations indicated losses of around 30% of the applied calcium during the first four years. Studies on the vegetation and soils revealed a greater than expected penetration of calcium to depth (10-20 cm) in the soil profile. Results suggest that source area liming at this rate has had minimal effects on the vegetation and by increasing the proportion of the catchment limed to 15% could have a much greater success in reducing the frequency of biologically damaging episodes.