Ecology of streams draining forested and non-forested catchments in an area of central Scotland subject to acid precipitation

Abstract

A study of 12 streams draining forested and non-forested catchments was made in an area of central Scotland where slow-weathering bedrock was predominantly quartzite, schists and slates. Sitka spruce (Picea sitchensis Carriere) was the most common tree species. Precipitation in the area had an annual mean pH in the range 4.3–4.5. Streams within the planted zone were always more acid than those outside and had higher concentrations of aluminium and manganese. With one exception, trout were absent from streams within long-established forests and planted salmon eggs (Salmo salar L.) died within a few weeks. A high proportion of such eggs survived in streams outside the forest. Siphlonurus lacustris Eaton was the only mayfly nymph found in the most acid streams in summer collections. In winter samples, mayfly nymphs, Heptagenia lateralis (Curtis) were found in only one forest stream but several species were present in the non-forested catchments. It is suggested that spruce forests can effectively collect acid pollutants which are subsequently washed out, thus accelerating the acidification of the soil. Streams therefore become increasingly acid as the neutralisation capacities of their catchments decrease.