Natural and land-use induced load of acidity, metals, humus and suspended matter in Lestijoki, a river in western Finland

Lestijoki, a river in western Finland, runs through areas with abundant peatlands and acid sulphate (AS) soils, which cause water quality problems. A natural population of sea trout (Salmo trutta m. trutta (L.)) and other valuable species still exist in the river, but the declining water quality is a serious threat to their survival. The AS soils and the peatlands cause a natural high load of acidity, metals, humus and suspended matter. This load is enhanced by human activities such as agriculture and draining, being 2–5 times higher than in Fennoscandian streams in general. During high flow, the load reaches levels deleterious for many species. The results of several large water quality surveys and routine monitoring show that low pH in Lestijoki (min. 4.9) and the feeder streams (min. 4.4), together with high metal contents (e.g., max. Fe 7.3 and 5.8 mg l–1, respectively) and high amounts of humic and suspended matter, occasionally cause conditions where reproduction failures of fish occur. The acidity and metal load from subsurface drained AS soils is especially deleterious for the biological conditions in the main stream.