Acidification and liming of River Vikedal, western Norway. A 20 year studyof responses in the benthic invertebrate fauna

This paper deals with benthic invertebrate population responses to acidific ation and liming in the lower part of River Vikedal. In 1979 the river show ed signs of increasing acidification. Highly sensitive invertebrates like t he mayfly Baetis rhodani were present in the river in low abundance, but di sappeared in the subsequent years. In order to re-establish a nontoxic wate r quality for fish, liming of the spring snowmelt to a minimum of pH 5.7 wa s started in 1987. During the later years liming has been successively incr eased. The invertebrate fauna showed a slow, but positive, response during the first years after liming, especially during autumn. B. rhodani recoloni zed the river in low density, but the spring cohort was still weak. Since 1 994 the lime dosage was increased to secure a minimum pH of 6.3 during spri ng snowmelt. This has resulted in an overall increased biodiversity in the limed section of the river. Several acid-sensitive species, like both cohor ts of B. rhodani and freshwater snails have colonized this part of the rive r. Simultaneously biodiversity in the unlimed reference sites has slightly improved during the last years. This is correlated with decreased sulphur d eposition and improved surface water chemical conditions.