Densities of benthic macroinvertehrates in upland Welsh streams of different acidity and land use

Benthic macroinvertebrates in six streams of contrasting acidity and land use were sampled quantitatively at nominal monthly intervals over three years. Abundances of the main taxonomic groups were assessed, and time series models fitted to the total densities. Total densities were highest in a circumneutral stream, mainly due to the presence of Ephemeroptera. Plecoptera was the most abundant taxon and had similar absolute abundances at all sites, including conifer afforested and moorland catchments. Dipterans were important at all sites, and in three acidic streams draining moorland the Oligochaeta also represented a major faunal component. In all streams the densities in each month showed low correlations between years, but the range of seasonal variations increased at low pH. There was no strong evidence of changes in stability with acidity, although the patterns of seasonal abundance may have implications for ecosystem interactions.