The distribution of micro‐arthropods in some southern English streams: the influence of physicochemistry

SUMMARY. 1. Micro‐arthropods were surveyed during October 1986 at thirty stony, stream riffle sites in the Ashdown Forest, southern England. 2. The importance of a number of physicochemical variables in determining both the distribution of micro‐arthropod taxa and community structure was assessed. 3. Acidic sites had an impoverished fauna. Total micro‐arthropod species richness and densities were highest under circumneutral conditions. and the same patterns were shown by the Hydrachnellae, Harpucticoida and Cladocera. A number of species seemed indifferent to acidic conditions and were widespread and, as a group, the cyclopoid copepods showed no relationship with pH. 4. Multiple regression showed that other environmental variables, in particular annual mean temperature and maximum discharge, were also important in explaining the between‐site distribution of separate micro‐arthropod groups and individual species. 5. Other multivariate techniques (ordination, classification and multiple discriminant analysis) showed that impoundment linkage, source distance and conductivity, along with pH, were the most important variables in explaining patterns of species composition between sites.