MICROCRUSTACEAN COMMUNITIES IN STREAMS FROM 2 PHYSIOGRAPHICALLY CONTRASTING REGIONS OF BRITAIN

Benthic microcrustaceans were sampled from forty-three streams in two physiographically contrasting regions of Britain: lowland southern Eng land and upland Wales. Lowland streams had a significantly higher spec ies richness than upland streams and, of the forty-three copepod and c ladoceran species identified, only fourteen (33%) were found in both t he lowlands and uplands. Canonical correspondence analysis revealed la rge differences in community structure between regions and between str eams within regions. Differences within regions were related to pH, th e distance downstream of a site and the presence of upstream impoundme nts. Variables underlying the large biological differences between reg ions are also discussed, including differences in chemistry (ionic con tent), physical parameters (e.g. flow), habitat availability and the i nfluence of species biogeography. The importance of understanding spec ies ecology and biogeography when assessing pollution impacts on strea m communities is emphasized and a model is proposed for predicting the composition of microcrustacean communities in temperate European stre ams.