The ordination and classification of macroinvertebrate assemblages in the catchment of the River Wye in relation to environmental factors

SUMMARY. 1. Macroinvertebrates were sampled from three habitats at forty‐five sites in the catchment of the Welsh River Wye. Species assemblages were ordinated by DECORANA. classified by TWIN‐SPAN and related to physico‐chemical factors using correlation and multiple discriminant analysis respectively. 2. DECORANA axis 1 was correlated with pH or total hardness, whilst axis 2 correlated with slope or distance from source. TWINSPAN groupings were also related to hardness and, to a lesser extent, slope. Assemblages at soft‐water sites (<15gm−3 CaCO3) were composed mostly of Plecoptera but at hard‐water sites, even at high slope (>10 m km−1), the fauna was dominated numerically by Ephemerop‐tera, net‐spinning Trichoptera, Mollusca and Crustacea. 3. We suggest that our data do not support the River Continuum Concept unless there are modifications to allow for multiple gradients.