Patterns of benthic invertebrate assemblages in rivers of northwestern North America

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Abstract
SUMMARY. 1. Biogeographical and on‐site. hydrological variables were evaluated to determine spatial distribution of benthic invertebrate assemblages at 100 river sites in northwestern North America.2. Results of cluster analysis suggested that the river sites comprised sixgroups (A‐F), each supporting a characteristic invertebrate assemblage.Distinct groups were best represented by abundant Tricorythidae (C), Amphipoda (F), Rhyacophilidae and Systellognatha (E), and Elmidae and Hydroptilidae (A). Brachycentridae (B) and Oligochaeta (D) were widespread throughout the study area.2. Both biogeographical and hydrological features contributed to the correct classification of site groupings characterized by distinctive fauna. However, biogeographical features were more useful than variables measured at the river site in discriminating among the site groupings.Groups C and F were most prevalent within the Hudson Bay drainage.Groups A, C and F were typically located within plains; group E sites were in mountainous regions.4. The hydrological variables most useful in delineating site groupings were mean current velocity and mean depth. Slow, deep waters characterized amphipod sites; shallow, fast flowing waters occurred at Rhyacophilidae Sysellognatha sites.5. Results substantiate previous views of a strong association between benthic invertebrates in small rivers and the terrestrial biome through which the river flows.