Large-scale macroinvertebrate assemblage patterns from least-disturbed wadeable stream sites across the 48 contiguous US states

We quantified the patterns in macroinvertebrate assemblages and their associated environmental gradients from 457 least-disturbed wadeable stream sites across the 48 contiguous United States sampled as part of US EPA's National Wadeable Stream Assessment. The majority of the variation in assemblage composition at the finest taxonomic resolution was related to substrate size, %fastwater habitat, water chemistry, as well as east-west geographic position and elevation. Sites were classified into 5 groups with cluster analysis, and group membership was predicted from environmental data using classification tree analysis (CTA). CTA correctly classified 69.1% of test sites and indicated that groups were distinguished by east-west location, and by factors distinguishing mountain streams from lowland/plains streams. Eastern and western groups that had similar environmental characteristics had very similar coarse scale taxa composition and convergent taxa traits. Ordinations confirmed that composition patterns using coarse level taxa resolution and taxa traits no longer reflected geographic distinctions, but were only related to non-geographic environmental factors. However, composition patterns based on traits, coarse taxa, and macroinvertebrates identified to the finest practical level were all correlated with the same dominant non-geographic environmental gradients.Original Abstract: Nous avons quantifie les schemas de repartition dans les assemblages de macro-invertebres et leurs gradients environnementaux associes a partir de 457 sites les moins perturbes possibles dans des cours d'eau echantillonnables a gue de 48 Etats americains contigus echantillonnes dans le cadre de l'US EPA's National Wadeable Stream Assessment. La majeure part de la variation de la composition des assemblages a la resolution taxonomique la plus fine est liee a la taille du substrat, au % d'habitat d'eau vive, a la chimie de l'eau, ainsi que d'est en ouest a la position geographique et a l'altitude. Les sites ont ete classes en 5 groupes avec une analyse par regroupement, et l'appartenance au groupe a ete predite a partir de donnees sur l'environnement en utilisant l'analyse par arbre de classification (CTA). CTA a classe correctement 69,1% des sites test et a montre que les groupes sont distingues par la localisation est-ouest, et par des facteurs qui distinguent les cours d'eau de montagne, des rivieres de basse altitude/plaines. Les groupes de l'est et de l'ouest qui ont les memes ca