Influence of tributaries on the longitudinal patterns of benthic invertebrate communities

There has been little effort to understand how tributaries influence mainstem rivers at large scales beyond the immediate influence of the tributary and downstream of the mixing zone. Such knowledge is needed to create breaks in stream networks that can aid in the classification of stream valley segments and conservation studies that rely on the delineation of zones. We use benthic invertebrate assemblages to infer longitudinal gradients and discontinuities and relate these patterns to confluence symmetry ratio (CSR; the size ratio of the tributary basin to the mainstem basin upstream of the confluence). In addition, we briefly explore reach and catchment‐scale environmental influences. We found evidence for both gradual and abrupt longitudinal changes in benthic invertebrate communities. There was not a smooth continuous gradient but a sawtooth pattern with an overarching trend. Two major discontinuities were found: one associated with a large CSR = 0.74 and reach scale factors including predominance of sand and an abundance of benthic organic matter that provided a unique habitat; and a second associated with a large CSR = 0.64 and a transition from coarse textured morainal deposits to glaciolacustrine deposits. There were synchronous additions of some benthic invertebrates (e.g., Eukiefferiella brehmi, Antocha, Hydropsyche morosa, and Oligochaeta) showing an affinity for downstream reaches, whereas others showed an affinity for headwater reaches (e.g., Simulium tuberosum, Baetis tricaudatus, and Micropsectra). Benthic invertebrate communities were driven by a combination of confluence symmetry ratio, landscape, and reach scale factors that can confound interpretation.