Influence of spatial positioning within stream networks on fish assemblage structure in the Kansas River basin, USA

We found that riverine confluences had localized effects (within 20 km) on stream fish assemblages of the Kansas River basin. The majority of variation in fish assemblages occurred from east to west and along a stream size gradient. After controlling for the influences of longitude and stream size, distance of sample sites from streams >= 5th order accounted for a small proportion of taxonomic variability. However, species richness was significantly higher and assemblage structure was different in tributary stream segments directly connected to larger-ordered streams, suggesting that the effects of spatial position within this stream network were greatest in close proximity to tributary-mainstem confluences. Fish collections from three intensively sampled tributaries also indicated an abrupt change in species assemblages between mainstem river sites and tributary sample sites above confluences, followed by a gradual taxonomic change with increasing distance up to 20 km from the confluence. Changes in fish assemblages were associated with the reduced abundance of adult stream species near the confluence with the mainstem, rather than the occurrences of riverine species in the tributary.