Scale-dependent community dynamics structure comparatively diverse yet internally homogenous invertebrate metacommunities in Grand Canyon tributaries.

Landscape discontinuities in river networks can induce abrupt changes in environmental conditions and community assemblages, although both of these ecosystem characteristics can shift within an uninterrupted reach as well. Differences in community structure can emerge due to dispersal-related dynamics or habitat filtering along elevational or environmental gradients, which can increase the capacity of the landscape to support biodiversity. Numerous low-order tributaries flow directly into the seventh-order mainstem of the Grand Canyon segment of the Colorado River, and these tributaries drain steep and heterogeneous landscapes. This raises the possibility that tributaries support diverse aquatic communities across elevational and longitudinal gradients. Here, we use eDNA metabarcoding to quantify aquatic invertebrate community structure at lower, middle, and upper reaches spanning two-kilometer intervals upstream from the confluences of fourteen tributaries in Grand Canyon, as well as within the mainstem Colorado River at each tributary confluence. Aquatic invertebrate communities differed between tributaries, but did not significantly differ in taxonomic richness or community dissimilarity between lower, middle, and upper reaches or show significant changes across within-tributary environmental gradients. The moderate patterns of dissimilarity that did occur within tributaries were primarily due to compositional turnover. Tributary basin-level heterogeneities are an important determinant of biodiversity distribution in Grand Canyon, more so than reach-level variation within tributaries, highlighting the scale-dependent nature of metacommunity dynamics in aquatic ecosystems.