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We aimed to determine which processes drive patterns of alpha and beta diversity in Arctic river benthic macrofauna across a broad latitudinal gradient spanning the low to high Arctic of eastern Canada (58 to 81(o)N). Further, we examined whether latitudinal differences in taxonomic composition resulted from species replacement with organisms better adapted to northerly conditions or from the loss of taxa unable to tolerate the harsh environments of higher latitudes. We used the bioclimatic envelope concept to provide a first approximation forecast of how climate warming may modify alpha and beta diversity of Arctic rivers and to identify potential changes in environmental variables that will drive future assemblage structure. Benthic macroinvertebrates, environmental supporting variables, and geospatial catchment data were collected to assess drivers of ecological pattern. We compared alpha diversity (i.e., taxonomic richness) across latitudes and partitioned beta diversity into components of nestedness and species turnover to assess their relative contributions to compositional differences. We found sharp declines in taxonomic richness along a latitudinal gradient. This alpha diversity pattern was not associated with a change in numerical abundance. beta diversity was highest when the most distant latitudes were compared, and pairwise latitudinal comparisons indicated that nestedness (loss of species) was the dominant contributor to compositional differences. Biotic-abiotic associations reflected both large-scale climatic drivers, including air temperature and prevalence of vegetated tundra, and small-scale secondary abiotic drivers of assemblage composition (e.g., substrate composition, water chemistry). The importance of nestedness to beta diversity across latitudes supports the physiological tolerance hypothesis that a change in environmental tolerance is a key driver of species richness declines with increasing latitude. Distinct taxonomic assemblages among low and high Arctic latitude sites were associated with large-scale, climate-related drivers (e.g., temperature trends, terrestrial vegetation), reflecting the primary structuring of assemblages by bioclimatic envelopes. The abiotic environment was the strongest driver of assemblage structure at high latitudes because of the extreme conditions. With continued warming, biodiversity differences along latitudinal gradients are expected to become less pronounced as temperatures and vegetation become more similar from south to north, with local-scale variables becoming dominant biotic drivers.