Abiotic variables drive different aspects of fish community trait variation and species richness across the continental United States.

Intraspecific trait variation (ITV) is an increasingly important aspect of biodiversity and can provide a more complete perspective on how abiotic and biotic processes affect individuals, species' niches and ultimately community-level structure than traditional uses of trait means. Body size serves as a proxy for a suite of traits that govern species' niches. Distributions of co-occurring species body sizes can inform niche overlap, relate to species richness and uncover mechanistic drivers of diversity. We leveraged individual-level body size (length) in freshwater fishes and environmental data from the National Ecological Observatory Network (NEON) for 17 lakes and streams in the contiguous United States to explore how abiotic and biotic factors influence fish species richness and trait distributions of body size. We calculated key abiotic (climate, productivity, land use) and biotic (phylogenetic diversity, trait diversity, community-level overlap of trait probability densities) variables for each site to test hypotheses about drivers of ITV in body size and fish diversity. Abiotic variables were consistently important in explaining variation in fish body size and species richness across sites. In particular, productivity (as chlorophyll) was a key variable in explaining variation in body size trait richness, evenness and divergence, as well as species richness. This study yields new insights into continental-scale patterns of freshwater fishes, possible only by leveraging the paired high frequency, in situ abiotic data and individual-level traits collected by NEON.