Shifts in fish habitat use and diet breadth across a modified large river's physical gradient follow river theories

Theories suggest that the physical environment of rivers affects their biological community's structure and interactions, but the directionality of effects differs across theories. Yet, most developed theories may not directly apply to modified rivers. As modified rivers become more frequent and river systems remain difficult to study, it is prudent to use appropriate river theories to guide our understanding of macrosystem structuring of rivers. To begin to test river theory in modified rivers, we used stable isotopes to quantify the resource use (i.e. habitat and diet breadth) of fish species and assemblages in the Upper Mississippi River System. We quantified the isotopic niche space, Layman community metrics, and isotopic niche overlap among eight fish species in five sampling reaches. We expected increased resource use breadth and reduced overlap moving downstream at both the species and assemblage levels potentially due to the accumulation of resources from upstream and an increase in ecosystem size. Results matched our expectations across the whole system at the species and assemblage level and across three physically similar reaches at the assemblage level, indicating longitudinal, network, or zonal theories can predict this system's structuring; these theories were previously thought to poorly characterize modified rivers. Contrarily, most species had similar resource use breadth metrics across physically similar reaches, indicating homogenization in resource use and consistent with the serial discontinuity concept, a modified river theory. More explicit spatial testing could determine mechanisms behind these patterns, which river theories best explain biological responses at finer spatial scales, and whether observed patterns are consistent across other modified rivers. Our data highlight that the fish assemblage in downstream reaches of the Upper Mississippi River System differed in habitat use and diet breadth compared to upstream reaches, suggesting concomitant differences in ecosystem functioning and that applied ecological actions should differ spatially.