AUTHORS: Shaley Valentine, Aquatic Ecology Lab, Ohio State University
ABSTRACT: River theory suggests predictable changes in biological resource availability occurs when the river’s physical structure shifts. However, little empirical evidence exists to know if river theories can predict shifts in resource use along environmental gradients and better understand the macrosystem structuring of difficult to study large rivers. Here, I quantified the prey and habitat use of eight native species across space and time using trace element, stable isotope, and diet analyses in the Upper Mississippi River System (UMRS). The UMRS varies in its physical structure from homogenous and complex in the upper reaches to (Pools 4, 8, 13) to physically simple and disparate in the lower reaches (Pool 26, the Open River reach, and the La Grange Pool of the Illinois River), and I expected the resource use of fishes to follow this complexity pattern. At the smallest scale, diets of predatory fishes were homogenous in the physically similar upper reaches. At a seasonal scale, contrary to expectations, resource use breadth increased and isotopic overlap decreased from upstream to downstream. And at life-long scales, fish more often originated from tributaries and other river reaches than the mainstem river moving downstream. Combined, these data suggest that the physical environment of the UMRS affects the habitat and prey use of native fishes. These results follow different riverine theories depending on scale: physically homogenous reaches exhibited similar resource use (Serial Discontinuity Concept) and physically disparate research exhibited stark contrasts in resource use (River Continuum Concept or Riverine Ecosystem Synthesis). Regarding macrosystem structuring, these stark differences suggest the upper and lower reaches of the UMRS have different ecosystem structures and functions and that management of this system should occur at both local and system-wide scales.
