River ecosystems are composed of a spatially and temporally heterogeneous matrix of aquatic, semi-aquatic, and terrestrial elements that are highly coupled via physical, chemical, and biotic linkages. Food webs in river ecosystems reflect this complexity as they can be highly variable over space and time and are characterized by longitudinal (upstream-downstream) and lateral (aquatic-terrestrial) connectivity, which provides energetic resources (e.g., nutrients, organic matter) to consumers in adjacent habitats. Food webs relate information about diversity, community composition, and ecosystem function. However, spatially-explicit food-web data remain scarce for river ecosystems and, thus, environmental determinants of food-web structure remain largely unresolved. In particular, anthropogenic gradients relating to river regulation and land-cover change disrupt hydrogeomorphic regimes, influence water quality, and alter fundamental properties of river ecosystems with potential consequences for cross-boundary linkages and food-web dynamics. At 12 riverine landscape study sites distributed along an urban-rural gradient of the Scioto River system, Ohio, I investigated the influence of anthropogenic and natural environmental factors on (1) reciprocal aquatic-terrestrial invertebrate flux dynamics, (2) trophic structure and food-chain length (FCL) of fish food webs, and (3) the contribution of aquatically-derived energy to terrestrial food webs.;Anthropogenic impacts relating to modified riparian landscape composition, water quality, and dams, as well as natural gradients in river size exerted strong influences on river food webs. A landscape composition gradient from urbanization to grassland-dominated riparian land cover explained patterns in aquatic insect emergence and terrestrial invertebrate inputs to the river. In particular, urban landscapes were positively associated with reduced emergence flux density and biomass and reduced biomass of terrestrial inputs, whereas riparian grasslands were negatively associated with these factors. Fish food webs were characterized by fewer predator species and smaller-bodied prey above dams than below while FCL was shorter above dammed reaches (3.88) than below (4.19). Fish FCL also responded positively to natural gradients in river size (e.g., channel width) ranging from 3.66 -4.13 at narrower reaches to 4.24 - 4.35 at the widest reaches. Aquatic energy represented a considerable contribution to terrestrial consumers that forage across different spatial scales ranging from sedentary shoreline tetragnathid spiders (76% from aquatic) to riparian swallows (67% from aquatic) and raccoons (59%) that forage over 100s to 1000s of meters. Moreover, terrestrial consumers relied on both benthic algae (33% reliance across all consumers) and phytoplankton (32% reliance), highlighting the importance of separate aquatic energy pathways to terrestrial food webs in mid-size river systems.;Collectively, my results indicate that anthropogenic changes to rivers can have appreciable impacts on riverine food webs. Additionally, I have shown that aquatic energy, in addition to terrestrially-derived energy, can fuel terrestrial food webs in mid-sized rivers and that the aquatic energetic signature can extend further from the river than previously considered. Finally, because of the inherent relationship between food webs and ecosystem function and stability, I anticipate results from this research will contribute to the ongoing development of a more holistic and functional approach to management and conservation of river-riparian ecosystems in the face of increasing anthropogenic influences.
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