Urban and suburban development is known to have marked impacts on the structure and composition of stream biotic assemblages, largely through altered habitat and food resources. However, mechanisms controlling these alterations remain less clear. To address how hydrogeomorphic (i.e., physical) instream alterations may drive biotic (i.e., food-web) shifts in urban streams, we investigated the impacts of urbanization-induced hydrogeomorphic characteristics on stream fish food web structure in 22 stream reaches in the Columbus Metropolitan Area, Ohio, USA. Reach lengths of approximately 20-30x bankfull widths were surveyed to calculate first- and second-order hydrogeomorphic variables, and were used to evaluate fish assemblage characteristics and collect individuals for stable-isotope (13C and 15N) analysis. Basal resources (i.e., algal and detrital samples) were also sampled. SIAR was used to determine fish (Creek Chub [Semolitus atromaculatus] and sunfish [Lepomis cyanellus and L. macrochirus]) reliance on aquatic (i.e., algal) resources and trophic position. Analyses focused on relationships between fish assemblage composition and food-web characteristics, and both hydrogeomorphic characteristics (e.g., incision ratio, shear stress) and geomorphic adjustment (e.g., widening, incision).
Results/Conclusions
Fish density was higher in less entrenched reaches with higher bankfull discharges and finer substrate, suggesting overbank flooding and floodplain access may influence the quantity of basal resources available to fish assemblages. Fish in larger, less incised stream reaches with coarser substrate were more reliant on aquatically-derived energy (i.e., energetic pathways derived from benthic algae), largely driven by an increase in the proportion of insectivorous and tolerant individuals and an increase in reliance on aquatically-derived energy among sunfish species (Lepomis cyanellus and Lepomis macrochirus). Both floodplain connectivity (characterized by incision and entrenchment ratios) and benthic substrate characteristics appear to be critical to fish-assemblage characteristics as well as major drivers of fish food-web structure. Our results contribute to a growing body of evidence of specific urban-associated physical alterations and their impact stream food webs, with potential impacts on ecological stability of these vulnerable systems.