Spatially-dependent human alterations determine fish assemblage composition in a modified river system

Abstract

Human landscape modifications (e. g., land-use change) and river regulation (e. g., dams) introduce multiple stressors that can disrupt natural spatially-structured environmental gradients along rivers and alter fish assemblage composition. At 12 river reaches in the Scioto River system of Ohio, USA, we surveyed fish assemblages and environmental variables (e. g., physical river characteristics, riparian land use and land cover, water chemistry) along an urban-to-rural landscape gradient. Mantel tests indicated that fish assemblage β-diversity was positively correlated with both environmental similarity and distance (river kilometers, rkm) between reaches. However, partial Mantel tests revealed no relationship between β-diversity and distance between study reaches after removing the influence of environmental variables; whereas similarity in environmental conditions remained a signifi cant predictor of β-diversity after removing the influence of distance between reaches. Redundancy analysis showed that physical river characteristics (e. g., channel width, floodplain area) and riparian land cover accounted for more variation in assemblage composition (42%) than did spatial location alone (7%), but that a portion of this variation (30%) was shared (i. e., spatially structured), suggesting a spatial dependence of environmental-fish relationships. The upstream concentration of dams played a critical role in disrupting patterns in fish assemblages along the longitudinal course of the river system. Overall, the strong infl uences of non-randomly distributed human disturbances appeared to govern fish diversity and assemblage composition. Thus, new paradigms may be needed to better understand and assess the multiple environmental and underlying spatial determinants of fish assemblage composition in highly modified, novel river ecosystems.