Environmental stressor gradients hierarchically regulate macrozoobenthic community turnover in lotic systems of Northern Italy

Environmental stressors present a hierarchical influence on freshwater organisms. This study investigates the hierarchy of environmental stressor gradients, which regulate the composition of instream macroinvertebrate communities of northern Italy (Po Valley and the south-eastern Alps). Species and environmental data were derived from 585 monitoring sites. Environmental parameters were split into three groups, describing (i) ecoregional, (ii) hydromorphological, and (iii) water quality attributes. Partial Redundancy Analysis (partial RDA) was used to hierarchically rank the group effects, which were expressed as unique (group specific) and joint effects (of two groups together). Overall, ecoregion explained more variance (30.2%) than hydromorphology (24.8%) and water quality (22.3%). Unique effects were generally low, but ecoregional unique effects were twice as high as those of the other groups. The analysis of single environmental variables highlighted significant effects of anthropogenic impact related to the substrate size composition, riparian vegetation, flow conditions, and Escherichia coli (surrogate descriptor of organic fecal pollution). Such stressor hierarchies can support biodiversity conservation plans, while the high joint effects of stressor groups suggested the need for combined management activities, addressing the respective stressors and stressor groups in concert. Management measures addressing only one stressor group isolated from others are likely to be less effective, or even ineffective.