Disentangling small dam impacts: macroinvertebrate metrics reveal local stressors in degraded agricultural streams

Agricultural land use and dams present prevalent, interacting stressors on freshwater ecosystems across Europe and globally. This study assessed the efficacy of established, taxonomy-based macroinvertebrate metrics for detecting localized dam impacts within streams already heavily modified by agriculture. Community responses were compared between rare riffle habitats and the dominant, slow-flow habitat types within the catchments. Metrics were based on the relative species richness of sensitive and tolerant taxonomic groups (calculated as the number of species within taxonomic groups divided by the total number of species). Despite a lack of significant differences in measured single-event physical habitat parameters between the dammed and reference sites, the macroinvertebrate assemblages exhibited clear responses. Dam impacts were most strongly detected in riffle habitats, aligning with a priori expectations of a greater number of sensitive taxa in these habitats. The general degradation metrics effectively teased out the dam signal, which shifted toward tolerant taxa (Oligochaeta, Diptera). The responses were habitat specific; for instance, filter feeding Trichoptera increased below the dams, while riffle specialist Plecoptera decreased. These findings underscore the utility of indices of general degradation when applied strategically with targeted, habitat-specific sampling, highlighting that effective biomonitoring and conservation strategies must prioritize the assessment and protection of rare, highly valued microhabitats in multi-stressed river networks.