The effects of weirs on structural stream habitat and biological communities.
Most of the world's rivers are affected by dams and weirs. Information on the quantitative and qualitative effects of weirs across biological communities is crucial for successful management and restoration of stream ecosystems. Yet, there is a lack of comprehensive studies that have analysed the serial discontinuity in direct proximity of weirs including diverse taxonomic groups from algae to fish. This study compared the abiotic stream habitat characteristics upstream and downstream of weirs as well as their effects on the community structure of periphyton, aquatic macrophytes, macroinvertebrates and fish at five different study rivers. Physicochemical habitat characteristics discriminated strongly between upstream and downstream sides of weirs in terms of water depth, current speed, substratum composition and the transition between free-flowing water and interstitial zone. Accordingly, abundance, diversity, community structure and functional ecological traits of all major taxonomic groups were indicative of serial discontinuity, but the discriminative power of individual taxonomic groups strongly differed between rivers. The simultaneous inclusion of abiotic habitat variables, taxonomic diversity and biological traits in multivariate non-metric multidimensional scaling was most comprehensive and powerful for the quantification of weir effects. In some cases, the intrastream discrimination induced by weirs exceeded the variation between geographically distant rivers of different geological origin and drainage systems. Community effects were generally detectable on high levels of taxonomic resolution such as family or order level. Synthesis and applications. River sections in spatial proximity to weirs are affected seriously and should be included in the ecological assessments of the European Water Framework Directive. Multivariate models that include several taxonomic groups and physicochemical habitat variables provide a universally applicable tool for the ecological assessment of impacts on serial discontinuity and other stressors on stream ecosystem health.