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Summary1. 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. 2. 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. 3. 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. 4. 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. 5. 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.

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Knowledge on the extent and mechanisms of fish damage caused by hydropower facilities is important for their ecological improvement. Herein, a novel field-based fish injury assessment protocol is proposed that includes vitality and four general health criteria, as well as nine lethal and sub-lethal injury types across 18 body parts. The protocol was validated using 3,087 specimens from four species of hatchery-reared fish, as well as 2,262 specimens from 32 species of wild fish. The protocol allowed a detailed and systematic evaluation of different fish injury types in the field. Injuries related to handling and to contact with different parts of the hydropower structure could be distinguished applying multivariate statistics. This approach allows quantification and comparison of fish injuries across sites, and can help to identify the technologies and operational procedures that minimise damage to fish. It may also be useful to assess fish health in other contexts including aquaculture. K E Y W O R D Sanimal welfare, fish conservation, fish mortality, fish passage, hydropower monitoring | INTRODUCTIONHydropower technology is considered a clean and renewable energy source of increasing worldwide importance (Zarfl, Lumsdon, Berlekamp, Tydecks & Tockner, 2015). Hydropower is generated by converting the kinetic and potential energy from falling water into rotating shaft power, which can be used to drive an electricity generator (Paish, 2002). Unfortunately, downstream moving fish often enter the hydropower structure where they are exposed to extreme risks of harm (Williams, Armstrong, Katopodis, Larinier & Travade, 2012).Various physical mechanisms can result in various forms of fish damage, including collisions with the machinery (Killgore, Maynord, Chan & Morgan, 2001), bar screens or cleaning devices (Adam & Brujis, 2006;Nettles & Gloss, 1987;Skalski, Mathur & Heisey, 2002), shear stress near the turbine blades, in the draft tube and in the tailrace (Čada, Garrison & Fisher, 2007), barotrauma caused by pressure changes , cavitation forces within the runner case, turbulences and fluid shear within the suction hose as well as in the tailrace (Abernethy, Amidan & Čada, 2001). Resulting injuries include scale loss, fin damage, haemorrhages, bruises, skin wounds, amputations of body parts or internal injuries, such as swim bladder rupture and emboli (Dedual, 2007;Ebel, 2013;Schneider, Hübner & Korte, 2012). As movement and migration are obligatory elements in the life cycle of many fishes (Lucas, Baras, Thom, Duncan & Slavík, 2001), the injuries and mortalities resulting from passage through turbines are still a major drawback of hydroelectric energy (Hogan, Čada & Amaral, 2014).The severity of the impact of power plant passage is dependent on technical characteristics such as the rotation speed of the turbine, turbine diameter, number of turbine blades, blade angle and the drop height, which determine the degree of pressure changes, shear stress, cavitation and the collision risk for fish (Ferguson, Ploskey,...

Stream substratum plays a key role for many riverine species and has become a focus topic in the context of structural habitat improvements. There is a lack of studies that compare the effectiveness of different substratum restoration measures. Herein, we compare four restoration techniques (two different gravel introductions, substratum raking and sickle-formed constrictor) that were carried out in six replicate rivers. Each measure was monitored for changes in physicochemical substratum quality and the effects of the construction work on downstream sites. Generally, the effects on physicochemical substratum quality were highly variable between restoration types and rivers and strongly decreased within 1 year. Most pronounced changes of substratum quality were detected for the gravel introductions. Substratum raking and the sickle-formed constrictor had the smallest effects, which were dependent on the original substratum composition of the restored sites. At the same time, substratum raking caused an average fine sediment deposition of 17 kg m À2 on downstream sites, being sixfold higher than for the other measures. Consequently, all of the investigated substratum restoration techniques are confined to short-term improvement of substratum quality. This finding, together with the observed damage on downstream sites, suggests that a rethinking of the currently applied restoration techniques is required, better considering catchment and natural substratum dynamics in river restoration.