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1. Population declines in freshwater mussels (Unionida) resulting from failed recruitment, together with episodic water pollution potentially leading to juvenile mussel mortality, are often suspected (but rarely well documented), even in the most strictly protected aquatic areas.2. The aim of this study was to test a robust design for an in situ investigation of the water pollution effects on juvenile freshwater pearl mussel (FPM, Margaritifera margaritifera) in the protected Malše River (Czech Republic), which acts as a model system. A 30-km river reach was delineated to include a section with previously recorded episodic multiple point-source pollution events, as well as downstream and upstream control sections. Juvenile FPMs in open water mesh cages were exposed to river water at 15 sites in 42 exposure units (342 juveniles in individual chambers), and their survival and shell growth were assessed after a 33-day period of low-flow and high-temperature conditions. The main physicochemical indicators related to municipal water pollution were measured weekly.3. The study results confirmed significantly impaired water quality indicated by episodic changes in total ammonia nitrogen (up to 1.037 mg L À1 ), total phosphorus (up to 0.516 mg L À1 ) and oxygen saturation (down to 75.0%) in the impaired section. While the growth of FPM (corrected for temperature effects) was significantly increased (shell length increment: 32.7 ± 9.7%impaired, 26.4 ± 10.0%control sections; mean ± SD), the survival (mean ± SD) of juveniles was significantly decreased in the impaired section (86.4 ± 12.5%) compared with the control sections (97.4 ± 5.7%).4. This study has demonstrated the capacity of applying a caged bivalve model at a fine spatial scale within a longitudinal river profile to document the impacts of water pollution. The methodology applied here may provide a useful tool for assessing and improving the level of water quality management and is applicable in FPM action plans and stream biotope conservation in general.

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