Summary We conducted a paired large‐scale predation experiment over 32 months in two streams being seasonally shaded by deciduous riparian trees, using the benthivorous fish species gudgeon (Gobio gobio) and stone loach (Barbatula barbatula) as top predators. The biomass of benthic grazers and periphyton in the presence/absence of fish was measured and the periphyton production was compared with the consumption rates using a model‐based approach. A three‐level trophic cascade from benthivorous fish via benthic grazers to periphyton was evident from the field experiment. Integrated over the whole study period, fish reduced the biomass of benthic grazers and indirectly increased the periphyton biomass. Scenario analyses, using a simple dynamic model, indicated top‐down control of periphyton to be strongest during autumn, when periphyton growth was light‐limited, and weaker in the spring, when periphyton growth was not light‐limited. The seasonal light supply variation was caused by shading due to deciduous riparian trees during the vegetation period. This asymmetry in temporal processes weakened the top‐down control in a natural benthic community. Even though grazer biomass is naturally reduced in summer, due to the emergence of the most abundant species (mayflies), a grazer biomasses high enough to reduce the spring periphyton peak could not be sustained by the low summer periphyton growth. We suppose that the temporal decoupling of grazer biomass from periphyton biomass might be caused by the very short generation time of the primary producers (days) compared with the long generation time of the primary consumers (mostly 1 year).

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The freshwater pearl mussel (FPM) is among the most endangered freshwater species worldwide. The few remaining populations suffer from low recruitment rates and are subject to habitat fragmentation, pollution, siltation, decline or loss of host fish populations, and climate change. Successful conservation strategies for FPM require a holistic understanding of its ecological requirements, life history, population dynamics, and habitat prerequisites. Although habitat requirements are well described, food requirements at different life stages have received less attention. Stable isotope analyses of FPM and potential food resources in three German streams were combined with mixing model analysis to quantify organic matter resources assimilated by juvenile (first year after encystment from host fish) and semi‐adult (10 years old, immature) individuals. There were only slight differences in dietary contributions between the two life stages, and terrestrial particulate organic matter and benthic organic matter contributed substantially to the diet. Tissue type was more important in explaining variation in dietary contributions than individual variation for semi‐adult FPM. The strong reliance on terrestrial resources sheds new light on the functional role of unionid mussels and the connection of streams to their riparian area. The dependence of FPM on terrestrial resources also emphasizes the need for a stronger focus on the restoration and protection of intact riparian areas, including wetlands with their specific vegetation, when planning conservation and management strategies for threatened FPM populations.