1. Recent advances in molecular methods foster the documentation of small spatial scale biological diversity over large geographical areas. These advances allow to correctly record α-diversity, but also enable biomonitoring that describes in

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Microplastic particles are found globally in all kinds of water bodies posing a serious threat to aquaticorganisms and ecosystems. However, plastic concentrations are not homogeneously distributed and can decreasealong a river course. Thus, potential sinks need to exist. Dams and reservoirs are proposed to be such potentialsinks for microplastics. We investigated plastic concentrations at the water surface upstream and downstream of areservoir inundated by a wastewater treatment plant effluent by filtering water with a net with 200 μm mesh size.We sampled at flow velocities ranging from 6 cm s–1 to 35 cm s–1. Upstream of the reservoir significantly higherconcentrations of microplastics (19.9 ± 7.3 particles per m– 3) than downstream (3.4 ± 2.2 particles per m– 3) wereidentified. Furthermore, the plastic concentrations increased with increasing flow velocities. Hence, reservoirs canbe sinks of microplastics as decreased flow velocities may lead to an increased sedimentation or decreased erosionpotential. Consequently, flow velocities are critical when identifying plastic loads. Single time or single spot measurementsmay not well represent plastic loads if flow velocities vary considerably in time or space.

Steadily increasing inputs of microplastics pose a growing threat to aquatic fauna, but laboratory studies potentially lack realism to properly investigate its effects on populations and ecosystems. Our study investigates the trophic and ontogenetic transfer of microplastics in a near-natural exposure scenario. The controlled outdoor freshwater mesocosms were exposed to polyamide (PA) 5–50 µm in size in concentrations of 15 and 150 mg L−1 and a control without microplastic addition. To verify the uptake of particles via the food chain, larvae and imagines of the midges Chaoborus crystallinus and C. obscuripes were examined, which feed on zooplankton during their larval stage. Larvae were captured after 117 days and imagines were caught in emergence traps that were emptied weekly. To detect the microparticles within the organisms, 200 larvae and 100 imagines per application were macerated and treated with fluorescent dye before investigation under a fluorescent microscope. We could detect up to 12 PA particles per individual larvae, while nearly no plastic was found in the imagines. This shows that, while Chaoborus sp. takes up microplastics via predation, most of the pollutant is egested through regurgitation and remains in the water, where it can further accumulate and potentially harm other organisms.