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Metal accumulation in aquatic food webs is mediated by physiochemical parameters of the environment and organismal traits. Trophic strategies influence an organisms’ exposure to metal pollution, but links between trophic ecology and exposure to divalent metals are relatively understudied. While organically bound metals are typically considered unavailable for uptake, organisms directly consuming dissolved organic carbon (DOC) and bacteria—via the microbial loop—must also be consuming organically bound metals. Hence, we predicted animals feeding within the microbial loop would accumulate metals through their diet. To test this prediction, we exploited dietary differences between two organisms, Simulium vittatum,a filter-feeding black fly and Hyalella azteca, a shredding detritivore. We exposed both species to three treatments of DOC (labile, recalcitrant, and no additional DOC) that were crossed with exposure to variable copper (Cu) concentrations (2–14 μg L–1) in laboratory microcosms. As predicted, H. aztecaexperienced a buffering effect by DOC. However, this pattern was not apparent for S. vittatum. Our results highlight the importance of considering trophic strategies when examining the impacts of metal pollution on aquatic communities, and demonstrate the potential for the microbial loop to facilitate metal uptake in freshwater food webs.

Quantifying the role that freshwater ecosystems play in the global carbon cycle requires accurate measurement and scaling of dissolved organic carbon (DOC) removal in river networks. We reviewed reach‐scale measurements of DOC uptake from experimental additions of simple organic compounds or leachates to inform development of aquatic DOC models that operate at the river network, regional, or continental scale. Median DOC uptake velocity (vf) across all measurements was 2.28 mm min−1. Measurements using simple compound additions resulted in faster vf(2.94 mm min−1) than additions of leachates (1.11 mm min−1). We also reviewed published data of DOC bioavailability for ambient stream water and leaf leachate DOC from laboratory experiments. We used these data to calculate and apply a correction factor to leaf leachate uptake velocity to estimate ambient stream water DOC uptake rates at the reach scale. Using this approach, we estimated a median ambient stream DOC vfof 0.26 mm min−1. Applying these DOC vfvalues (0.26, 1.11, 2.28, and 2.94 mm min−1) in a river network inverse model in seven watersheds revealed that our estimated ambient DOC vfvalue is plausible at the network scale and 27 to 45% of DOC input was removed. Applying the median measured simple compound or leachate vfin whole river networks would require unjustifiably high terrestrial DOC inputs to match observed DOC concentrations at the basin mouth. To improve the understanding and importance of DOC uptake in fluvial systems, we recommend using a multiscale approach coupling laboratory assays, with reach‐scale measurements, and modeling. Better understanding of DOC processing in river networks is neededReach‐scale DOC uptake velocity is faster for simple compounds than leachatesReach‐scale DOC uptake velocities are implausibly fast at the network scale