Summary Water abstraction is rapidly increasing worldwide in order to respond to escalating demands for water, food and energy. Abstraction can alter the hydrological regime of streams and rivers, reduce in‐stream habitats, change water quality and affect fluvial communities. It can also impair ecosystem functioning, although this aspect has been seldom assessed. We experimentally tested the effect of water abstraction on stream functioning in a headwater mountain stream using a before and after control‐impact (BACI) design. We quantified changes in water physicochemical properties, biofilm biomass and activity (exo‐enzymes and metabolism), and ecosystem processes (nutrient cycling, organic matter retention and breakdown). Water abstraction did not affect water physicochemical properties, but reduced the biomass and the alkaline phosphatase activity of biofilm per surface unit, the areal uptake of nutrients, and the travel distance of organic matter. It did not affect the biofilm metabolism and breakdown of submerged leaf litter, but decreased the total breakdown. The impacts were larger when analysed per unit of channel length. All variables, except benthic chlorophyll‐a, were significantly reduced by abstraction, due to the contraction of the wetted channel. The decrease in the biomass and activity of biofilm, as well as in nutrient uptake and total organic matter breakdown observed in this study could increase nutrient concentration in downstream reaches and reduce the energy transfer to higher trophic levels. Ecosystem services of permanent headwater streams are likely to be compromised with the reduction of wetted‐channel width.

Large variability in dissolved organic carbon (DOC) uptake rates has been reported for headwater streams, but the causes of this variability are still not well understood. Here we assessed acetate uptake rates across 11 European streams comprising different ecoregions by using whole‐reach pulse acetate additions. We evaluated the main climatic and biogeochemical drivers of acetate uptake during two seasonal periods. Our results show a minor influence of sampling periods but a strong effect of climate and dissolved organic matter (DOM) composition on acetate uptake. In particular, mean annual precipitation explained half of the variability of the acetate uptake velocities (VfAcetate) across streams. Temperate streams presented the lowest VfAcetate, together with humic‐like DOM and the highest stream respiration rates. In contrast, higher VfAcetate were found in semiarid streams, with protein‐like DOM, indicating a dominance of reactive, labile compounds. This, together with lower stream respiration rates and molar ratios of DOC to nitrate, suggests a strong C limitation in semiarid streams, likely due to reduced inputs from the catchment. Overall, this study highlights the interplay of climate and DOM composition and its relevance to understand the biogeochemical mechanisms controlling DOC uptake in streams.

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