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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Ongoing global warming is expected to alter temperature-dependent processes. Nevertheless, how co-occurring local drivers will influence temperature sensitivity of plant litter decomposition in lotic ecosystems remains uncertain. Here, we examined the temperature sensitivity of microbial-mediated decomposition, microbial respiration, fungal biomass and leaf nutrients of two plant species varying in litter quality. We also assessed whether the type of microbial community and stream water characteristics influence such responses to temperature. We incubated alder (Alnus glutinosa) and eucalypt (Eucalyptus globulus) litter discs in three streams differing in autumn–winter water temperature (range 4.6–8.9 °C). Simultaneously, in laboratory microcosms, litter discs microbially conditioned in these streams were incubated at 5, 10 and 15 °C with water from the conditioning stream and with a water control from an additional stream. Both in the field and in the laboratory, higher temperatures enhanced litter decomposition rates, except for eucalypt in the field. Leaf quality modified the response of decomposition to temperature in the field, with eucalypt leaf litter showing a lower increase, whereas it did not in the laboratory. The origin of microbial community only affected the decomposition rates in the laboratory, but it did not modify the response to temperature. Water quality only defined the phosphorus content of the leaf litter or the fungal biomass, but it did not modify the response to temperature. Our results suggest that the acceleration in decomposition by global warming will be shaped by local factors, mainly by leaf litter quality, in headwater streams.

Intermittent streams, dominant in arid and semi-arid regions, are suggested to be more representative of the global river network than perennial rivers. Even so, the impacts of constant changes in hydrological regime on the functioning of these streams and riparian areas remain to be elucidated. In this study, two native deciduous litter species were used to compare microbialdecomposition patterns between the channel of an intermittent stream and its riparian area over one year. Overall, the stream channel presented higher decomposition rates and fungal biomass than the riparian area, for both litter species. Despite a prolonged absence of streambed surface water (254 days), differences in hydrological conditions in the wetter seasons (autumn and winter) led to lingering effects, shaping and differentiating decomposition dynamics in both zones throughout the whole hydrological cycle. As the present results highlight the importance of the "hydrological imprint" for the leaves degradation process, long term studies seem to be advisable over short-term ones to better understand the functioning of intermittent streams.

These authors contributed equally to the development of this work. The rest of the authors are listed in random order. AbstractCoordinated distributed experiments (CDEs) enable the study of large-scale ecological patterns in geographically dispersed areas, while simultaneously providing broad academic and personal benefits for the participants. However, the effective involvement of early-career researchers (ECRs) presents major challenges. Here, we analyze the benefits and challenges of the first CDE exclusively led and conducted by ECRs (i.e. ECR-CDE), which sets a baseline for similar CDEs, and we provide recommendations for successful CDE execution. ECR-CDEs achieve most of the outcomes identified in conventional CDEs as well as extensive benefits for the young cohort of researchers, including: (i) receiving scientific credit, (ii) peer-training in new concepts and methods, (iii) developing leadership and communication skills, (iv) promoting a peer network among ECRs, and (v) building on individual engagement and independence. We also discuss the challenges of ECR-CDEs, which are mainly derived from the lack of independence and instability of the participants, and we suggest mechanisms to address them, such as resource re-allocation and communication strategies.