The salinization of freshwaters is a global water quality problem that leads to the biological degradation of aquatic ecosystems. However, little is known about the spatial extent of freshwater salinization and the relative contribution of each human activity (e.g. agriculture, urbanization, mining or shale-gas extraction). Here, we investigated environmental factors that explain spatio-temporal patterns of water salinity and examined the causes, the extent and the degree of salinization of Spanish rivers. Results showed a strong variation in water salinity among river typologies and between river reaches in good and poor ecological status according to the Water Framework Directive. The variation in water salinity was largely explained by a combination of natural (i.e. climate and geology) and anthropogenic (i.e. land use) factors. By contrast, land use factors as urbanization and agriculture were the main drivers of salinization, which affected more than one quarter of the rivers and streams in Spain, especially those in the most arid regions (central and southern regions) and in the main courses of the largest rivers such as the Ebro, Douro and Tajo rivers. The information provided here can be relevant to set priority regions and actions to ameliorate freshwater salinization. This article is part of the theme issue ‘Salt in freshwaters: causes, ecological consequences and future prospects’.

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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Coordinated 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 reallocation and communication strategies. We conclude that ECR-CDEs can be a relevant tool to empower ECRs across disciplines by fostering their training, networking and personal well-being.

Understanding how different food resources sustain stream food webs is fundamental towards increasing our knowledge on trophic structure and energy flow pathways in fluvial ecosystems. Food webs in small mountain streams are sustained by autochthonous (instream primary production) and allochthonous (inputs from the terrestrial ecosystem) organic resources, with their relative importance highly dependent on catchment land cover. This study aimed to understand how catchment land cover determines food resource type (autochthonous, allochthonous) and quantity in mountain streams, and how this affects energy flow pathways and food web structure. We hypothesised that food resource type and quantity would reflect catchment land cover. Thus, changes in food resources would lead to shifts in macroinvertebrate assimilation of autochthonous and allochthonous food resources and consequently in dominant energy flow pathways. We further hypothesised that changes in food resources will have strong effects on dominant feeding groups and community biomass distribution among taxa in food webs. Energy flow pathways were quantified by combining macroinvertebrate biomass measures and assimilation of food resources estimated from δ2H and δ15N in 10 streams along a forest cover gradient, located in the Cantabrian Mountains (northern Spain). Results showed that grassland/shrub dominated streams had a higher proportion autochthonous food resources and a lower proportion of allochthonous food resources, whereas forested streams showed the opposite pattern. Changes in food resources with forest cover resulted in shifts in food resource assimilation and dominant energy flow pathways. Forested streams were mainly sustained by allochthonous resources, while streams flowing through grassland/shrub landscapes were mostly sustained by autochthonous resources. Food resource assimilation differed between feeding groups. Detritivores showed a fixed assimilation of allochthonous resources independent of resource quantity, while omnivore assimilation was determined by the dominant food resource. This was reflected in food‐web structure. There was an increase in detritivore biomass and conservation of omnivore biomass with increasing forest cover, leading to a more equal distribution of community biomass among macroinvertebrates comprising individual food webs. The dependence of stream food webs on dominant food resources highlights the importance of catchment land cover in determining energy flow pathways and food web structure in low order mountain streams. These findings will improve our predictions on the effects of land cover change on the functioning of mountain stream ecosystems.

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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.