Abstract: 1. 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. 2. 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. 3. 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. 4. Energy flow pathways were quantified by combining macroinvertebrate biomass measures and assimilation of food resources estimated from ?2H and ?15N in 10 streams along a forest cover gradient, located in the Cantabrian Mountains (northern Spain). 5. 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. 6. 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. Thiswas 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. 7. 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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