Stable isotopes reveal food web modifications along the upstream-downstream gradient of a temperate stream

The upstream-downstream gradient (UDG) is a key feature of streams. For instance food webs are assumed to change from upstream to downstream. We tested this hypothesis in a small European river catchment (937 km(2)), and examined whether food web modifications are related to structural (i.e. food web composition) or functional changes (i.e. alteration of linkages within the web). We adopted a double approach at two levels of organisation (assemblage and species levels) using two isotopic metrics (isotopic space area and isotopic niche overlap), and proposed a new hypothesis-testing framework for exploring the dominant feeding strategy within a food web. We confirmed that the UDG influenced stream food webs, and found that food web modifications were related to both structural and functional changes. The structural change was mainly related to an increase in species richness, and induced functional modifications of the web (indirect effect). In addition, the UDG also modified the functional features of the web directly, without changing the web composition. The proposed framework allowed relating the direct effect of the UDG to a diet specialisation of the species, and the indirect effect via the structural changes to a generalist feeding strategy. The framework highlights the benefits of conducting the double approach, and provides a foundation for future studies investigating the dominant feeding strategy that underlies food web modifications.