Running water ecosystems are widely recognized as being disproportionately affected by a diverse range of anthropogenic activities. Humans impact such ecosystems via myriad interacting pressures, including land use modification, water abstraction, in-stream construction, and nutrient and waste inputs. Such stressors often act in unison to negatively affect riverine communities, and can ultimately modify fundamental ecosystem attributes such as food chain length. Here, we used a stable isotopic approach to assess food chain length variability in relation to human disturbance at differential spatial scales across a large suite of streams and rivers throughout Switzerland. This allowed us to identify major gradients of stress, interactions among stressors, and their effects on this unifying ecosystem property.
Results/Conclusions
Based on isotopic analyses of nearly 2400 individual fish collected from over 200 sites across Switzerland, we reveal dramatic variation in δ15N values in predatory fish, with raw δ values spanning ~1 – 18 ‰, a difference equivalent to several whole trophic levels. These data were contextualised with relevant aquatic and terrestrial baseline data from each site, in order to assign site-specific food chain length estimates, and allow direct inter-site comparisons. Whilst a significant portion of the variance in raw δ15N values could be explained by differential baseline content, marked differences in food chain length still persisted between streams following correction. Preliminary analyses suggest that food chain length co-varies with human stressor gradients, with food chains typically simplified and shortened in the most impacted sites, whilst more pristine systems displayed greater natural variability.