Trophic structure of macroinvertebrates in alpine non-glacial streams

Alpine landscapes are changing rapidly due to environmental change, ultimately affecting the structure and function of running water ecosystems. This study examined the trophic structure, using stable isotopes of carbon and nitrogen, of macroinvertebrate assemblages in 32 non-glacial headwaters in 4 glacierized alpine catchments in Switzerland in summer and autumn. We predicted a simple trophic structure (primary and secondary consumers) in streams with consumer isotopic signatures reflecting predominant primary producers (autochthonous versus allochthonous sources) in each season. Principal components analysis revealed seasonal shifts in water physico-chemistry in each catchment, mostly due to changes in phosphorous constituents. Catchment differences in stream physico-chemistry reflected differences in local geologies; i.e., gneiss versus dolomite geologies. Trophic structures, based on nitrogen isotope ratios (delta N-15), were indeed simple, comprising primary consumers (mean delta N-15 = -2.2 +/- 0.4 SE) of basal resources and secondary consumers (predators; mean delta N-15 = 1.4 +/- 0.6 SE). All streams were fishless. Carbon ratios (delta C-13) of primary resources ranged on average from -27.2 +/- 1.4 SE (riparian plants) to -24.6 +/- 0.6 SE (periphyton), with only riparian plants showing a significant difference between summer and autumn. Carbon ratios (delta C-13) of macroinvertebrates ranged from -22.2 to -27.9 with higher values found in autumn than summer for Limnephiliidae and Perlodidae. The results indicated that allochthonous resources provided a significant energy base to these headwater streams, in contrast to glacial-fed alpine streams having a dominant autochthonous resource base. The results showed that primary consumers in these alpine streams were mostly generalists (omnivores), adjusting diets to capitalize on changes in available resources. Omnivory may buffer macroinvertebrate assemblages in alpine streams to shifts in basal resources due to landscape transformation resulting from ongoing environmental change.