Stable isotope analysis of macroinvertebrates and their food sources in a glacier stream

1. Food sources and trophic structure of the macroinvertebrate community along a longitudinal gradient were examined in a glacier stream of the Swiss Alps (Val Roseg). Analysis of multiple stable isotopes (delta (13)C and delta (15)N) and measurement of C : N ratios were used to differentiate between allochthonous and autochthonous organic matter. 2. Although isotopic signatures of algae varied widely among sites and dates, it was possible to discriminate between allochthonous and autochthonous food sources using a site-specific approach. 3. Dominant food sources of herbivorous invertebrates in all main channel sites were epilithic diatoms and the filamentous gold alga Hydrurus foetidus. Allochthonous organic matter was of some importance only in a groundwater-fed stream close to the floodplain margin. 4. Seasonal changes in the delta (13)C signature of the macroinvertebrates corresponded with seasonal changes in delta (13)C of the gold alga H. foetidus. This indicated that the energy base remains autochthonous throughout the year. 5. Because of limited food sources, feeding plasticity of the invertebrate community was high. Both grazers and shredders fed predominantly on algae, whereas gatherer-collectors seemed to be omnivorous. 6. The overall enrichment of delta (15)N was 2.25 parts per thousand. (r(2) = 0.99) per trophic level. On a gradient from the glacier site to a downstream forested site trophic enrichment was constant but variation in delta (15)N within trophic levels decreased.