With 10 figures and 5 tables in the text Abstract: Val Roseg in the Swiss Alps is a complex alluvial valley formed in glacial outwash. The braided flood plain, 2.6 km long and 130-510 m wide, begins 1.2 km downstream of the glacier terminus and extends to a "knickpoint" at 1990 m a.s.!. where water upwells before entering a constrained reach. A long-term study has been initiated to investigate habitat heterogeneity and how such heterogeneity (I) contribu tes to the biodiversity of benthos, groundwater fauna, and periphyton in a harsh envi ronment and (2) influences ecosystem processes such as productivity and decomposi tion dynamics. As a first step we have distinguished different channel types based on the correspondence between hydrological connectivity and physico-chemical attrib utes. This functional characterization will serve as a habitat template to structure future ecological research in the Val Roseg flood plain. Six distinct channel types have been identified within the fl oodplain ecosystem: (i) Main channel. (ii) Side channels, (iii) Intermittently-connected channels. (iv) Mixed channels, (v) Ground water channels, and (vi) Tributaries. Distinct seasonal and daily runoff patterns, caused by ice melt, change the hydrological connectivity between individual channel types. Results clearly demonstrate that the whole flood plain shifts from dominance by surface water at high summer discharge to a groundwater-controlled system in winter. Temporal variability, rather than the means of environmental values, has been used to differentiate between individual floodplain channel types. Groundwater chan nels exhibit the highest spatial but the lowest temporal variability. In contrast, inter mittently-connected channels are characterized by a low spatial but an extraordinary temporal variability. High spatio-temporal heterogeneity resulting from a diversity of channel types is believed to play a major role in maintaining what appears to be re markably high biodiversity in this glacial flood plain.

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 (δ13C and δ15N) 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 δ13C signature of the macroinvertebrates corresponded with seasonal changes in δ13C 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 δ15N was 2.25‰ (r2=0.99) per trophic level. On a gradient from the glacier site to a downstream forested site trophic enrichment was constant but variation in δ15N within trophic levels decreased.

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1. Seasonal changes in longitudinal patterns of environmental conditions and macroinvertebrate community distributions were examined in an alpine glacial stream (Roseg River, Switzerland). 2. Physico‐chemical parameters reflected seasonal changes in glacial influence via shifts in water sources and flowpaths (glacial meltwater versus ground water), and were best described by turbidity, particulate phosphorus and specific conductance. High nitrogen concentrations indicated snowmelt was the main water source in June. 3. Macroinvertebrate densities and taxon richness were highest during spring (4526 m–2 and 16 taxa, all sites combined) and late autumn/early winter (8676–13 398 m–2 with 16–18 taxa), indicating these periods may be more favourable for these animals than summer when glacial melting is maximal. Diamesa spp. (Chironomidae) dominated the fauna at the upper three sites (>95% of zoobenthos) and were abundant at all locations. Other common taxa at lower sites (1.2–10.6 km downstream of the glacier terminus) included other chironomids (Orthocladiinae, Tanytarsini), the mayflies Baetis alpinus and Rhithrogena spp., the stoneflies Leuctra spp. and Protonemura spp., blackflies (Simulium spp., Prosimulium spp.), and Oligochaeta. 4. Co‐inertia analysis revealed a strong relationship between environmental conditions and benthic macroinvertebrate assemblages. Furthermore, it elucidated temporal variability in longitudinal response patterns, as well as a similarity in temporal patterns among individual sites. 5. Our results suggest that zoobenthic gradients are not solely related to temperature and channel stability. Seasonal shifts in sources and pathways of water (i.e. extent of glacial influence), and periods of favourable environmental conditions (in spring and late autumn/early winter) also strongly influenced zoobenthic distributions.

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