Seasonal and functional variation in the trophic base of intermittent Alpine streams

In high-altitude Alpine streams, seasonal cycles of snowmelt, glacial melt, and rainfall drive variation in the availability of algal food resources. Yet high-altitude streams also exhibit varying degrees of flow intermittency, from solely winter-drying streams to others that dry periodically throughout summer and autumn. These environmental drivers may interact in different ways to determine the functional trophic base of macroinvertebrates inhabiting high-altitude streams. Here, we estimated the proportional contribution of autochthonous resources to the assimilated diets of benthic macroinvertebrates in 26 headwater streams of Val Roseg, a high Alpine glacial catchment, using stable isotope analysis (δ13C and δ15N) of different macroinvertebrate families and their potential food sources. We compared dietary estimates along a gradient of flow intermittency and across 3 seasons (Alpine spring, summer and autumn). Assimilation from autochthonous sources was highest for collector-gatherers and filter feeders in spring, and for grazers in summer. Grazers had higher estimated assimilation from autochthonous sources in intermittent streams than in perennial streams, particularly in summer, while collector-gatherers showed little effect of flow intermittency on dietary estimates. However, responses were highly taxon-specific, with different responses to variation in flow intermittency and season across families within functional groups. Our results suggest that frequent summer drying events represent trade-offs between greater access to algal food resources and a higher risk of desiccation, but that differing life-history and functional feeding traits across macroinvertebrate taxa drive marked variation in the risks or benefits associated with inhabitants of drying streams.

Methods
Our paper presents an observational study of aquatic macroinvertebrate diets within 30 intermittent headwater streams of Val Roseg, a glacierized Alpine catchment, in 2017 and 2018. Flow intermittency was measured from July 2017 using electrical resistance sensors to log presence/absence of water. We collected macroinvertebrates, and their potential basal food resources, from within the streams across three seasons: Alpine autumn (September 2017), spring (June 2018), and summer (August 2018). We estimated the relative contributions of autochtonous resource (periphyton and filamentous algae) to aquatic invertebrates using analysis of C and N stable isotopes. Information on data processing is summarised in the associated ReadMe file.