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

Abstract

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, ranging 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 (δ 13 C and δ 15 N) of different macroinvertebrate families and their potential food sources. We compared dietary estimates along a gradient of flow intermittency and across three 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 tradeoffs 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.

Long‐term reconstruction of energy fluxes in an alpine river: Effects of flow regulation and restoration

“…A notable difference is that FPOM retained a terrestrial signature from the reservoir in Colorado, whereas FPOM appears to be a combination of terrestrial and aquatic sources in the Spöl. Alpine River systems are, in general, limited in terrestrial organic matter inputs and constrained by autochthonous resource availability (Siebers et al, 2022). Our results suggest that these limitations might also control energy fluxes in the food web.…”