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Flow regulation of montane and alpine headwater streams can fundamentally alter food web structure and energy flows through changes in productivity, resource availability, and community assembly. Dam flow‐release schemes can be used to mitigate the environmental impacts of flow regulation via environmental flows, which can increase discharge variability and other ecologically important hydrological properties. In particular, managed floods can reintroduce disturbance to the system and stimulate the reactivation of physical habitat dynamics. However, how managed floods might restore ecosystem processes is virtually unknown. In this study, we examined patterns in potential energy fluxes before, during and after a long‐term experimental flood program on the river Spöl, a regulated alpine River in southeast Switzerland. We used benthic samples collected during long‐term monitoring and stable isotope analysis (δ13C and δ15N) of macroinvertebrates and their potential food sources to reconstruct secondary production, and potential energy fluxes, over a 20‐year study period. The experimental floods did not alter the relative importance of basal resources but resulted in a considerable decline in secondary production, which remained low after the discontinuation of the floods. Our data suggest that a lack of recolonization by mosses following the discontinuation of the experimental flood program on the river Spöl may have driven patterns in energy fluxes by limiting macroinvertebrates using mosses for habitat. The effects of environmental flows on energy flows in this system thus depend on flood disturbance and the environmental context following the discontinuation of floods. [ABSTRACT FROM AUTHOR]

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 (δ13C and δ15N) 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. [ABSTRACT FROM AUTHOR]

Intermittent and ephemeral streams in dryland regions often fragment into isolated pools as they dry. In general, pool fragmentation provides favorable conditions for aquatic algae and plant growth and should therefore result in foodweb reliance on aquatic algal resources over comparatively-recalcitrant terrestrial plant litter. However, differences in pool hydrology and morphology might substantially alter organic matter dynamics and, thus, alter the trophic base of aquatic consumers. We tested whether 1) taxa composition or 2) pool hydrology and morphology might affect the trophic base of macroinvertebrate assemblages in dryland stream pools. We used stable isotope analysis to assess the hydrological regime (δ18O and δ2H) and trophic structure (δ13C and δ15N) of fragmented pools within an ephemeral dryland stream. We compared differences in pool morphology, evaporation rates, and groundwater connectivity to dietary-mixing model results for macroinvertebrate assemblages over 2 seasons. Differences in macroinvertebrate assemblage composition were not consistently related to pool hydrology, morphology, or season. We found that filamentous algae and aquatic biofilms supported macroinvertebrate assemblages to a greater extent than did terrestrial plant litter. Biofilm assimilation was higher in highly-evaporated pools, and terrestrial plant litter assimilation decreased as pools became smaller and aquatic vegetation cover decreased. Overall trophic diversity decreased with increasing aquatic vegetation cover. This study suggests that hydrological and morphological gradients across pools drive differences in the trophic base of macroinvertebrate assemblages, potentially through variation in organic matter productivity and biomass. Consequently, natural variability in groundwater connectivity and pool morphology likely enhances trophic diversity across this ephemeral riverscape. [ABSTRACT FROM AUTHOR]