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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]

More than one‐third of the world's rivers cease to flow and go dry on a periodic basis—so‐called intermittent rivers. The frequency and duration of flow intermittency in running waters are increasing due to climate change and water demands for human use. Intermittency effects on stream biodiversity and ecosystem functioning are dramatic and are expected to become increasingly prevalent in alpine landscapes in the near future. This project used modified field sensors to measure flow intermittency, temperature, and water origin (groundwater, precipitation, glacier) at high spatio‐temporal resolution throughout an alpine fluvial network (Val Roseg, Switzerland). We continuously recorded water presence in 30 tributary streams and validated sensor performance with field‐collected measures. Three different flow regimes were observed in the network, including periodically intermittent, seasonally intermittent, and permanently flowing streams. Twenty‐four streams (80% of recorded streams) dried at least once during the sampling period. Principal components analysis along with generalized additive models showed alpine streams with low average temperature and high conductivity (groundwater‐fed) were prone to permanent flow, whereas streams with higher average temperature and low conductivity (glacier‐fed) typically had intermittent flow. The field sensors proved precise for simultaneously measuring flow intermittency, temperature, and water origin at high resolution throughout the river network. Overall, this approach provides an effective way to develop eco‐hydrological models that examine the effects of flow intermittency on biodiversity and ecosystem functioning in riverine networks. [ABSTRACT FROM AUTHOR]

Abstract: While light availability plays a critical role in seagrass growth and distribution, there is limited understanding of how changes in light exposure impact belowground processes. We investigated the effect of prolonged and fluctuating reductions in light on root growth and exudation by three colonizing seagrasses: Cymodocea serrulata, Halophila ovalis, and Halodule uninervis. Seagrasses were grown in mesocosms under continuous full light (control), under continuous light reduction (medium or low), or under fluctuating light (10 d of low and 4 d of high light, repeated three times). Plants were harvested (1) 6 weeks after light treatments (impact), and (2) after an additional 4 weeks of continuous full light (recovery). Root exudates were collected from trap solutions and measured for dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and dissolved organic matter (DOM) excitation/emission fluorescence spectroscopy. Root biomass decreased in all shading treatments. The most notable impact of light treatment was an increase in root exudation of DOC, protein‐like DOM and humic‐like DOM under fluctuating light for all species. After 4 weeks of recovery, exudation of DOC, and protein‐like DOM observed under fluctuating light returned to the control light levels. However exudation of DOC and protein‐like DOM by H. ovalis grown in continuous low light remained greater than the control, likely due to root death. This study suggests the belowground environment of seagrasses is sensitive to light reduction. Monitoring changes in root exudation of seagrasses can provide an effective and rapid method to assess light stress and short‐term recovery of seagrasses. [ABSTRACT FROM AUTHOR]