Effects of an experimental increase in flow intermittency on an alpine stream

Abstract

Flow intermittency occurs naturally in alpine streams. However, changing rainfall patterns and glacier retreat are predicted to increase the occurrence of flow intermittency in alpine catchments, with largely unknown effects on ecosystem structure and function. We conducted a flow manipulation experiment within a headwater stream of Val Roseg, a glacierized alpine catchment, to determine the effects of increased flow intermittency on aquatic macroinvertebrates, periphyton, benthic organic matter, and trophic structure. Compared to an adjacent reference channel, an increase in flow intermittency reduced macroinvertebrate density, taxa richness, and the proportion of rheophilic taxa. Density and richness remained low in the manipulated channel after resumption of natural flow. Flow intermittency did not affect organic matter standing stocks, but increased assimilation of periphyton by aquatic macroinvertebrates.Predation on aquatic invertebrates by riparian spiders also increased. We attribute many of these patterns to the timing of drying, which likely excluded summer-growing cohorts of rheophilic, aerial dispersers. This study suggests that reductions in summer glacial melt and rainfall events might increase flow intermittency and lead to fundamental changes in diversity and function of alpine fluvial networks.

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

“…First, we (1) grouped the isotope signatures of filamentous algae and periphyton (hereafter: periphyton) based on source δ 13 C and δ 15 N overlap and ecological similarity (Phillips et al 2005), (2) excluded consumer isotope values ( n = 125 out of 481 total) that were unsuitable for mixing model analysis (Smith et al 2013) (Supplementary Information Methods 1), and (3) eliminated sites ( n = 4) from the dataset where the source signatures of periphyton and CPOM (i.e., the two most distal isotope signatures; Siebers et al 2019a) overlapped for both δ 13 C and δ 15 N (i.e., if the mean ± standard deviation values overlapped for both elements), as we expected that consumers from these sites would present unrealistically diffuse dietary estimates (Phillips et al 2014). Basal resource isotope values were aggregated for each site across seasons (Supplementary Table S1), as: (1) we previously observed little variation in resource δ 13 C and δ 15 N from spring to autumn in Val Roseg (Siebers et al 2020; although c.f. Sertić Perić et al 2021); yet (2) we considered our sampling design (periphyton collected at a single time point per season) might have resulted in unrealistically low variance in estimated basal resource δ 13 C and δ 15 N, particularly periphyton, if season‐specific values were used ( see Jardine et al 2014).…”

“…Overlaying these seasonal patterns in basal resource availability are the ecological effects of drying events. A higher frequency of drying events can reduce the quality of terrestrial organic matter within streams (Datry et al 2018) and restrict the overall abundance of macroinvertebrates (Siebers et al 2020), thereby increasing the relative value of algae to macroinvertebrates as well as reducing competition for this higher‐quality food resource. Benthic macroinvertebrates in high‐altitude streams often exhibit high dietary flexibility in response to environmental variability (Fell et al 2017; Niedrist and Füreder 2017).…”

Predicting flow intermittence in France under climate change

“…Changes in the seasonality of peak and low flows may strongly affect community composition, particularly in snowdominated hydrological systems (see Mustonen et al 2018) such as Alpine streams. Summer intermittence may particularly affect the communities (Piano et al 2019, Siebers et al 2020 and the functioning of Alpine stream ecosystems (Siebers et al 2019) that have historically been little exposed to such events. Strategies that confer resistance to freezing could also serve to resist drying (Tolonen et al 2019); however, such change in seasonality is likely to override species' adaptation capacity, leading to important changes in freshwater community composition (Mustonen et al 2018).…”

Beta diversity of aquatic macroinvertebrate assemblages associated with leaf patches in neotropical montane streams

“…Mountain headwater streams play a fundamental role in processing and transporting terrestrial and aquatic organic matter, and usually harbor high biodiversity (Boyero et al., 2016). Typically, aquatic communities in mountaintop headwater streams reflect adaptations to local environmental conditions (e.g., low temperature and nutrient availability, high current velocity or wind, high channel slopes, coarse substrates, physical habitat diversity, and riparian zone, and terrestrial ecosystem integrity) (Siebers et al., 2020). It is important to focus on both spatially extensive ecology (i.e., beta diversity) and site‐extent ecology (alpha diversity) because management of ecological systems must be extensive and local.…”