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Thermal regimes of aquatic ecosystems are predicted to change as climate warming progresses over the next century, with high-latitude and high-elevation regions predicted to be particularly impacted. Here, we have modelled alpine stream water temperatures from air temperature data and used future predicted air temperature trajectories (representative concentration pathway [rcp] 4.5 and 8.5) to predict future water temperatures. Modelled stream water temperatures have been used to calculate cumulative degree days (CDDs) under current and future climate conditions. These calculations show that degree days will accumulate more rapidly under the future climate scenarios, and with a stronger effect for higher CDD values (e.g., rcp 4.5: 18-28 days earlier [CDD = 500]; 42-55 days earlier [CDD = 2000]). Changes to the time to achieve specific CDDs may have profound and unexpected consequences for alpine ecosystems. Our calculations show that while the effect of increased CDDs may be relatively small for organisms that emerge in spring-summer, the effects for organisms emerging in late summer-autumn may be substantial. For these organisms, the air temperatures experienced upon emergence could reach 9°C (rcp 4.5) or 12°C (rcp 8.5) higher than under current climate conditions, likely impacting on the metabolism of adults, the availability of resources, including food and suitable oviposition habitat, and reproductive success. Given that the movement of aquatic fauna to the terrestrial environment represents an important flux of energy and nutrients, differential changes in the time periods to achieve CDDs for aquatic and terrestrial fauna may de-couple existing predator-prey interactions., (Global Change Biology© 2024 The Author(s). Global Change Biology published by John Wiley & Sons Ltd.)

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. [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]

Alpine streams can exhibit naturally high levels of flow intermittency. However, how flow intermittency in alpine streams affects ecosystem functions such as food web trophic structure is virtually unknown. Here, we characterized the trophic diversity of aquatic food webs in 28 headwater streams of the Val Roseg, a glacierized alpine catchment. We compared stable isotope (δ13C and δ15N) trophic indices to high temporal resolution data on flow intermittency. Overall trophic diversity, food chain length and diversity of basal resource use did not differ to a large extent across streams. In contrast, gradient and mixing model analysis indicated that primary consumers assimilated proportionally more periphyton and less allochthonous organic matter in more intermittent streams. Higher coarse particulate organic matter (CPOM) C:N ratios were an additional driver of changes in macroinvertebrate diets. These results indicate that the trophic base of stream food webs shifts away from terrestrial organic matter to autochthonous organic matter as flow intermittency increases, most likely due to reduced CPOM conditioning in dry streams. This study highlights the significant, yet gradual shifts in ecosystem function that occur as streamflow becomes more intermittent in alpine streams. As alpine streams become more intermittent, identifying which functional changes occur via gradual as opposed to threshold responses is likely to be vitally important to their management and conservation. [ABSTRACT FROM AUTHOR]

1. The dissolved organic matter (DOM) biogeochemistry of intermittent rivers is affected by periods of disconnection and drying after cessation of surface flow, with a pronounced concentra tion of solutes as pools evaporate and contract. However, inputs of alluvial ground water to stream pools may offset the effects of evaporation and influence the quantity and composition of DOM. 2. We used δ18 O and δℑ²H values of surface water and alluvial ground water (AW) together with DOM fluorescence excitation-emission spectroscopy to investigate changes in DOM biogeochemistry of pools from four intermittent streams in semi-arid north-west Australia. Our objective was to deter mine whether the fluorescence characteristics of DOM in pools are driven by their connectivity to alluvial ground water. 3. Pool water δ18O values ranged from -8 to 13.1‰ & and showed progressive enrichment as pools became disconnected from AW and subsequently evaporated. Parallel factor analysis of DOM fluo rescence revealed that DOM in pools was generally dominated by humic-like components, with les ser contributions from tryptophan-like and tyrosine-like components. 4. DOC concentrations were higher in more evaporated pools. Humic-like and tryptophan-like DOM were also highest in pools isolated from AW, suggesting that inputs of AW had a dilution effect while also moderating evaporative concentration of solutes. In contrast, tyrosine-like DOM was only weakly correlated with the hydrological status of pools. 5. The proportion that each component contributed to total DOM fluorescence remained mostly consistent across both AW-connected and evaporating pools. However, some pools with AW through-flow had higher contributions from tyrosine-like fluorescence and lower contributions from humic-like fluorescence. 6. Disconnection from AW and subsequent evaporative concentration of solutes is an important control on DOM fluorescence characteristics in these intermittent streams. However, production and removal of DOM were only rarely influenced by AW connectivity, likely reflecting the high hydrological variability of intermittent rivers, even following cessation of surface flow. [ABSTRACT FROM AUTHOR]