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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. [ABSTRACT FROM AUTHOR]

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]

Streams in hot, arid environments often exist as a series of isolated pools along main channels. During these periods, shallow alluvial through flow may strongly influence key ecological processes within pools. We measured diel changes in δ13C values of dissolved inorganic carbon (DIC) and dissolved oxygen (DO) in two pools of ephemeral, dryland streams. We quantified alluvial water connectivity through stable isotope analysis (δ18O and δ2H) of pool and alluvial water. We also estimated gross primary productivity (GPP) and ecosystem respiration (ER) rates across a wider set of pools in both streams. δ13CDIC values displayed regular diel cycles, where both pools displayed small but similar daily amplitude (0.7–0.9‰) despite contrasting amplitudes of change in DO (0.8 mg L−1 vs. 2.8 mg L−1) and contrasting alluvial water connectivity (connected vs. disconnected). Water temperature was the strongest predictor of both δ13CDIC values and rates of change in δ13CDIC across both pools. Across both streams, all pools were net heterotrophic. GPP (0.35–1.73 g O2 m−2 d−1) and ER (0.49–2.64 g O2 m−2 d−1) rates were linked to aquatic vegetation cover. The disconnect between diurnal amplitudes of δ13C values and DO concentrations thus suggests that ecological drivers of gas exchange became increasingly localised as pools contracted. [ABSTRACT FROM AUTHOR]

Characterising food‐web responses to environmental factors could greatly improve our understanding of environment‐biota relationships, and especially in floodplains where trophic interactions can be particularly important during phases of hydrological disconnection. The effects of floodplain hydrology and environmental attributes on structural aspects of biotic assemblages have been extensively studied, but responses at the functional level remain largely unknown.Here, we characterised a central aspect of food‐web architecture, the food chain length, as the maximum trophic position within 24 macroinvertebrate communities of parafluvial habitats in the Maggia river floodplain, in Switzerland. We investigated how the food chain length changed with different levels of habitat size, primary productivity and disturbance, the three factors potentially affecting food chain length in both theoretical and empirical studies.We found that food chain length was lower in frequently flooded habitats and immediately after a flood. We also showed that trophic omnivory, where predators fed at lower trophic levels after flooding, and in more frequently flooded habitats, may explain these changes.These findings show that trophic omnivory may explain how predators resist disturbance and are maintained in highly dynamic landscapes. More importantly, given that trophic omnivory may overall weaken trophic linkages and thus increase food‐web stability, this suggests that it could be a key mechanism in sustaining biodiversity in river floodplains. [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]

Seagrass roots host a diverse microbiome that is critical for plant growth and health. Composition of microbial communities can be regulated in part by root exudates, but the specifics of these interactions in seagrass rhizospheres are still largely unknown. As light availability controls primary productivity, reduced light may impact root exudation and consequently the composition of the root microbiome. Hence, we analyzed the influence of light availability on root exudation and community structure of the root microbiome of three co-occurring seagrass species, Halophila ovalis, Halodule uninervis and Cymodocea serrulata. Plants were grown under four light treatments in mesocosms for 2 weeks; control (100% surface irradiance (SI), medium (40% SI), low (20% SI) and fluctuating light (10 days 20% and 4 days 100%). 16S rDNA amplicon sequencing revealed that microbial diversity, composition and predicted function were strongly influenced by the presence of seagrass roots, such that root microbiomes were unique to each seagrass species. Reduced light availability altered seagrass root exudation, as characterized using fluorescence spectroscopy, and altered the composition of seagrass root microbiomes with a reduction in abundance of potentially beneficial microorganisms. Overall, this study highlights the potential for above-ground light reduction to invoke a cascade of changes from alterations in root exudation to a reduction in putative beneficial microorganisms and, ultimately, confirms the importance of the seagrass root environment -- a critical, but often overlooked space. [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]