Simulating rewetting events in intermittent rivers and ephemeral streams: a global analysis of leached nutrients and organic matter

Climate change and human pressures are changing the global distribution and extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56‐98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached organic matter. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events

Une synthèse globale de la biodiversité face au recul des glaciers

International audienceAt the interface between the cryosphere, hydrosphere, pedosphere, glacier-influenced ecosystems are particularly sensitive to the impacts of climate change. Global warming has hastened both the rate and extent of glacial melting, which foreshadows global-scale modification in biodiversity patterns and functions and make these ecosystems critically endangered. Usually, these glacier-fed systems are referred as hostile environments characterized by low diversity and productivity. However, glacial influence also creates specific habitats sheltering unique ecosystems. Although our understanding of the ecological consequences of glacier retreat has improved significantly in the past decade, we still lack a comprehensive framework that can predict biodiversity responses to glacier retreat globally. To address these gaps, we conduct a meta-analysis of biodiversity change across glacial influence using 2100 observations from more than 200 published studies in the three major glacier-influenced systems: tidewater glacier-fed fjords, glacier-fed freshwaters, and terrestrial glacier forefields. We show that on average glacial influence has a negative effect on taxa abundance and richness, thereby forecasting a general increase in local diversity as glaciers retreat (winners). However, we also observe a significant heterogeneity among community and population responses to glacial influence, with 6-11% of studied populations threatened by the glacier retreat (losers). Most losers are specialised taxa, adapted to glacial conditions while winners are generalist taxa colonizing from downstream. Our analysis further identifies key geographical variables (glacier cover, isolation and melting rates but not latitude nor altitude) and species traits (body size and trophic position) that would modulate taxa sensitivity to glacia

Une synthèse globale des réponses de la biodiversité à la retraite glaciaire

International audienceGlaciers cover about 10% of the Earth's land area but they are retreating rapidly and many will disappear within decades. Glacier retreat is a worldwide phenomenon increasing the threat to water resources, biodiversity and associated ecosystem services for hundreds of millions of people, mostly in developing countries. Our understanding of the ecological consequences of glacier retreat has improved significantly in the past decade, but we still lack a comprehensive framework for predicting biodiversity responses to glacier retreat globally, across diverse habitats and taxa. By conducting a global meta-analysis of 234 published studies, including more than 2,100 biodiversity surveys covering marine, freshwater and terrestrial assemblages, we show here that taxon abundance and richness generally increase at lower levels of glacier influence, suggesting that diversity increases locally as glaciers retreat. However, significant response heterogeneity was observed between study sites and species: 6-11% of the studied populations, particularly in fjords, would lose out from glacier retreat. Most of the losers are specialist species, efficient dispersers, uniquely adapted to glacial conditions, whereas the winners are generalist taxa colonizing from downstream. Our global analyses also identify key geographic variables (glacier cover, isolation and melting rates, but not latitude or altitude) and species traits (body size and trophic position) likely to modulate taxon sensitivity to glacial retreat. Finally, we propose mechanistic diagrams for model development to predict biodiversity change following glacier retreat

Une influence glaciaire réduite conditionne la composition et la qualité du périphyton dans les cours d'eau alpins

[Departement_IRSTEA]Eaux [TR1_IRSTEA]QUASARE [ADD1_IRSTEA]Systèmes aquatiques soumis à des pressions multiplesInternational audienceGlacier retreat alters both physical and chemical characteristics of glacially influenced streams, with consequences on the biological communities colonizing these headwaters. While reduced glacial influence is known to alter the structure of alpine stream invertebrate communities, effects on other biotic groups such as primary producers remain poorly quantified. In 68 study sites along a gradient of glacial influence, we characterized both periphyton community structure [biomass and the relative abundance of diatoms (+ chrysophytes), green algae, and cyanobacteria] and functioning (photosynthetic and enzymatic activities), and measured algal growth rate. Using comprehensive data sets from glacier-fed streams in the Eastern and the Western Alps we linked these periphyton features to climate-change associated environmental modifications. In addition to known effects of declining glaciers on algal diversity, we here provide quantitative estimations of periphyton quality and the relative coverage of algal and bacterial groups on streambed surfaces in response to environmental changes. We found, for example, that harsh environmental conditions (e.g., cold water temperatures and high turbidity) favor the dominance of diatoms and chrysophytes within the periphyton, which groups are considered to be of higher nutritional quality for grazing invertebrates when compared to cyanobacteria. These results will thus help to anticipate changes in food resources, and thereby food webs in alpine catchments as response to declining glaciers

Réponses écologiques aux fluctuations de débit dans les cours d'eau tropicaux des Andes

International audienceGlacier-fed streams display very specific flow regime characterized by a strong temporal variability at various temporal scales due to the complexity of the water storages and releases by glaciers. However, the acceleration of glacier shrinkage under the ongoing climate change results in a strong alteration of glacier runoff. In particular, we expect an increase in both low and high extreme flow events. In a tropical glacierized catchment, we examine the relationship between flow fluctuation and aquatic community composition from the diurnal to the inter-annual time scales to identify the mechanisms driving the community structure and predict the aquatic biodiversity response to potential changes in flow regime under global warming. Based on observational and experimental studies, we show that the benthic fauna is highly adapted to the natural flow fluctuation and display both resistance and resilience to high flow events. However, our studies suggest that an increase in frequency of those extreme flow events might prevent communities to fully recover leading to irreversible shifts; and that reduction in glacier runoff would strongly affect the remarkable mountain stream biodiversity

Réponses écologiques à la fluctuation multi-échelles du débit dans les cours d'eau tropicaux alimentés par des glaciers

International audienceGlacier-fed streams display very specific flow regime characterized by a strong temporal variability at various temporal scales due to the complexity of the water storages and releases by glaciers. However, the acceleration of glacier shrinkage under the ongoing climate change results in a strong alteration of glacier runoff. In particular, we expect an increase in both low and high extreme flow events. In a tropical glacierized catchment, we examined the relationship between flow fluctuation and aquatic community composition from the diurnal to the inter-annual time scales to identify the mechanisms driving the community structure and predict the aquatic biodiversity response to changes in flow regime under global warming. Based on long-term observational studies, we showed that the benthic fauna was highly adapted to the natural flow fluctuation and displayed both resistance and resilience to high-flow events. On the contrary, a 4-year flow manipulation study indicated that the benthic ecosystem was affected by low-flow conditions. Flow reduction induced a rapid shift in invertebrate community composition characterized by an increase in generalist species density while the system required 30 times longer to return to its initial state. Our studies suggested that the increase in frequency of low-flow events might prevent communities to fully recover leading to irreversible shifts. Thus, the reduction in glacier runoff might induce a homogenization of the aquatic fauna in glacierized catchments, leading to a reduction of the remarkable mountain stream biodiversity

Écosystèmes aquatiques de haute montagne: menaces et conséquences ?

International audienceThough high mountain freshwaters are generally referred as pristine ecosystems, they have been and will be affected by multifaceted anthropogenic pressures combined with the effects of the ongoing climate change. While the acceleration of glacier shrinkage and the reduction in snow cover induce a continuous, relatively slow flow reduction at a global scale; anthropogenic activities generate abrupt but local disturbances. However, the current boom in hydropower plant constructions and the growing water needs of ski resorts for drinking water and artificial snow, particularly during low-water periods, end up multiplying these local disturbances, generating thus flow alteration at the watershed scale. However, because high mountain watersheds were historically (and are still) considered as remote unspoiled areas with unlimited water resources, eco-hydrological scientific studies are scarcer in these regions and the development of management tools have been neglected. Based on observational and experimental studies in glacierized catchments in both the Alps and the Andes, we analysed the processes that drive the distribution of aquatic-invertebrate communities. Our results show that the dynamic of the different water source contribution to mountain stream networks (glacier and snow meltwater, groundwater, rain) generate a mosaic of connected habitats that engender a strong taxonomic heterogeneity at the watershed scale, through local (environmental filtering, species interaction) and spatial (dispersal limitation) processes. These studies indicate that multiple flow alterations modify both quality and availability of aquatic habitats as well as the connectivity among them, altering the freshwater community composition, function, and dynamic. Improving our understanding of the underlying processes driving these ecosystems is crucial to provide environmental flow rules for freshwaters and develop appropriate management tools to mitigate the impacts of flow alteration and maintain these ecosystems and their resulting ecosystem services

Effet de la variabilité temporelle du débit sur les écosystèmes aquatiques de montagne

International audienceGlacier-fed streams display very specific flow regime characterized by a strong temporal variability at various temporal scales due to the complexity of the water storages and releases by glaciers. However, the acceleration of glacier shrinkage under the ongoing climate change results in a strong alteration of glacier runoff. In particular, we expect an increase in both low and high extreme flow events. In a tropical glacierized catchment, we examine the relationship between flow fluctuation and aquatic community composition from the diurnal to the inter-annual time scales to identify the mechanisms driving the community structure and predict the aquatic biodiversity response to potential changes in flow regime under global warming. Based on observational and experimental studies, we show that the benthic fauna is highly adapted to the natural flow fluctuation and display both resistance and resilience to high and low flow events. However, our studies suggest that an increase in frequency of those extreme flow events might prevent communities to fully recover leading to irreversible shifts; and that reduction in glacier runoff would strongly affect the remarkable mountain stream biodiversity

Réponse des métacommunautées aquatiques de montagne à l'altération d'écoulements à plusieurs échelles

International audienceMountain watersheds exhibit particularly complex hydrographic networks characterized by both a high spatial and temporal environmental heterogeneity linked to the dynamic of the different water source contributions (glacier and snow meltwater, groundwater, rain). Mountain freshwater ecosystems shelter a singular biodiversity characterized by rare species, often endemic, and display a high spatial variability in aquatic community composition linked to this high level of habitats availability. However, these ecosystems are threatened by the ongoing climate change and associated flow alterations linked to both the accelerated glacier shrinkage and the modification in snow cover. In addition, although mountain regions were historically assimilated to water towers of the surrounding low lands with unlimited water resources; these regions are on the contrary facing unprecedented anthropogenic pressures on the resource, especially due to the current boom in hydropower projects but also the growing water needs of ski resorts for drinking water and artificial snow particularly during low-water periods. These flow alterations induce changes in the hydrological regime at the local scale; but also affect, at the catchment scale, the environmental heterogeneity in habitats and the connectivity of the stream network. These multi-scale alterations might have considerable effects on both the structure and functioning of mountain aquatic ecosystems. In this study, we examined the spatial distribution of aquatic invertebrate communities in two mountain glacierized catchments, with low (3%) and high (30%) glacial influence located respectively in the Andes and the Alps. We sampled benthic invertebrate communities and characterized geographical, physico-chemical and food resource conditions in 51 and 29 stream sites in the Andean and Alpine catchment, respectively. Using both distance decay relationship and variation partitioning analyses, we evaluated the relative contribution of local environmental conditions and spatial processes in structuring the aquatic metacommunity. This study allowed a better understanding of the mechanisms driving the spatial organization of the aquatic communities. This knowledge would permit to develop multi-scale environmental flows in mountains watersheds and management tools to mitigate the impacts of flow alteration and maintain these mountain freshwater ecosystems and their resulting ecosystems services