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International audience; Glacier 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.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]QUASARE [ADD1_IRSTEA]Systèmes aquatiques soumis à des pressions multiples; International audience; Optimizing biological monitoring (long-term surveys, restoration assessments) in large rivers and other environments.

International audience; Glacier retreat is a worldwide phenomenon with important consequences for the hydrological cycle and downstream ecosystem structure and functioning. To determine the effects of glacier retreat on aquatic communities, we conducted a 4-year flow manipulation in a tropical glacier-fed stream. Compared with an adjacent reference stream, meltwater flow reduction induces significant changes in benthic fauna community composition in less than 2 weeks. Also, both algal and herbivore biomass significantly increase in the manipulated stream as a response to flow reduction. After the flow reduction ceased, the system requires 14-16 months to return to its pre-perturbation state. These results are supported by a multi-stream survey of sites varying in glacial influence, showing an abrupt increase in algal and herbivore biomass below 11% glacier cover in the catchment. This study shows that flow reduction strongly affects glacier-fed stream biota, prefiguring profound ecological effects of ongoing glacier retreat on aquatic systems.