Sub-basin and temporal variability of macroinvertebrate assemblages in Alpine streams: when and where to sample?

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L'environnement alpin face au changement climatique: glaciers, rivières et paysages, quels enjeux?

Alors que tous s’accordent dans l’idée de conduire une gestion environnementale optimale, c’est-à-dire de déterminer des mesures efficaces qui permettent à la balance énergie-écologie de couvrir l’ensemble des objectifs, l’enjeu réside parfois dans le manque de connaissances concernant le fonctionnement des systèmes naturels, et des écosystèmes notamment. Les environnements alpins, moins bien maitrisés, devraient faire l’objet d’une attention toute particulière, d’autant plus en cette année des Risques 2018. Cet article expose les principaux résultats de recherche du groupe AlpWISE ( wp.unil.ch / alpenv ) des scientifiques de l’Université de Lausanne, sous la direction du Professeur Dr. Stuart Lane. Il s’agit de déterminer les réponses des éléments du paysage au changement climatique rapide par une approche intégrative regroupant en particulier géographie, géomorphologie et écologie. La figure 1 présente un résumé des interactions du paysage discutées, avec le Val d’Arolla pour terrain d’étude

Summer is in winter: Disturbance-driven shifts in macroinvertebrate communities following hydroelectric power exploitation

In Alpine streams, humans have strongly modified the interactions between hydraulic processes, geomorphology and aquatic life through dams, flow abstraction at water intakes and river channel engineering. To mitigate these impacts, research has addressed both minimum flows and flow variability to sustain aquatic ecosystems. Whilst such environmental flows might work downstream of dams, this may not be the case for water intakes. Intakes, generally much smaller than dams, are designed to abstract water and to leave sediment behind. Sediment accumulation then results in the need to flush intakes periodically, often more frequently than daily in some highly glaciated basins. Sediment delivery downstream is then maintained through short duration floods with very high sediment loads. Here we tested the hypothesis that sediment flushing, and the associated high frequency of bed disturbance, controls in-stream habitat and macroinvertebrate assemblages. We collected macroinvertebrates over a 17-month period from an Alpine stream as well as a set of lateral unperturbed tributaries that served as controls. In contrast to established conceptual models, our results showed that the stream is largely void of life during summer, but that populations recover rapidly as the frequency of intake flushing falls in early autumn, producing richer and larger populations in winter and early spring. The recovery in autumn may be due to the recruitment of individuals from tributaries. We conclude that intake flushing in summer inverts expected summer-winter macroinvertebrate abundances, and questions the extent to which environmental flows in intake-impacted Alpine streams will lead to improvements in instream macrofauna unless sediment also is managed

Lidar measurement of surface melt for a temperate Alpine glacier at the seasonal and hourly scales

This study shows how a new generation of terrestrial laser scanners can be used to investigate glacier surface ablation and other elements of glacial hydrodynamics at exceptionally high spatial and temporal resolution. The study area is an Alpine valley glacier, Haut Glacier d'Arolla, Switzerland. Here we use an ultra-long-range lidar RIEGL VZ-6000 scanner, having a laser specifically designed for measurement of snow- and ice-cover surfaces. We focus on two timescales: seasonal and daily. Our results show that a near-infrared scanning laser system can provide high-precision elevation change and ablation data from long ranges, and over relatively large sections of the glacier surface. We use it to quantify spatial variations in the patterns of surface melt at the seasonal scale, as controlled by both aspect and differential debris cover. At the daily scale, we quantify the effects of ogive-related differences in ice surface debris content on spatial patterns of ablation. Daily scale measurements point to possible hydraulic jacking of the glacier associated with short-term water pressure rises. This latter demonstration shows that this type of lidar may be used to address subglacial hydrologic questions, in addition to motion and ablation measurements