Influence of water abstraction on the macroinvertebrate community gradient within a glacial stream system: La Borgne d'Arolla, Valais, Switzerland

Abstract

SUMMARY1. Water abstraction from glacial rivers is an important characteristic of hydroelectric power schemes in Alpine regions. Streams in the Valais region of Switzerland have been particularly affected.2. Invertebrate distributions are described in La Borgne d'Arolla, a glacial stream with icemelt‐, snowmelt‐ and groundwater‐dominated tributaries. The icemelt‐dominated streams have been affected by abstractions for more than 30 years.3. The glacial streams contain only Chironomidae (Diamesa), and are devoid of fauna for between 200 and 500 m below the glacier snouts.4. Immediately below the water intakes the streams are intermittent, flowing only during system purges and high floods, and are devoid of fauna for short distances (<1.5km).5. Further downstream, abstraction of glacial meltwater increases the importance of snowmelt and groundwater, increasing water temperatures, improving water clarity and increasing the length of krenal/rhithral streams at the expense of kryal streams.6. A community including Chironomidae, Simuliidae, Baetidae, Nemouridae, Limnephilidae and Chloroperlidae occurs as soon as a permanent flow is maintained by tributary runoff, and the channel becomes stable.7. A wide range of taxa inhabit snowmelt‐ and groundwater‐dominated tributary streams with stable channels, often at much higher altitudes than the main river. The tributaries provide sources for rapid colonization of the main channel following ice retreat or physical disturbance.8. Purges and high floods are important disturbances within the main channel. Recovery may be rapid because of drift from tributaries, but sites influenced by frequent disturbances have reduced faunas in comparison to stable channel sites.9. This study supports the model proposed by Milner & Petts (1994) and shows that deterministic responses of macroinvertebrate communities may be observed to changes of temperature, turbidity, flow regime and channel stability.