INFLUENCE OF WATER ABSTRACTION ON THE MACROINVERTEBRATE COMMUNITY GRADIENT WITHIN A GLACIAL STREAM SYSTEM - LA BORGNE DAROLLA, VALAIS, SWITZERLAND

1. Water abstraction from glacial rivers is an important characteristi c of hydroelectric power schemes in Alpine regions. Streams in the Val ais region of Switzerland have been particularly affected. 2. Inverteb rate distributions are described in La Borgne d'Arolla, a glacial stre am 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 (Diam esa), and are devoid of fauna for between 200 and 500 m below the glac ier snouts. 4. Immediately below the water intakes the streams are int ermittent, flowing only during system purges and high floods, and are devoid of fauna for short distances (< 1.5 km). 5. Further downstream, abstraction of glacial meltwater increases the importance of snowmelt and groundwater, increasing water temperatures, improving water clari ty 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 groundw ater-dominated tributary streams with stable channels, often at much h igher altitudes than the main river. The tributaries provide sources f or rapid colonization of the main channel following ice retreat or phy sical disturbance. 8. Purges and high floods are important disturbance s within the main channel. Recovery may be rapid because of drift from tributaries, but sites influenced by frequent disturbances have reduc ed faunas in comparison to stable channel sites. 9. This study support s the model proposed by Milner & Petts (1994) and shows that determini stic responses of macroinvertebrate communities may be observed to cha nges of temperature, turbidity, flow regime and channel stability.