RESPONSE OF INVERTEBRATES TO LOTIC DISTURBANCE - IS THE HYPORHEIC ZONE A PATCHY REFUGIUM

1. Natural experiments, in the form of disturbance from spates, were u sed to study the resistance and resilience of interstitial communities . Investigations were conducted in a by-passed section of the Rhone Ri ver characterized by an artificial hydrology with frequent spates sepa rated by regular minimum discharge of 30 m(3) s(-1). 2. Three areas of a bar were studied, upwellings at the head of the bar (stations 1 and 2), and downwelling at the tail of the bar (station 3). In the head o f the bar the substratum was characterized by stable cobbles, while mo bile gravels dominated in the tail of the bar. At each station, sample s were derived from four depths (0.5, 1.0, 1.5 and 2.0 m below the sur face of the substratum). Fifteen spates occurred during the study peri od whose peak discharge ranged from 50 to 1640 m(3) s(-1). Temporal va riations of the fauna were studied by comparing the spate effect obser ved 1 day (resistance), 7 days (resilience) and 17 days after the spat e. Within-class correspondence analysis was used to compare the tempor al variability of the fauna within each class (station/depth). 3. The fauna differed markedly between the three stations, and the relative d ensity of stygobionts (i.e. hypogean fauna) decreased from 55% at stat ion 1 to 4% at station 3. The spatio-temporal variability increased dr amatically from station 1 to station 3. 4. The results suggest that th e hyporheic zone acts as a patchy refugium: the stations were more or less active refugial zones, depending on hydrology (upwelling or downw elling), substratum stability and spate amplitude. 5. The downwelling station was the main refugium area for benthic taxa. Important migrati ons of benthic groups (e.g. Gammarus, Cladocera) or hyporheic taxa (e. g. Cyclopoida and Harpacticoida) were observed deep into the sediment (2 m). Vertical movements of stygobionts (Niphargus, Niphargopsis) wer e also observed at high amplitude spates. These movements were very im portant (great numbers of individuals migrated) at low and medium magn itude spates, but were unimportant at high discharge, when the thresho ld of sediment instability was exceeded. In this case the substratum b ecame mobile and induced drift of benthic organisms. 6. Conversely, in the upwelling stable stations, accumulation was less important (lower number of species and lower densities) but more constant with increas ing discharge, suggesting that substratum stability is also a key fact or. 7. Generally recovery was rapid at all stations (within 7 days) bu t no relationships were found between resilience (rate of recovery) an d the amplitude of spates.