Variations in the structure of stygobiont crustacean populations (Niphargus rhenorhodanensis and Proasellus valdensis) within the sediments of a karst outflow

The interstitial fauna living within the sediments of a cave outflow situated in Verna (France) was mainly represented by two stygobiont species, the amphipod Niphargus rhenorhodanensis and the isopod Proasellus valdensis. Their population dynamics was observed by means of artificial substrates buried in the alluvia. Except for some individuals of Niphargus neither of these species was observed outside the cave and had low densities inside (0 to 30 individuals per species and per basket). These densities varied with time and depth within the sediment. The more superficial levels were more populated than the deeper ones. The reduction with depth was more marked for Proasellus than for Niphargus. Two distinct periods separated by a violent spate appeared during this study. The first period was characterized by high and relatively changing densities and the second by a low and constant one (except for the last sample of June 1989). Thus the recolonization of the sediment after the spate was very low. No correlations were observed between particulate organic matter (> 150 µm) and the number of individuals. The interpretation of the results took into account biological cycle features and preferential occupation of space for both species as well as the disturbance in the hydrological cycle and the phenomenon of competition.