REPEATED GRADIENTS IN SUBTERRANEAN LANDSCAPE - EXAMPLE OF THE STYGOFAUNA IN THE ALLUVIAL FLOODPLAIN OF THE RHONE RIVER (FRANCE)

Interstitial habitats were previously considered as homogeneous compar ed to epigean biotopes, but are now recognized as heterogeneous, mainl y because of the influence hydrology exerts on the physical, chemical and biological structure of the system. The aim of this study is to re veal several spatial gradients in subterranean landscape but at a floo dplain scale. Three types of transects were sampled: (1) a transverse profile across the floodplain, from the fluvial axis to the margins of the floodplain; (2) two longitudinal profiles at different spatial sc ales (a gravel bar and a Rhone River section) and (3) vertical profile s, from subsurface sediments (0.5 m deep) to 2.5 m deep, in the main c hannel and in the alluvial plain. Comparison of these gradients emphas izes repeated structures within the landscape: the qualitative distrib ution of the stygofauna was the same along all three gradients. The ub iquitous hypogean organisms occurred as well near the main channel as near the surface or in downwelling zones, and the strict phreatobites occurred as well on the margins of the floodplain as in the deep sedim ent or in upwelling zones. The temporal dimension appeared to be a sup plementary source of repetition, explained by the sensitivity of stygo bites to environmental fluctuations. In alluvial floodplains of Europe an large rivers, where the stygofauna is generally well diversified an d abundant, hypogen organisms could be used as tools in studies of lan dscape change and for environmental management. In this context, we pr opose an ordinated list of taxa, which could be adapted to diverse bio geographical situations.