A GRADIENT OF INTERSTITIAL NIPHARGUS-RHENORHODANENSIS POPULATIONS IN 2 KARST FLOODPLAIN TRANSITION ZONES OF THE FRENCH JURA

The Bou-Rouch technique was used to sample two small alluvial plains i n two different karst areas. One was mainly supplied by karst water (V erna), the other by both karst and surface water (Pissoir). Physical a nd chemical characteristics varied with time (seasons) and location of sampling sites at both stations. The distribution of Niphargus rhenor hodanensis populations was influenced by the geogaphical characteristi cs of the outlets. Only a small number of individuals was found near t he outlet.and none in the floodplain when the outlet was located at a base-level (Verna). Individuals were more numerous and their presence constant at all sites and dates when the outlet was located at the ove rflow level of the massif (Pissoir).