Abstract : Ecological criteria are needed for a comprehensive evaluation of groundwater ecosystem health by including biological components with the physical and chemical properties that are already required by European directives. Two methodological approaches to assess the ecological status of groundwater ecosystems were combined in two alluvial plains (the Ariège and Hers Rivers, southwestern France) varying in agriculture intensity (from grassland to crop rotation including maize and sunflower, and to maize monoculture). In the first approach, the composition of invertebrate assemblages (only obligate-groundwater crustaceans, i.e. stygobionts) sampled in 28 wells differing in their land use contexts was analysed. Abundance, species richness, and assemblage composition significantly changed with agricultural land use or urbanization around the wells. In the second approach, we tested an in situ exposure of sentinel organisms to quantify their response to the environmental pressures. The epigean and native amphipod species Gammarus cf. orinos was used as the sentinel species. Amphipods (30 individuals in each of 10 wells) were exposed for one week to the in situ conditions at two seasons with contrasted concentrations of pollutants. The Ecophysiological Index (EPI) synthetizing the survival rates and energetic storage decreased in wells with low oxygen and high nitrate concentrations, but only during the highest contamination period. Atrazine-related compounds negatively impacted sentinel health whatever the season. The combination of these two approaches may have major applications for orientating groundwater ecosystem management.
