Are surface water characteristics efficient to locate hyporheic biodiversity hotspots ?

International audienceLocation of river-groundwater exchange zones and biodiversity hotspot is essential for a river management plan,especially for Mediterranean karstic rivers. This location is often difficult and time-consuming when long riversectors are considered. We studied a 13 km-long sector of the Cèze River (Southern France) located in a karsticcanyon.We compared five indicators of river-groundwater exchanges: longitudinal profiles of temperature, electricalconductivity and 222Rn concentrations in the surface water of the river, chemical characteristics of thehyporheic water and hyporheic biodiversity. Upwelling zones occurred downstream of geomorphological heterogeneities(here at the tail of gravel bars). Surface water chemistry, especially electrical conductivity and222Rn concentrations, clearly traces large scale gaining sections, which were not associated to valley narrowingbut with lateral springs, suggesting a crucial role of the geological structuration of the karstic plateau ofMéjanne-le-Clap. Hyporheic water chemistry fits with the large-scale hydrological pattern, but with a high variabilitycorresponding to local heterogeneities. The stygobite fauna (obligate groundwater organisms) and benthicEPTC (Ephemeroptera, Plecoptera, Trichoptera and Coleoptera) occurred preferentially in the gainingsections fed by groundwater, likely because of oligotrophicwater and cooler temperature. The spatial distributionof river-groundwater exchange zone and hyporheic biodiversity may be thus predicted using changes in surfacewater chemistry, especially for electrical conductivity and 222Rn concentrations.zab