Are surface water characteristics efficient to locate hyporheic biodiversity hotspots ?
Résumé
Location 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 river
sectors are considered. We studied a 13 km-long sector of the Cèze River (Southern France) located in a karstic
canyon.We compared five indicators of river-groundwater exchanges: longitudinal profiles of temperature, electrical
conductivity and 222Rn concentrations in the surface water of the river, chemical characteristics of the
hyporheic water and hyporheic biodiversity. Upwelling zones occurred downstream of geomorphological heterogeneities
(here at the tail of gravel bars). Surface water chemistry, especially electrical conductivity and
222Rn concentrations, clearly traces large scale gaining sections, which were not associated to valley narrowing
but with lateral springs, suggesting a crucial role of the geological structuration of the karstic plateau of
Méjanne-le-Clap. Hyporheic water chemistry fits with the large-scale hydrological pattern, but with a high variability
corresponding to local heterogeneities. The stygobite fauna (obligate groundwater organisms) and benthic
EPTC (Ephemeroptera, Plecoptera, Trichoptera and Coleoptera) occurred preferentially in the gaining
sections fed by groundwater, likely because of oligotrophicwater and cooler temperature. The spatial distribution
of river-groundwater exchange zone and hyporheic biodiversity may be thus predicted using changes in surface
water chemistry, especially for electrical conductivity and 222Rn concentrations.
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