Are surface water characteristics efficient to locate hyporheic biodiversity hotspots?

Abstract

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 timeconsuming 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 oligotrophic water 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.

Drivers of functional diversity in the hyporheic zone of a large river

“…Niphargus species (here N. casparyi, N. rhenorhodanensis and N. fontanus) are known to live in the hyporheic zone of other tributaries of the Rhône River, especially in upwelling areas of gaining sectors (Marmonier et al, 2019(Marmonier et al, , 2020. In the Ain River, they were mostly sampled downstream of meanders located in the central part of the sector (station 12D and 15D) or close to the morainic hills (stations 26D, 28U-D).…”

“…In addition to their importance in the total taxonomic richness (e.g. Dole-Olivier et al, 2015;Marmonier et al, 2020), their role in ecological functions must be considered with attention (Ercoli et al, 2019;Fattorini et al, 2020;Di Lorenzo et al, 2021).…”

“…In addition, taxa such as stygobites are obligated to live in groundwater habitats (Williams, 1984;Marmonier et al, 1993). The relative composition of these three groups of species may determine zones of high diversity (hyporheic hotspots, Marmonier et al, 2020;Hutchins et al, 2020). Furthermore, the hyporheic fauna is very diverse in food preferences ranging from fine sediment feeders, to particulate organic matter-feeders (detritivores) and predators (Williams et al, 2010).…”

“…especially the location of large-scale draining sectors (i.e. sections of several kilometers where local upwelling zones are fed by deep groundwater,Marmonier et al, 2020).…”

Are hyporheic oligochaetes efficient indicators of hydrological exchanges in river bed sediment? A test in a semi‐natural and a regulated river

“…Physical and chemical indicators can help with locating the down and upwelling zones, respectively (Ward, Malard, Tockner, & Uehlinger, 1999) because groundwater inflows generally have a constant temperature, a high solute content (e.g., Ca ++ , Na + , and K + ; Grasby, Hutcheon, & McFarland, 1999; Cook, 2013), thereby inducing changes in surface water electrical conductivity (Hill & Neal, 1997). Chemical changes induced by groundwater supplies may, however, be hidden by tributaries or by temporal cycles (Marmonier et al, 2020). To avoid these problems, biological indicators of river‐groundwater exchanges were proposed for hyporheic assemblages (Graillot et al, 2014; Lafont, Camus, & Rosso, 1996; Leigh, Stubbington, Sheldon, & Boulton, 2013).…”