Hyporheic flowpaths and interstitial invertebrates associated with stable and eroded river sections: interactions between micro- and meso-scales
Résumé
Erosion of the river beds linked to human activities is currently a crucial issue. By inducing strong changes in river dynamics, this erosion alters many of the related ecological processes. The modification of the relationships between the river and its aquifer due to erosion is not really understood. The large scale consequences of erosion for the interactions between river and the underlying aquifer must be clarified, together with the small scale effects of river erosion on hyporheic flowpaths. The role of water exchange within the hyporheic zone on the distribution of interstitial invertebrates is universally recognized, but nothing is known on the potential links between riverbed erosion, the subsequent modification of water exchange at a local scale and their consequences on the distribution and biomass of hyporheic invertebrates. Two sections, stable versus eroded, were studied on the Herbasse, a third-order stream, by sampling hyporheic water and invertebrates at two depths within the substrate (−15 and −50 cm deep), upstream and downstream of ten riffles. Seven physical and chemical parameters were used to identify the hydrological flowpaths at both the riffle (up and downwelling areas) and the section scales (eroded vs stable) and to specify the level of nutrients in the hyporheic habitat. Significant differences were observed between the stable and eroded sections both in the vertical and longitudinal dimensions, with for example available oxygen decreasing with depth and along hyporheic flowpaths, and revealing more obvious differences in the eroded section gaining groundwater than in the stable section. Most of the hypogean organisms (Niphargus sp., Salentinella juberthiei, and Proasellus sp.) were most abundant at the deepest level, in the upwelling zones and the eroded section, while typical epigean taxa (Gammarus sp., Plecoptera and Ephemeroptera) were mostly present near the surface and in the stable section, suggesting that erosion processes may influence the development of hyporheic assemblages at riffle and reach scales.