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

International audienceRiver-groundwater exchanges are crucial for several ecological processes, but diffi- cult to localize. Hyporheic oligochaetes may represent efficient biological indicators of these exchanges, but hydrological disturbance may alter oligochaete distribution and blur the spatial patterns of river-groundwater exchanges. Oligochaete assem- blages were compared in the Drôme River, which is a semi-natural braided river with an unmodified discharge, and the Ain River, which has reduced sediment mobility and hydropeaking due to several upstream dams. In the semi-natural river, the oligo- chaete distribution was mostly controlled by the origin of the water: some stygoxene species (e.g., Chaetogaster spp. and Vejdovskyella intermedia) were restricted to or more abundant in downwellings, whereas a set of stygophiles (e.g., Stylodrilus sp. and Cernovitoviella sp.) and stygobites (e.g., Aberrantidrilus cuspis) were present or more abundant only in upwellings. Species living in fine sediment (e.g., Potamothrix sp. and Tubifex sp.) also occurred in upwellings due to their location at the border of the braided strip. Conversely, in the regulated river, oligochaete assemblages were abun- dant and diversified, with species living in fine sediment being dominant in the par- tially clogged stations. The spatial distribution of most species did not follow typical surface-hyporheic exchange patterns: most stygophiles (e.g., Proppapus volki and Mar- ionina argentea) and some stygobites (e.g., Haber turquini) were found in both upwell- ings and downwellings or even at stations without water exchanges (e.g., Rhyacodrilus balmensis). In rivers subjected to hydrological disturbance and hydropeaking, oligochaetes became poor indicators of river-groundwater exchanges