RESEARCH ARTICLE Are hyporheic oligochaetes efficient indicators of hydrological exchanges in river bed sediment? A test in a semi-natural and a regulated river Michel Creuzé des Châtelliers | Sylvain Doledec | Michel Lafont | Marie-José Dole-Olivier | Lara Konecny | Pierre Marmonier Univ Lyon, Université Claude Bernard Lyon 1, CNRS, ENTPE, UMR 5023 LEHNA, Villeurbanne, France Correspondence Michel Creuzé des Châtelliers, Univ Lyon, Université Claude Bernard Lyon 1, CNRS, ENTPE, UMR 5023 LEHNA, F-69622 Villeurbanne, France. Email: michel.deschatelliers@univ-lyon1.fr Funding information Electricité de France, Grant/Award Number: Convention 109651; French Water Agency, Grant/Award Number: 2013-0412; Graduate School H2O'Lyon, Grant/Award Number: ANR-17-EURE-0018; Zone Atelier Bassin du Rhône (ZABR) Abstract River-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. KEYWORDS aquatic oligochaeta, braided river, ecological indicator, hyporheic zone, invertebrates, meandering river 1 | INTRODUCTION Exchanges of water between rivers and surrounding aquifers are cru- cial (Hancock, 2002), mostly because groundwater inflows control the stream temperature (Olsen & Young, 2009) and nutrient inputs (Malard, Tockner, Dole-Olivier, & Ward, 2002), whereas surface water infiltration inside sediment regulates the amount of dissolved oxygen and biodegradable organic matter coming from the surface to the hyp- orheic zone (Claret, Marmonier, & Bravard, 1998). As a result, the composition of hyporheic assemblages strongly relies on these Received: 22 September 2020 Revised: 16 November 2020 Accepted: 18 November 2020 DOI: 10.1002/rra.3758 River Res Applic. 2020;1–9. wileyonlinelibrary.com/journal/rra © 2020 John Wiley & Sons Ltd 1