Do benthic invertebrates use hyporheic refuges during streambed drying ? A manipulative field experiment in nested hyporheic flowpaths

The existence of resilience mechanisms related to a disturbance, such as invertebrate migrations into the hyporheic zone (HZ, saturated subsurface interstices), promotes persistence of benthic communities in river ecosystems. Water exchanges through the HZ, which influence the distribution of biota, are heterogeneous at different scales, determining nested hyporheic flowpaths. The effect of these nested exchanges on the use of the hyporheic refuges by benthic invertebrates is still unknown. We simulated streambed drying in a stream section where hydrological exchanges were considered at riffle (upstream or downstream of riffles) and floodplain (downstream end of a floodplain) scales. Physicochemical indicators determined that local hyporheic flowpaths (up-and downwelling zones) were nested in a large-scale downwelling section of the river. In this situation, the effect of 24 h of experimental drying on the distribution of invertebrates was examined at three sediment depths to follow their migrations into the HZ. Whereas invertebrate assemblages did not change in the control channel, abundance of benthic invertebrate increased in the HZ of the impact channel (up to seven-fold). Changes occurred rapidly (15-24 h) and only upstream of riffle where surface water down-welled. The migration was taxon-specific and concerned the most abundant benthic taxa that temporarily colonize the HZ ("temporary hyporheos," e.g., Leuctra cf. fusca, Baetis sp., Caenis sp., Orthocladiinae, Tanypodinae). In the context of climate change, hyporheic refuge use will promote persistence of communities facing the increasing frequency of extreme hydrological events. Improved knowledge about the distribution and function of these refuges is becoming crucial for river managers