A permeable hyporheic zone may contribute to buffer the effects of a drying event on prey availability for salmonid juveniles

Abstract

Climate change is increasing the intensity and frequency of hydrological events in freshwater. Summer droughts and drying events drastically reduce the favourable habitat for aquatic organisms shaping interactions among species. Macroinvertebrates are usually less abundant during severe summer low flows, reducing prey availability for carnivorous fish, such as brown trout (Salmo trutta). To quantify the consequences of a surface water drying event on fish performance, we conducted an experiment in 6 experimental channels naturally fed by water and macroinvertebrates. After a colonisation period for macroinvertebrates, the water level was dropped to the benthic surface for two weeks in 3 channels, while it remained constant in the 3 other channels. After water level restoration, juvenile brown trout were reared for three weeks in the control and dry channels. The drying event did not reduce the abundance of macroinvertebrates. Survival and growth of juvenile trout were unaffected by drying. Our experiment provided circumstantial evidence that the hyporheic zone could potentially serve as an effective refuge for macroinvertebrates, mitigating the effects of drying events on food webs, although this mitigation would strongly depend on sediment characteristics, habitat quality and the composition of the macroinvertebrate community.