Mesocosm experiments reveal the direction of groundwater-surface water exchange alters the hyporheic refuge capacity under warming scenarios

Climate change is expected to affect hydrologic and thermal regimes of river ecosystems. During dry periods when river flows decrease and water temperatures increase, the hyporheic zone (HZ) can provide a refuge to surface aquatic invertebrates and enhance the resilience capacity of riverine ecosystems. However, shifts from up- to downwelling flow conditions in the HZ could jeopardise this capacity. Using laboratory mesocosms and high-resolution fibre-optic distributed temperature sensing, we explored the combined effects of five different increased surface water temperature treatments (from 15 to 27 degrees C at 3 degrees C intervals) and the direction of water exchange on the ability of Gammarus pulex (Crustacea: Amphipoda: Gammaridae) to migrate into the HZ as a response to warming. We determined the survival rates of this ubiquitous hyporheic dweller and its rates of consumption of alder (Alnus glutinosa; Betulaceae) leaf litter in the HZ. Results showed that at increasing surface water temperature, leaf-litter breakdown was observed at a greater depth in the sediments under downwelling flow conditions, that is, G.pulex migrated deeper into the HZ compared with upwelling conditions, resulting in greater survival rates (6411 vs. 44 +/- 10%). However, under both upwelling and downwelling conditions, we found evidence for potential use of the hyporheic zone as a thermal refuge for G.pulex. Below sediment depths of 25cm, temperatures remained low (<22 degrees C) even when surface waters were at 27 degrees C, so temperatures deep in the hyporheic zone never exceeded critical thermal thresholds for G.pulex. This study provides evidence that alterations to the direction of groundwater-surface water exchange can alter the capacity of the HZ to provide a refuge for benthic invertebrates, thereby affecting the resilience of river communities to warming under climate change.