Effects of a short-term dry fall of streams on oxygen dynamics in the hyporheic zone

Climate change and water resource management may result in the temporary drying up of streams or stream reaches. Such flow interruption has significant effects on biogeochemical processes, particularly on oxygen dynamics in the hyporheic zone. We hypothesized that the hyporheic zone may quickly return to the pre-interruption oxygen pattern once the original flow conditions are restored. To test this, oxygen concentrations were measured in situ in the surface water and pore water of the hyporheic zone using flow-through cells (every 2 cm down to 14 cm in the sediment) and a planar optode placed in the streambed of an urban river. Measurements were taken five days before and fifteen days after a one-day flow interruption at different streamwater velocities ranging from 0.1 m/s to 0.5 m/s. We found that the flow interruption reduced the diurnal amplitude of oxygen in the surface water. At high flow velocities (> 0.3 m/s), the changes in surface water oxygen concentration propagated into the pore water, leading to lower diurnal oxygen amplitudes throughout the sediment depth profile. This alteration affects the biogeochemical milieu in the hyporheic zone and thus the nutrient dynamics and functioning of the ecosystem as a whole. Contrary to the hypothesis, the oxygen dynamics did not return to the pre-interruption oxygen pattern, even three weeks after the streamflow interruption. Both, surface water and pore water had lower oxygen concentrations, which were about 2 mg/L O₂ lower than before the flow interruption. The altered vertical gradient and the two-dimensional oxygen patterns in the hyporheic zone caused by even short dry fall of streams highlight the impact on the oxygen dynamics of river ecosystems. It also emphasizes the need for sustainable water management strategies to mitigate the long-term ecosystem consequences of flow intermittency.