A Novel In Situ Experimental Setup for Studying the Impact of Bedform Celerity on 2D Oxygen Distribution in the Hyporheic Zone of Streams

Hydrodynamic and morphodynamic forces interacting across the sediment-water interface control the biogeochemistry in the hyporheic zone. When investigating the redox zonation within streambeds, dissolved oxygen (O₂) is considered a key solute to the understanding of river ecosystems. However, no field studies have measured the spatiotemporal O₂ distribution linked to bedform celerity induced by changes in stream water velocity. Therefore, we developed and tested an innovative in situ setup in the River Erpe, Germany. The setup combines a planar O₂ optode and O₂ flow-through cells for eight sediment depths to capture the variability in O₂ dynamics, and a laser scanner to capture bedform morphodynamics, which was used to calculate bedform celerity. The setup was tested under different stream water velocities between 0.1 and 0.5 m/s. We found that O₂ patterns in the streambed depend on stream water velocity. At low velocities, bedforms were stationary and a stable redox zonation with limited O₂ penetration in the streambed (up to 4 cm) was observed. As we increased the velocity up to 0.3 m/s, the spatiotemporal variability of O₂ distribution across the bedform increased, with anoxic patches moving along the migrating bedforms. At the highest velocity tested (0.5 m/s), the sediment bed was constantly oxygenated with deeper O₂ penetration as compared to slower velocities. The present study provides proof of concept for in situ O₂ measurements in small rivers, which helps to refine laboratory and mesocosm experiments, improve the knowledge of the processes involved in natural environments, and develop more sustainable river management strategies.