Spatial and Temporal Dynamics of Hyporheic Respiration Under Variable Discharge Conditions

The hyporheic zone is the site of intensive biogeochemical cycling in streams. However, the controls on spatio-temporal variability in hyporheic processing, and the impact of this hyporheic processing on reach-scale processing, are largely unknown. We aimed to evaluate spatial variability in hyporheic respiration along an upland river over the course of a flood event using the reactive tracer resazurin (Raz). Raz, a weakly fluorescent dye, irreversibly transforms to resorufin (Rru) under mildly reducing conditions, providing a proxy for aerobic respiration in the hyporheic zone. Eight conductivity loggers and in-situ fluorometers, measuring in-stream concentrations of Raz, Rru, fluorescein, and turbidity, were evenly spaced along a 1km reach of the Selke River, a gravelly, third-order river in north-central Germany. Sub-reaches between fluorometers differed in the number of streambed structures (ex. pool-riffle sequences and gravel bars) hypothesized to impact hyporheic exchange, residence time distributions, and the development of biogeochemical hotspots. Discharge over the 5 days of the experiment in the Selke River ranged from baseflow conditions of 0.3 m³/s to peak flows of 2.6 m³/s. Seven in-stream slug injections of Raz, NaCl and the conservative tracer fluorescein were conducted at discharge conditions of 0.3, 0.8, 2.5, 2.1, 1.3, 1.0, and 0.9 m³/s. Aerobic respiration rates and residence time distributions in the reach and sub-reaches are evaluated relative to the changing discharge conditions. Preliminary results indicate that although reach-scale tracer travel times decrease with increasing discharge, the reach-scale transformation of Raz to Rru is lowest at intermediate discharge and highest at during baseflow and peak flow conditions. This suggests that the highest transformation rates occur during high discharge.