CO2 and O2 dynamics in moving sandy sediments as a response to natural and regulated flow regimes in streams

Streams represent a small fraction of the earth's overall surface area, but contribute significantly to global biogeochemical cycles. Streams are subject to discharge fluctuations that can be of both natural or anthropogenic origin. These fluctuations influence hyporheic exchange fluxes, sediment movement, and biological activity within the streambed. The objective of the present study is to quantify the processes that link flow dynamics, bedform migration, and stream metabolism. This was done in an indoor flume system by applying specific discharge fluctuations and measuring the dynamics of O₂ consumption and CO₂ production at the sediment-water interface using planar optodes and in-stream sensors. Streambed morphodynamics were observed by using high-frequency time-lapse photography and a bottom tracking sensor. Preliminary results show that O₂ consumption and CO₂ production in the sediment are a function of stream water velocity and bedform celerity. CO₂ accumulates in the sediment under stationary streambed conditions and is transported to the surface water by hyporheic exchange fluxes as well as by diffusion. Higher stream flow velocity and bedform celerity causes the moving fraction of the sediment and the surface water to act as one mixed hydrological compartment, where exchange with deeper sediment layers is limited. Furthermore, dynamic surface water flow regimes cause a net increase in O₂ consumption and CO₂ production rates compared to constant flow regimes. Natural flood regimes cause a sudden release of CO₂ from the streambed as increased velocities cause high turnover of the sediment. The results of this study emphasize the importance of integrating bedform motion, which is commonly overlooked, into biogeochemical investigations of streams. The understanding gained from the present study is expected to improve existing models for analysing nutrient and reactive transport under unsteady flow conditions and to inform us on how human-controlled flow in streams influences carbon cycling and water quality.