Technical Note: Testing pore-water sampling, dissolved oxygen profiling and temperature monitoring for resolving dynamics in hyporheic zone geochemistry

Abstract. The hyporheic zone (HZ) is of major importance for carbon and nutrient cycling as well as for the ecological health of stream ecosystems. However, biogeochemical observations in this ecotone are complicated by a very high spatial heterogeneity and temporal dynamics. Especially the latter are difficult to observe without disturbing the system. In this field study, we tested and combined three less common methods for time-resolved measurements with high vertical resolution. We installed Rhizon samplers for repeated pore-water extraction, an optical sensor unit for in-situ measurements of dissolved oxygen, and a depth-resolved temperature monitoring system in the HZ of a small stream. While Rhizon samplers were found to be highly suitable for pore-water sampling of dissolved solutes, measured gas concentrations, here CH₄, showed a strong dependency of the pump rate during sample extraction, and an isotopic shift in gas samples became evident. This was presumably caused by a different behaviour of water and gas phase in the pore-space. The manufactured oxygen-sensor could locate the oxic-anoxic interface with very high precision. This is ecologically important and allows to distinguish aerobic and anaerobic processes. Temperature data could not only be used to estimate vertical hyporheic exchange, but also depicted sedimentation and erosion processes. Overall, the combined approach was found to be a promising tool to acquire data for the quantification of biogeochemical processes in the HZ with high spatial and temporal resolution.