Dissolved oxygen gradients in hyporheic zones depend on fine sediment and associated respiration rates

Dissolved oxygen (DO) in the hyporheic zone (HZ) is a crucial parameter for the survival of many stream organisms and is involved in a multitude of aerobic chemical reactions. However, HZ DO budgets are easily perturbed by climate change and anthropogenic processes that have caused increased deposition of fine sediments (< 2 mm) in many stream beds. The fine sediment fraction hampers exchange of DO-rich stream water with the HZ. In this study we performed a raster sampling approach (0.90 cm length x 1.50 cm width; 30 cm distance between sampling points) at sediment depths of 10 and 25 cm with a focus on DO and its stable isotopes (δ¹⁸O_DO). The aim was to analyze small-scale turnover patterns in a forested (site 1) and an anthropogenically influenced stream section (site 2) in a 3^rd order stream in southern Germany. Grain size analyses showed similar average fine sediment fractions at site 1 (42.5 ±13.7 %) and site 2 (46.3 ±10.8 %). They increased with depth at both sites (38.5 ± 6.3 %, 0-15 cm; 46.5 ± 17.4 %, 15-30 cm at site 1 and 40.6 ±4.5 %, 0-15 cm; 52.0 ±12.2 %, 15-30 cm at site 2). DO concentrations in the HZ ranged from 1.4 to 4.5 mg L^-1 (2.0 ±0.7 mg L^-1) and 1.5 to 1.8 mg L^-1 (1.7 ±0.1 mg L^-1) at site 1 and from 1.2 to 2.9 mg L^-1 (1.6 ±0.5) and 1.0 to 2.4 mg L^-1 (1.6 ±0.4) at site 2 at 10 and 25 cm depth, respectively. The low DO concentrations in the HZ suggest high DO consumption rates and reduced exchange with stream water. This is possibly a result of increased fine sediment proportions. However, other factors such as organic carbon contents and increased respiration rates may also influence DO gradients. In contrast, the stream water had an average DO concentration of 9.8 ±0.2 mg L^-1. Associated δ¹⁸O_DO values of the open water (23.4 ±0.1 ‰) differed from those of sediment waters that showed averages of +22.5 ±0.5 ‰ and +22.4 ±0.3 ‰ at site 1 and +22.5 ±0.4 ‰ and +22.3 ±0.2 ‰ at site 2 at 10 and 25 cm depth, respectively. These sedimentary values indicated dominant photosynthesis, even though due to absence of light in the subsurface this process seems unlikely. Therefore, kinetically-driven processes such as diffusion, interactions with Fe or unknown DO sources within the HZ might have caused such ¹⁶O-enriched values. Our findings suggest that the analyses of DO, δ¹⁸O_DO and fine sediment gradients in the HZ should be combined with stable carbon isotope measurements to further our understanding of hyporheic processes relevant for stream biota.