Understanding the hyporheic methane cycle based on field investigations in a small stream

Global methane emission estimates from aquatic ecosystems, especially from rivers, remain highly uncertain due to a lack of high-resolution (temporal and spatial) data. Over three years, we have gathered data on the methane cycle in the river Moosach, a small stream in Southern Germany, to increase our conceptual understanding of the methane cycle in the hyporheic zone.

The methane distribution in the streambed was measured at ten geochemical profiles with a 1 cm vertical resolution during different seasons and at several locations in the stream. Measurements of the stable carbon isotopes of methane (¹³C), in conjunction with analyses of the microbial community distribution, were used to decipher pathways of methane production and oxidation. To unravel the relevance of different transport pathways, methane ebullition was monitored weekly for one year at four test sites and compared to diffusive fluxes across the sediment-water and water-air interfaces. Quantifying the oxidation of methane to CO₂ proved to be the most challenging part of the project. Especially when it came to distinguishing which reduction processes the oxidation was coupled to. This is because the observed geochemical gradients in the hyporheic zone were very steep, and dissolved oxygen reduction and denitrification zones often overlapped.

Dissolved methane concentrations were generally high and reached up to 1000 µmol L^-1 but had a heterogeneous distribution. Ebullition transported up to 30 times more methane to the atmosphere than diffusive fluxes, although this was also highly site-specific and subject to significant seasonal variations. From the isotopic difference in ¹³C between dissolved methane in the surface water and in gas bubbles, we estimated that up to 44% of the methane transported diffusively was oxidized.

Taken together, these results show a high methane production in the hyporheic zone of river Moosach. The highly depleted stable carbon isotope composition of methane suggests a large contribution from hydrogenotrophic methanogenesis but the abundance of certain microbial groups and high ebullition in winter also indicate that methanol could be a substrate for methane production. Methane was oxidized at the top of the hyporheic zone, as shown by a clear isotopic enrichment in ¹³C of methane in several geochemical profiles, but could only marginally reduce greenhouse gas emissions since most methane escaped to the atmosphere as gas bubbles. Dissolved oxygen, nitrate, and nitrite were possible electron acceptors for methane oxidation. Factors favoring methane emissions were higher temperatures, high organic carbon contents in the hyporheic zone, and a fine-grained but permeable bed substrate that ensured anoxic conditions while allowing good exchange with the surface water. Spatial heterogeneity appeared to be larger than temporal variations, which renders extrapolation from point measurements a challenge for the overall assessment of greenhouse gas emissions.