DOI: 10.11588/data/mu3con
title: Methane cycling in a eutrophic lake characterised by multiple stable isotope and flux measurements [data]

description: Recent estimations demonstrate that methane (CH4) emissions from aquatic systems are responsible for up to half of global CH4 emissions. Lakes represent one of the largest CH4 sources, and emissions are predicted to increase due to global warming. However, there is a large uncertainty associated with CH4 emissions from freshwater environments to the atmosphere. Emissions result from the interplay between physical transport, CH4 production and consumption processes, which remain poorly understood. Especially the recently discovered CH4 production in oxic water layers (OMP) questions the traditional understanding of CH4 formation processes and might constitute a major driver of limnic CH4 emissions. However, the underlying mechanisms and pathways of OMP are largely unknown. In the scope of this study, a combination of field measurements and laboratory incubation experiments as well as concentration measurements and stable isotope techniques were conducted to disentangle the complex processes involved in the CH4 cycle of small, seasonally stratified, eutrophic Lake Willersinnweiher, which is situated in south-west Germany. Methane oversaturation in the water column of Lake Willersinnweiher was found throughout the whole year. However, seasonal and spatial variations of dissolved CH4 depending on lake stratification were observed. Spatial disparities of epilimnic CH4 concentrations indicated that lateral input of CH4 originated mainly from a few selected shallow sites and not from the whole littoral area of the lake. Inflowing groundwater characterised by high CH4 concentrations, which were unusually enriched in 13C and deuterium (2H), further contributed to CH4 supersaturation in the water column. During the stratification period, accumulation of CH4 just below the thermocline coincided with chlorophyll-a peaks, thus indicating the potential presence of OMP in connection with phytoplankton blooms at Lake Willersinnweiher. The potential occurrence of OMP was further validated by a mass-balance approach and by identifying methyl phosphonate, methylamine, and methionine as three potential precursor compounds of CH4 in the oxic water column. In the sediment, the stable carbon (δ13C-CH4) and stable hydrogen (δ2H-CH4) isotope values of CH4 revealed the contribution of different pathways of methanogenic CH4 production depending on water depth. However, sulphate (SO42-)-dependent anaerobic methane oxidation (AOM), revealed by flux measurements, stable isotope evidence of CH4 and dissolved inorganic carbon (DIC) acted as a sink of sedimentary CH4 that substantially diminished CH4 fluxes from the sediment into bottom waters. Furthermore, CH4 from the anoxic CH4-rich hypolimnion was largely depleted by aerobic methane oxidation (MOx) at the oxic-anoxic interface. Methane fluxes from the water column to the atmosphere as well as its stable isotopic values showed strong seasonal dependence and variations, with CH4 being more enriched in 13C and 2H during the mixing period in winter compared to the stratification period in summer. While diffusive CH4 fluxes dominated total CH4 emissions during the mixing period, ebullition was the main contributor to CH4 emissions during the stratification period. Furthermore, the recently introduced isotope indicator Δ(2,13) was applied to characterise CH4 sources at Lake Willersinnweiher. Δ(2,13) values implied that littoral and groundwater inputs of CH4 mainly contributed to CH4 supersaturation at Lake Willersinnweiher during the mixing period. However, lateral and vertical CH4 inputs alone could not explain CH4 supersaturation during the stratification period. Therefore, OMP might be another important CH4 source. This study shows that all investigated sinks and sources of CH4 are subject to strong variations based on lake stratification, physicochemical conditions, and lake depth. Hence, the results of this study suggest that a combination of dual isotope and concentration measurements of CH4 is a promising tool in order to untangle seasonal and spatial dynamics of CH4 sources and sinks in the complex CH4 cycle of lakes.