Temporal water column dynamics control microbial methane oxidation above an active cold seep (Doggerbank, North Sea)

Methane is a potent greenhouse gas with strongly increasing atmospheric concentrations since industrialisation. In the ocean, methane is most dominantly produced in sediments and is of microbial and/or thermogenic origin. Uprising methane may escape from the ocean floor to the overlying water column where it can be oxidized by methane oxidizing bacteria. The aerobic methane oxidation (MOx) is thus an important final barrier, which can mitigate methane release from the ocean to the atmosphere where it contributes to global warming. Nevertheless, there is rather little knowledge on the temporal dynamics of the microbial methane filter capacity in the water column. To gain a better understanding of the dynamics, we conducted two 48 hours’ time-series experiments during highly stratified conditions in summer and and mixed water column conditions in autumn above an active methane seep in the North Sea (Doggerbank, 41m water depth). At Doggerbank, dissolved CH₄  δ¹³C-values (<-65 ‰) indicate a microbial CH₄ origin, and seismic data suggest a gas pocket at >50 m sediment depth. Our time series measurement showed that CH₄ concentrations were highly elevated with up to 2100 nM in bottom and 350 nM in surface waters under stratified conditions. The maxima showed a ~12h periodicity, indicating that the flux of CH₄ from the seep was linked to tidal dynamics with the lowest CH₄ concentrations at rising tide and enhanced flux at falling tide. In contrast, during mixed water column conditions we found lower maxima of only up to 450 nM. Yet, during mixed conditions we found that surface water methane concentrations were on average XX-fold higher compared to stratified conditions, suggesting a higher methane efflux to the atmosphere during this time period.  MOx activity showed a similar temporal behaviour suggesting that tidal dynamics are an important control on the efficiency of the microbial CH₄ filter in the water column. Under stratified conditions MOx rates were highest in bottom waters (<5.7 nM/day), however we also found high MOx rates in near-surface waters at times of elevated seep activity during stratified (<3.2 nM/day) and mixed water column conditions (<16.2 nM/day). Our results indicate that the efficiency of the microbial filter is affected by temporal dynamics and seasonality.