Seasonal methane accumulation and release from a gas emission site in the central North Sea

Abstract

Abstract. We investigated dissolved methane distributions along a 6 km transect crossing active seep sites at 40 m water depth in the central North Sea. These investigations were done under conditions of thermal stratification in summer (July 2013) and homogenous water column in winter (January 2014). Dissolved methane accumulated below the seasonal thermocline in summer with a median concentration of 390 nM, whereas during winter, methane concentrations were typically much lower (median concentration of 22 nM). High-resolution methane analysis using an underwater mass-spectrometer confirmed our summer results and was used to document prevailing stratification over the tidal cycle. We contrast estimates of methane oxidation rates (from 0.1 to 4.0 nM day−1) using the traditional approach scaled to methane concentrations with microbial turnover time values and suggest that the scaling to concentration may obscure the ecosystem microbial activity when comparing systems with different methane concentrations. Our measured and averaged rate constants (k') were on the order of 0.01 day−1, equivalent to a turnover time of 100 days, even when summer stratification led to enhanced methane concentrations in the bottom water. Consistent with these observations, we could not detect known methanotrophs and pmoA genes in water samples collected during both seasons. Estimated methane fluxes indicate that horizontal transport is the dominant process dispersing the methane plume. During periods of high wind speed (winter), more methane is lost to the atmosphere than oxidized in the water. Microbial oxidation seems of minor importance throughout the year.

The Distribution of Dissolved Methane and Its Air-Sea Flux in the Plume of a Seep Field, Lingtou Promontory, South China Sea

“…Similar results have been reported in other gas seepage areas, such as the North Sea, south of the Dogger Bank, and Tommeliten, the central North Sea [45]. The difference value of the dissolved methane concentrations between surface waters and bottom waters may be attributed to removal by microbial methane oxidation and lateral dispersion by physical transport, favored by strong tidal currents and ocean current [43,[45][46][47]. Exceptionally high methane concentrations usually point to hydrocarbon seepage.…”

Response of marine methane dissolved concentrations and emissions in the Southern North Sea to the European 2018 heatwave

“…In coastal waters, the largest source of CH 4 seems to be sedimentary and leads to an enrichment of CH 4 in bottom waters, the fate of which depends on water column depth. Concentrations of CH 4 in surface waters and related CH 4 emissions to the atmosphere are higher in shallow regions of the continental shelf (Borges et al 2016), and, in deeper areas, CH 4 in bottom waters is dispersed by lateral transport (advection or turbulent mixing) or removed by microbial oxidation, and is transported very slowly across the thermocline to surface waters (Schneider von Deimling et al 2011;Mau et al 2015;Graves et al 2015). Overall, this leads to a negative relation between CH 4 concentration and depth (in absolute values) both locally (Borges et al 2016; and globally (Weber et al 2019).The future evolution of CH 4 emissions from coastal waters in response to warming and eutrophication (and related expansion of hypoxia) remains largely unconstrained and unquantified (Naqvi et al 2010).…”

Effects of low oxygen concentrations on aerobic methane oxidation in seasonally hypoxic coastal waters

“…Model results also predict that such oxygen-depleted zones will expand in the future since increasing surface water temperatures will lead to enhanced stratification and, thus, to less oxygen supply to bottom waters (Keeling et al, 2010;Friedrich et al, 2014). Several definitions for water column oxygenation levels were suggested in the literature (Diaz and Rosenberg, 2008;Canfield and Thamdrup, 2009;Middelburg and Levin, 2009;Naqvi et al, 2010). Here, we adopt the threshold adapted by Middelburg and Levin (2009) and Naqvi et al (2010), where hypoxia is defined as [O 2 ] < 63 µmol L −1 .…”