Description: Coastal seas may account for more than 75% of global oceanic methane emissions. There, methane is mainly produced microbially in anoxic sediments from where it can escape to the overlying water column. Aerobic methane oxidation (MOx) in the water column acts as a biological filter reducing the amount of methane that eventually evades to the atmosphere. The efficiency of the MOx filter is potentially controlled by the availability of dissolved methane and oxygen, as well as temperature, salinity, and hydrographic dynamics, and all of these factors undergo strong temporal fluctuations in coastal ecosystems. In order to elucidate the key environmental controls, specifically the effect of oxygen availability, on MOx in a seasonally stratified and hypoxic coastal marine setting, we conducted a 2-year time-series study with measurements of MOx and physico-chemical water column parameters in a coastal inlet in the southwestern Baltic Sea (Eckernförde Bay). We found that MOx rates generally increased toward the seafloor, but were not directly linked to methane concentrations. MOx exhibited a strong seasonal variability, with maximum rates (up to 11.6 nmol l-1 d-1) during summer stratification when oxygen concentrations were lowest and bottom-water temperatures were highest. Under these conditions, 70-95% of the sediment-released methane was oxidized, whereas only 40-60% were consumed during the mixed and oxygenated periods. Laboratory experiments with manipulated oxygen concentrations in the range of 0.2-220 µmol l-1 revealed a sub-micromolar oxygen-optimum for MOx at the study site. In contrast, the fraction of methane-carbon incorporation into the bacterial biomass (compared to the total amount of oxidised methane) was up to 38-fold higher at saturated oxygen concentrations, suggesting a different partitioning of catabolic and anabolic processes under oxygen-replete and oxygen-starved conditions, respectively. Our results underscore the importance of MOx in mitigating methane emission from coastal waters and indicate an organism-level adaptation of the water column methanotrophs to hypoxic conditions.

Description: We developed a purge-and-trap system to measure CH_4 with high accuracy in the surface water and the water column of the Kiel Bight, close to the monthly time series station Boknis Eck. The small-scale variability of CH_4 was investigated from samples taken in the Baltic Sea during the two Alkor cruises AL510 and AL516 in the in early summer and autumn 2018 when two gas exchange experiments (GasEx) were carried out around the Boknis Eck time series station. This station has been sampled for CH_4 on a monthly basis since 2007.During the GasEx cruises, special emphasis was put on the sampling of the upper water column and the mixed layer, with high-resolution sampling to determine potential gas gradients within the mixed layer that are not captured with conventional sampling approaches. Water column CH_4 samples were collected from Niskin bottles mounted to a CTD-water-rosette-sampler, Dinghy samples were collected using an aquarium pump, Underway samples were taken from the ship's sea water supply. All samples were filled bubble-free into dark brown 20 mL glass vials by overflowing the approximate threefold volume and crimp-sealed with rubber stoppers and aluminium caps.