As part of the MOSES Project, in April 2023 methane measurements were started in the north-western part of the island Heligoland in the German Bight (North Sea). The objective was to complement the measurements of the "Sternfahrten" to identify the carbon cycle and its flow from the start of the Elbe river into the North Sea. Therefore, a Contros methane sensor for dissolved methane was deployed under water at about 10 to 12 meter depth (depending on the tide) close to the underwater observatory (UW-OBS) MarGate (54°11' N, 7°52' E), from the COSYNA Project. To ensure correct values latter was cleaned frequently from growing organisms by scientific divers. The present data contains the data from 2023, the first year running the sensors. Based on the concentrations of dissolved methane the methane emissions (diffusive flux) was calculated.

After the underwater methane sensor was retrieved it was calibrated by the company (4HJena). Thereby a drift from 0.3 ppm was detected and corrected. For the following calculations only values were used when the power of the sensor integrated pump was between two and eight watt.For the calculation of the methane concentration, first the solubility of the methane was determined (Guinasso et al., 1979). This was based on corresponding temperature and salinity data from specific sensors also deployed in the UW-OBS MarGate at this time. Afterwards the concentration was calculated according to the manufacturers' specifications.For the calculation of the methane flux, atmospheric methane concentration and wind data were taken from the ICOS station. Since the MarGate field is protected from the west side by the island, in case of westerly wind the following corrections were made:• direction 140° - 240°, wind force multiplied by factor 0.5• direction 241° - 280°, wind force multiplied by factor 0.7Those values were applied to the Nightingale formula calculating the diffusive flux for the year 2023.For more information about the used data and bibliography see additional metadata.