Air-Sea interaction of methane and nitrous oxide in shallow brackish inshore waters of the Baltic Sea

Maysoon Lundevall Zara, Stockholm University, Department of Geological Sciences, Stockholm, Sweden and Volker BrĂ¼chert, Stockholm University, Geological Sciences and Bolin Centre for Climate Research, Stockholm, Sweden
In this study we report on water and air-side concentrations of methane and nitrous oxide from very shallow inshore areas of the Baltic Sea. The goal of the study was to establish the role of the less than two-meter deep inshore area of the Baltic Sea as a source of methane and nitrous oxide for the atmosphere and offshore waters for different seasons of the year and sites with varying vegetation and sediment types for data collected between 2008 and 2019. During 2019 measurements were made before, during and after annually occurring cyanobacterial bloom in the period June-October 2019. Air measurements have been done using flux chambers at water surface, with and without bubble shield to establish the contribution of bubbles emitted from sediments.

All measurements were conducted in areas with water depths of less than 1 meter. The initial concentration of methane in the air-flux chamber at the time of deployment was on average 2.55 ppm. Initial results show the concentration of methane in the flux chamber after 3 days to be on average more than 60 ppm. On the other hand specific locations with clearly visible bubbles the methane concentration was more than 1000 ppm. The highest concentrations have been measured in flux chambers without bubble shield, followed by flux chambers indicating the contribution of methane bubbles to the methane flux. About half of the methane fluxes from initial analyses were between 0.7-1.7 mg/m2/day, but a number of flux values were significantly higher than that, up to 40 mg/m2/day. The high fluxes were observed for periods with high wind speeds in the area when rising methane bubbles were observed at the water surface indicating destabilization of trapped shallow gas in sediments of the near-shore area. Our conclusion is that the inshore methane fluxes exceed those measured in offshore coastal areas by to a factor of 2 to 200 with high variability in space and time. The very shallow inshore areas of the Baltic Sea are therefore an important contributor to the total methane flux emitted from the Baltic Sea.