| dc.description.abstract | Methane is an important greenhouse gas, with a greenhouse warming potential that is 34 times higher than for CO2 over a 100 year horizon. Anoxic sediments, containing a lot of (degrading) organic matter, are sources of methane. However, the majority of the methane released from the sea floor is oxidized to CO2 in the water column and does not reach the atmosphere. In the Wadden Sea a lot of organic matter is present and the waters are shallow, potentially allowing a lot of methane produced at the sea floor to be emitted into the atmosphere. Methane concentrations in and above the Dutch Wadden Sea and methane emissions from the Dutch Wadden Sea have never been measured before. To quantify the methane emissions from the Dutch Wadden Sea, methane concentrations in the water, the sediment and the atmosphere, methane oxidation rates in the water, isotopic signatures of methane in the air and water, and methane emissions were measured together with meteorological information and water properties. These variables were measured at an 48 hour station where the research vessel, the Navicula, anchored during summer (July 2019) and autumn (November 2019). Average methane emission rates determined with a bucket technique were 6.5 ± 6.3 nmol/s (23 ± 22 nmol/m2/s) in summer and 2.0 ± 1.0 nmol/s (7.1 ± 3.5 nmol/m2/s) in autumn. Based on methane isotopic signatures in air samples, taken from the bucket, and in water samples, it can be concluded that the emissions measured originate from biogenic processes in the Wadden Sea. Emissions calculated with the bucket techniques are larger than the calculated sea-air flux based on dissolved and atmospheric concentrations, but both show the similar temporal changes, indicating that the bucket emissions are representative for what happens in the atmosphere, but the response is magnified. Measured methane emissions are higher at higher dissolved methane concentrations. Based on the sea-air flux, it can be concluded that higher salinity, water and air temperature, wind speed, roughness length and lower methane concentrations in the atmosphere also cause higher methane emissions from the Dutch Wadden Sea. Dissolved methane concentrations in autumn peak just after low and high tide. For dissolved methane concentrations of about 20 nmol/L and higher, methane oxidation rates increase with concentration. The higher oxidation rates result in isotope enrichment of the dissolved methane in these samples, and the observed fractionation constant is consistent with microbial oxidation as the main removal process. Dissolved methane concentrations and methane oxidation rates in summer are, respectively, about four and eight times higher than in autumn. Patterns in atmospheric methane mixing ratios and isotopic signatures in air samples cannot be explained by the temporal cycles of the variables measured during this research project. They are likely caused by passing synoptic-scale plumes with enhanced methane mixing ratios, but this could not be proven in this report. Future expeditions to the Dutch Wadden Sea, during which the same variables are measured, could reduce uncertainties still present after this research project, for example on emission rates and their variation over a year. In addition, measuring the isotopic signatures of atmospheric air samples could help identifying the origin of the methane elevations in the atmosphere. | |
