Seasonal study of the small-scale variability in dissolved methane in the western Kiel Bight (Baltic Sea) during the European heatwave in 2018

[摘要] Methane (CH 4 ) is a climate-relevant atmospheric trace gas which isemitted to the atmosphere from coastal areas such as the Baltic Sea. Theoceanic CH 4 emission estimates are still associated with a high degreeof uncertainty partly because the temporal and spatial variability in theCH 4 distribution in the ocean surface layer is usually not known. Inorder to determine the small-scale variability in dissolved CH 4 we setup a purge and trap system with a significantly improved precision for theCH 4 concentration measurements compared to static headspaceequilibration measurements. We measured the distribution of dissolvedCH 4 in the water column of the western Kiel Bight and EckernfördeBay in June and September 2018. The top 1 m was sampled in high resolutionto determine potential small-scale CH 4 concentration gradients withinthe mixed layer. CH 4 concentrations throughout the water column of thewestern Kiel Bight and Eckernförde Bay were generally higher inSeptember than in June. The increase in the CH 4 concentrations in thebottom water was accompanied by a strong decrease in O 2 concentrationswhich led to anoxic conditions favourable for microbial CH 4 productionin September. In summer 2018, northwestern Europe experienced a pronouncedheatwave. However, we found no relationship between the anomalies of watertemperature and excess CH 4 in both the surface and the bottom layer atthe site of the Boknis Eck Time Series Station (Eckernförde Bay).Therefore, the 2018 European heatwave most likely did not affect theobserved increase in the CH 4 concentrations in the western Kiel Bightfrom June to September 2018. The high-resolution measurements of theCH 4 concentrations in the upper 1 m of the water column were highlyvariable and showed no uniform decreasing or increasing gradients with waterdepth. Overall, our results show that the CH 4 distribution in the watercolumn of the western Kiel Bight and Eckernförde Bay is stronglyaffected by both large-scale temporal (i.e. seasonal) and small-scalespatial variabilities which need to be considered when quantifying theexchange of CH 4 across the ocean–atmosphere interface.

[学科分类] 大气科学