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

Abstract

Abstract. Methane (CH4) is a climate-relevant atmospheric trace gas which is emitted to the atmosphere from coastal areas such as the Baltic Sea. The oceanic CH4 emission estimates are still associated with a high degree of uncertainty partly because the temporal and spatial variability in the CH4 distribution in the ocean surface layer is usually not known. In order to determine the small-scale variability in dissolved CH4 we set up a purge and trap system with a significantly improved precision for the CH4 concentration measurements compared to static headspace equilibration measurements. We measured the distribution of dissolved CH4 in the water column of the western Kiel Bight and Eckernförde Bay in June and September 2018. The top 1 m was sampled in high resolution to determine potential small-scale CH4 concentration gradients within the mixed layer. CH4 concentrations throughout the water column of the western Kiel Bight and Eckernförde Bay were generally higher in September than in June. The increase in the CH4 concentrations in the bottom water was accompanied by a strong decrease in O2 concentrations which led to anoxic conditions favourable for microbial CH4 production in September. In summer 2018, northwestern Europe experienced a pronounced heatwave. However, we found no relationship between the anomalies of water temperature and excess CH4 in both the surface and the bottom layer at the site of the Boknis Eck Time Series Station (Eckernförde Bay). Therefore, the 2018 European heatwave most likely did not affect the observed increase in the CH4 concentrations in the western Kiel Bight from June to September 2018. The high-resolution measurements of the CH4 concentrations in the upper 1 m of the water column were highly variable and showed no uniform decreasing or increasing gradients with water depth. Overall, our results show that the CH4 distribution in the water column of the western Kiel Bight and Eckernförde Bay is strongly affected by both large-scale temporal (i.e. seasonal) and small-scale spatial variabilities which need to be considered when quantifying the exchange of CH4 across the ocean–atmosphere interface.