Beschreibung:
<jats:p>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.
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