24 May 2023
 | 24 May 2023
Status: this preprint is open for discussion.

Diel and seasonal methane dynamics in the shallow and turbulent Wadden Sea

Tim René de Groot, Anne Margriet Mol, Katherine Mesdag, Pierre Ramond, Rachel Ndhlovu, Julia Catherine Engelmann, Thomas Röckmann, and Helge Niemann

Abstract. The Wadden Sea is a coastal system fringing the land-sea borders of Denmark, Germany, and the Netherlands. The Wadden Sea is extremely productive and influenced by strong variations in physical and biological forcing factors that act on time scales of hours to seasons. Productive coastal seas are known to dominate the ocean’s methane emission to the atmosphere, but knowledge on controls and temporal variations of methane dynamics in these vastly dynamic systems are scarce. Here we address this knowledge gap by measuring methane inventories and methanotrophic activity at a temporal resolution of two hours over a time period of two days, repeatedly during four successive seasons in the central Dutch Wadden Sea. We found that methane dynamics varied between colder and warmer seasons, with generally higher water column methane concentrations and methanotrophic activity in the warmer seasons. Efflux of methane to the atmosphere was, on the other hand, lower in the warmer seasons because of lower wind speeds. On a diel scale, tides controlled methanotrophic activity, which increased ~40 % at low tide compared to high tide. We estimate that methane oxidizing bacteria reduce the methane budget of the Dutch Wadden Sea by only 2 %, while ~1/3 escapes to the atmosphere and ~2/3 are flushed out into the open North Sea at ebb tide. Our findings indicate that tides play a key role in controlling methane dynamics and methanotrophic activity and highlight the importance of high resolution and repeated sampling strategies to resolve methane dynamics in fast-changing coastal systems.

Tim René de Groot et al.

Status: open (until 05 Jul 2023)

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Tim René de Groot et al.

Tim René de Groot et al.


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Short summary
This study investigates methane dynamics in the Wadden Sea. Our measurements revealed distinct variations triggered by seasonality and tidal forcing. Methane budget was higher in warmer seasons, but remained surprisingly high in colder seasons. Methane dynamics were amplified during low tides, flushing the majority of methane into the North Sea or released to the atmosphere. Methanotrophic activity was also elevated during low tide but mitigated only a small fraction of the methane efflux.