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

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.