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

The UNESCO world heritage Wadden Sea is a highly productive coastal system, rich in biodiversity and influenced by physical and biological forcing. Such coastal seas are known to dominate ocean’s methane emissions, but knowledge on variations and controls of methane and the efficiency of the microbial methane filter in these vastly dynamic systems are scarce. We conducted high frequency sampling over a 2-days’ period during four seasons to determine diel and seasonal effects on methane dynamics in the Dutch Wadden Sea. We found that waters were charged with methane throughout the year with maximum concentrations of up to 155 nM in summertime. Methane concentrations were generally lower at high tide and in the colder seasons, whereas MOB activity increased by ~2-fold during low tide compared to high tide. On average, only a minor fraction of the WZ methane budget (~2%) is retained by MOBs in the WZ itself, while ~1/3 escapes to the atmosphere and ~2/3 are flushed out into the open North Sea where it may be consumed by microbes or is liberated to the atmosphere.