Methane in the southern North Sea: Sources, spatial distribution and budgets

Measurements of methane (CH 4) so far have always shown supersaturation in the entire North Sea relative to the atmospheric partial pressure and the distribution of surface CH 4 reveals a distinct increase towards the shore. Since North Sea sediments presumably are an insignificant source for CH 4 the coastal contribution via rivers and tidal flats gains in importance. In this work, CH 4 data from the River Weser, the back barrier tidal flats of Spiekeroog Island (NW Germany), and the German Bight are presented. Results from the River Weser are compared to other rivers draining into the German Bight. Measurements in the tidal flat area of Spiekeroog Island highlight this ecosystem as an additional contributor to the overall CH 4 budget of the southern North Sea. A tidally driven CH 4 pattern is observed for the water column with maximum values during low tide. Tidal flat sediments turn out to be the dominating source because pore waters discharged during low tide are highly enriched in CH 4. In contrast, the freshwater contribution to the tidal flats by small coastal tributaries has almost no impact on water column CH 4 concentrations. The CH 4 level seems to be disturbed irregularly by wind forcing due to elevated degassing and prevention of advective flow when tidal flats remain covered by water. Based on our data, two model calculations were used to estimate the impact of tidal flats on the CH 4 budget in the German Bight. Our results demonstrate that the back barrier tidal flats of the east Frisian Wadden Sea contribute CH 4 in an order of magnitude between the Wash estuary and River Elbe and thus have to be considered in budget calculations.