Methane in the southern North Sea: Sources, spatial distribution and budgets Maik Grunwald a, * , Olaf Dellwig a, b , Melanie Beck a , Joachim W. Dippner b , Jan A. Freund a , Cora Kohlmeier a , Bernhard Schnetger a , Hans-Ju ¨ rgen Brumsack a a Carl von Ossietzky University of Oldenburg, Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl-von-Ossietzky-Str. 9-11, 26111 Oldenburg, Germany b Leibniz Institute for Baltic Sea Research (IOW), Seestr. 15,18119 Rostock, Germany article info Article history: Received 5 July 2008 Accepted 27 November 2008 Available online 6 December 2008 Keywords: methane North Sea Wadden Sea Weser pore water budget calculation abstract 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. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Methane (CH 4 ) is the most abundant organic compound in the Earth’s atmosphere. It contributes largely to the greenhouse effect by absorption of infrared radiation and photochemical reactions (Crutzen and Zimmermann, 1991; Lelieveld et al., 1993). CH 4 affects tropospheric ozone, hydroxyl radicals, and carbon monoxide concentrations as well as stratospheric chlorine. Principal atmo- spheric reactions of CH 4 are described in detail by e.g. Cicerone and Oremland (1988). Methane is produced in stomachs of ruminant mammals and by termites, but most biogenic CH 4 originates from bacterial production under anaerobic conditions in wetlands, swamps, rice fields, and landfills (Cicerone and Oremland, 1988; Sass et al., 1990; Fung et al.,1991; Bartlett et al., 1992; Roulet et al., 1992a,b). Marine environments may account for up to 10% of the global atmospheric CH 4 budget (Oremland et al.,1987; Cicerone and Oremland, 1988; Bange et al., 1994). Marine CH 4 produced within the water column originates from bacterial production in the digestive tracts of zooplankton (Bianchi et al., 1992; Marty, 1993; DeAngelis and Lee, 1994) and from sinking organic particles (Karl and Tilbrook, 1994). However, the highest amount of CH 4 is produced by methanogenesis in deeper sediment layers of productive coastal areas (Scranton and McShane, 1991; Hovland et al., 1993). Further sources are river runoff (DeAngelis and Lilley, 1987; Scranton and McShane, 1991; Middelburg et al., 2002) and groundwater discharge (Bugna et al., 1996; Kim and Hwang, 2002). Besides microbial oxidation (Ward et al.,1987; Jones,1991), removal of dissolved CH 4 occurs mainly via sea-air flux by both diffusive and turbulent transfer as well as bubble effervescence (Upstill-Goddard, 2006). Increasing atmospheric CH 4 concentrations during the last two centuries (Ehhalt et al., 2001) document that production by above-mentioned processes is higher than removal. Although shelves and estuaries represent only a small part of the world’s ocean area they contribute about 75% to the global oceanic CH 4 flux (Bange et al., 1994). In this contribution we focus on the CH 4 distribution pattern in the German Bight (southern North Sea) and the adjacent coastal environment. Riverine input is regarded as * Corresponding author at: GKSS Research Centre, Max-Planck-Str. 1, 21502 Geesthacht, Germany. E-mail address: maik.grunwald@gkss.de (M. Grunwald). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2008.11.021 Estuarine, Coastal and Shelf Science 81 (2009) 445–456