Continental Shelf Research 18 (1998) 1795 — 1806 Groundwater seepage in Eckernfo¨rde Bay (Western Baltic Sea): Effect on methane and salinity distribution of the water column I. Bussmann*, E. Suess GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany Abstract The effluent activity from a well-known pockmark structure in Eckernfo¨rde Bay was monitored for methane, salinity, and temperature signals in the water column intermittently over three years between 1991, 1993 and 1994. Groundwater discharge from an aquifer into the brackish waters of the western Baltic, dilutes bottom water salinities to values as low as 2.9. Seasurface height and the amount of precipitation preceding sampling periods by 5 days correlated significantly with the rate of groundwater discharge. Concentrations of methane in bottom water at the pockmark site were strongly influenced by seepage intensity. At two sampling sites (control and pockmark site) distinctly lower methane concentrations were observed towards the sea surface, although the entire water body of Eckernfo¨ rde Bay appears to be affected by methane seeping from the sediments. This is supported by high methane concentrations above equilibrium with atmospheric methane throughout most of the year. Maximum concentration above the equilibrium value in surface waters was 2800. Methane flux from surface waters into the atmosphere follows strong seasonal variations, with maximum values in the winter (200 — 400 mol m d). The study reveals the important role of coastal oceans in the global methane cycle, as an intense but variable source of methane of largely unknown magnitude.  1998 Elsevier Science Ltd. All rights reserved 1. Introduction The role of the world’s ocean in the global methane budget is considered to be minor (Cicerone and Oremland, 1988). More specific evaluations, however, revealed that shallow marine waters do contribute significantly (8—65 Tg CH  /yr) to the * Corresponding author. Present address: Alfred-Wegener Institut fu¨r Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany. E-mail: ibussman@awi-bremerhaven.de 0278—4343/98/$ — See front matter  1998 Elsevier Science Ltd. All rights reserved PII: S0278 — 4343(98)00058 — 2