Groundwater seepage in Eckernforde Bay (Western Baltic Sea): Effect on methane and salinity distribution of the water column

The effluent activity from a well-known pockmark structure in Eckernforde B ay was monitored for methane, salinity, and temperature signals in the wate r column intermittently over three years between 1991, 1993 and 1994. Groun dwater discharge from an aquifer into the brackish waters of the western Ba ltic, dilutes bottom water salinities to values as low as 2.9 parts per tho usand. Seasurface height and the amount of precipitation preceding sampling periods by 5 days correlated significantly with the rate of groundwater di scharge. Concentrations of methane in bottom water at the pockmark site wer e strongly influenced by seepage intensity. At two sampling sites (control and pockmark site) distinctly lower methane concentrations were observed to wards the sea surface, although the entire water body of Eckernforde Bay ap pears to be affected by methane seeping from the sediments. This is support ed by high methane concentrations above equilibrium with atmospheric methan e throughout most of the year. Maximum concentration above the equilibrium value in surface waters was 2800 parts per thousand. Methane flux from surf ace waters into the atmosphere follows strong seasonal variations, with max imum values in the winter (200-400 mu mol m(-2) d(-1)). The study reveals t he important role of coastal oceans in the global methane cycle, as an inte nse but variable source of methane of largely unknown magnitude. (C) 1998 E lsevier Science Ltd. All rights reserved.