Groundwater seepage in the marine environment:role for mass flux and bacterial activity

In Eckernforde Bay (western Baltic Sea) pockmark structures are induced by groundwater seeping out of the sediment.On 3 occasions in winter and spring 1993-94 we investigated the influence of groundwater on the reduction of salinity, on porewater chemistry, and on bacterial activities (methane oxidation and sulphate reduction).In 2 out of 3 sampling campaigns groundwater discharge could be detected.The concentration gradients of Cl-and SO,= are moved towards the sediment surface by the vertical advection of groundwater during seep tunes.Without groundwater discharge the porewater chemstry resembled the control site.Compared to the control site, the methane oxidation and sulphate reduction rates were elevated at the pockmark site, reaching maximum values of 49 and 269 pm01 I-' d-' respectively The groundwater venting from the pockmark had a n end member composition of 80 mM Na', 1.0 mM Ca" and was depleted in Mg".Due to mixing of these major cations along the groundwater/seawater ~nterface, no C a C 0 3 precipitation was found around the pockmark site KEY WORDS.Methane oxidation rate .Sulphate reduction rate .Baltic Sea .Salinity .