Spatial distribution and budget for submarine groundwater discharge in Eckernförde Bay (Western Baltic Sea)

Submarine groundwater discharge (SGD) from subseafloor aquifers, through muddy sediments, was studied in Eckernförde Bay (western Baltic Sea). The fluid discharge was clearly traced by 222Rn enrichment in the water column and by the chloride profiles in pore water. At several sites, a considerable decrease in chloride, to levels less than 10% of bottom-water concentrations, was observed within the upper few centimeters of sediment. Studies at 196 sites revealed that .22% of the seafloor of the bay area was affected by freshwater admixture and active fluid venting. A maximal discharge rate of .9 L m22 d21 was computed by modeling pore water profiles. Based on pore water data, the freshwater flow from subseafloor aquifers to Eckernförde Bay was estimated to range from 4 3 106 to 57 3 106 m3 yr21. Therefore, 0.3–4.1% of the water volume of the bay is replaced each year. Owing to negligible surface runoff by rivers, SGD is a significant pathway within the hydrological cycle of this coastal zone. High-resolution bathymetric data and side-scan sonar surveys of pockmarks, depressions up to 300 m long, were obtained by using an autonomous underwater vehicle. Steep edges, with depths increasing by more than 2 m within 8–10 m in lateral directions, equivalent to slopes with an angle of as much as 118, were observed. The formation of pockmarks within muddy sediments is suggested to be caused by the interaction between sediment fluidization and bottom currents. Fluid discharge from glacial coastal sediments covered by mud deposits is probably a widespread, but easily overlooked, pathway affecting the cycle of methane and dissolved constituents to coastal waters of the Baltic Sea. For coastal areas, Sonrel (1868) reported the discharge of freshwater from submarine springs and speculated on their use and risks for sailors. Since then a few studies have considered the importance of fluid discharge from sediments for nutrient budgets of coastal environments, formation of offshore plankton blooms, hydrological cycles, or the release of trace elements and gases such as radon from the seafloor (Johannes 1980; Valiela et al. 1990; Moore 1996; Cable et al. 1997; Laroche et al. 1997). For the South Atlantic Bight, on the eastern coast of the United States, Moore (1996) deduced that the quantity of submarine groundwater discharge (SGD; here taken as fresh plus salt water) represented 40% of the river input into the study area. Although these figures probably overestimate the contribution of submarine seepage 1 Corresponding author ([email protected]). Acknowledgments We thank the captains and crews of RVs Littorina, A. v. Humbold, and Alkor for their support during several cruises. We are grateful to W. Lemke and J. Harff from the Institute of Baltic Research (IOW, Warnemünde) for providing the vibro corer system. Jayne Wolf-Welling is gratefully acknowledged for improving the English text. We thank DeBeers Marines/Maridan for conducting the AUV dives. Application of GIS was supported by Dr. A. Schäfer. The paper benefited from the comments and suggestions of two anonymous reviewers. This research was funded as part of the EU shared cost project Sub-GATE (contract no. ENV4-CT97-0631). in this area (Younger 1996), this study revealed the significance of this pathway for marine chemistry. Reports concerning fluid discharge in coastal areas, e.g., off Italy, Greece, Crete, Japan, Israel, Lebanon, Florida, and the Baltic proper, underlined the worldwide occurrence of this transport pathway from land to the ocean (Kohout 1966; Zektser 1996; Schlüter 2002; Taniguchi et al. 2002). Considering the different regional settings, different modes of fluid flow from sediments can be distinguished: (1) focused flow along fractures in karst and rocky areas, (2) dispersed flow through soft sediments, and (3) recirculation of seawater through sediments. The compositions of fluids range from nearly pure freshwater, as is the case in karst areas, to the seepage of saline water, as reported for regions off Florida (Cable et al. 1997). Techniques used to localize discharge sites and to determine flow rates include: visual observation by towed camera systems, direct measurements by different types of seep meters, pore water investigations and modeling, measurements of sediment temperature, as well as water column studies of trace elements and naturally occurring radionuclides such as 222Rn and 226Ra (Moore 1996; Albert et al. 1998; Burnett et al. 2001; Taniguchi et al. 2002). Nevertheless, in most cases, the origin and subseafloor pathways of fluids and their impact on pore water composition is not well understood. In this study we examine submarine groundwater discharge (SGD) in Eckernförde Bay (western Baltic Sea). For