Water and material exchange at the seawater - freshwater interface, southern Baltic Sea

Julia Westphal1, Iris Schmiedinger1, Jan Scholten2, Willard. S. Moore3, Feng Hsu2, Michael. E. Böttcher4
1 Leibniz Institute for Baltic Sea Research Warnemünde
2 Kiel University
3 University of South Carolina
4 Leibniz Institute for Balt

P 9.22 in Groundwater-surface water-interactions - processes and methods

Submarine groundwater discharge (SGD) is an important pathway of dissolved element transport from the terrestrial to the marine environment. Little is known about the controls of this transport from landside to the Baltic Sea and vice versa, and the associated biogeochemical reactions at the seawater/ freshwater interfacial zone.

In the present study we investigate water exchange and associated biogeochemical processes at the coastal fen of the nature reserve Heiligensee & Hütelmoor, southern Baltic Sea, which is the study area of the DFG research training group BALTIC TRANSCOAST. The compartments under consideration include the coastal water column and vertical pore water and solid phase profiles in sediments and soils along the coast as well as in the fen peatland.

Pore waters were retrieved via push-pull pore water lances and permanent pore water samplers in the shallow water area. 224/223Ra isotope investigations in the water column and in the pore water are used for a general detection of benthic-pelagic water exchange. A focus of the investigations was set on the development of concentration gradients of redox-sensitive elements, nutrients and the stable isotope (O, H, C) composition. Physical parameters, major and trace elements have been included.

The study area displays a spatial and temporal heterogeneity in pore water compositions due to a complex lithology with permeable sands and impermeable peat layers. The preliminary results of the hydro geochemical and isotope investigations show SGD contributions along the coastline of the study area. 224/223Ra measurements indicate that SGD along the coast is highly episodic.

The pore water´s carbon isotope composition of dissolved inorganic carbon indicates intense anaerobic oxidation of dissolved organic carbon and/or methane which may be influenced by the presence of below-ground peat layers. Dissolved sulfide accumulates already in shallow sediments (below 30 cmbsf) due to high rates of sulphate reduction and/or pore water fluxes.

To further quatify local pore water dynamics (`benthic-pelagic coupling`) it is further planned to apply benthic chambers to allow for a flux determination of water and elements and analyses of the dominat biogeochemical processes.

Acknowledgement: The research is supported by DFG within the Research graduate project BALTIC TRANSCOST and Leibniz IOW.