Groundwater Discharge to the Southern Baltic Sea

Submarine groundwater discharge (SGD) has been recognized as an important exchange pathway between hydrologic reservoirs due to its impact on biogeochemical cycles of the coastal oceans. In the southern Baltic Sea (polish coastline) groundwater discharge has been identified as a vital factor changing the pore water and seawater concentrations of the measured water constituents. The end-members of the system, namely the seawater and groundwater, characterized with significantly different chemical concentrations. The mixing of groundwater and seawater occurred already in the sediment and consequently, a salinity gradient was formed. Thus, concentration changes/variations of selected water components in pore water samples had been observed. The mixing between seawater and groundwater represented the conservative or not-conservative trend depending on the measured water component. The groundwater was a source of NH₄⁺, PO₄^3-, Cd, Co, Cr Mn, Zn, DIC, DOC and was diluting the Hg, Ni, Cu, NO₃^- and NO₂^- concentrations in pore water and, as a result, diluting the chemical concentrations in the water column. According to the estimates above the groundwater discharge the Baltic Sea compared with total runoff is very small, around or above 1%. However, very locally, as in the Bay of Puck, the contributions of groundwater were significant. The estimated fluxes of dissolved inorganic nitrogen (DIN) -49.9 ± 18.0 t yr^− 1 and PO₄^‐ -56.3 ± 5.5 t yr^− 1 into the Bay of Puck via SGD were significant in comparison with loads delivered to the bay from other, well-recognized sources. The groundwater discharge was also a substantial source of metals (Cd, Co, Cr Mn, Zn) in comparison with rivers. The carbon specific flux into the Bay of Puck was estimated at 850 mg m^-2 yr^-1 while DIC and DOC fluxes via SGD to the Baltic Sea were projected at 283.6±66.7 kt yr^-1 and 25.5±4.2 kt yr^-1. The SGD derived carbon load to the Baltic Sea is an important component of the carbon budget, which gives the sea a firmly heterotrophic status.