ALBERT

Description: Excess nutrient supply by the rivers and the atmosphere are considered as the major causes for the persistently poor ecological status of the Baltic Sea. More than 97% of the Baltic Sea still suffers from eutrophication due to past and present inputs of nitrogen and phosphorus. One of the poorly quantified nutrient sources in the Baltic Sea is submarine groundwater discharge (SGD). Through seepage meter deployments and a radium mass balance model, a widespread occurrence of SGD along the coastline of Eckernförde Bay was detected. Mean SGD was 21.6 cm d −1 with a calculated freshwater fraction of 17%. Where SGD was detected, pore water sampled by a piezometer revealed a wide range of dissolved inorganic nitrogen (DIN: 0.05–1.722 µmol L −1 ) and phosphate (PO 4 3− : 0.03–70.5 µmol L −1 ) concentrations. Mean DIN and PO 4 3− concentrations in non-saline (salinity 〈 1) pore waters were 59 ± 68 µmol L −1 and 1.2 ± 1.9 µmol L −1 , respectively; whereas pore water with salinities 〉 1 revealed higher values, 113 ± 207 µmol L −1 and 6 ± 12 µmol L −1 for DIN and PO 4 3− , respectively. The nutrient concentrations along the salinity gradient do not suggest that land-derived groundwater is the definitive source of nutrients in the Baltic Sea. Still, SGD may contribute to a major autochthonous nutrient source, resulting from remineralization or dissolution processes of organic matter in the sediments. The DIN and PO 4 3− fluxes derived from SGD rates through seepage meters are 7.9 ± 9.2 mmol m −2 d −1 and 0.5 ± 0.4 mmol m −2 d −1 , lower by a factor of ~ 2 and ~ 5 when compared to the fluxes derived with the radium mass balance model (mean DIN: 19 ± 28 mmol m −2 d −1 ; mean PO 4 3− : 1.5 ± 2.7 mmol m −2 d −1 ). Assuming that these mean radium-based nutrient fluxes are representative for the coastline of Eckernförde Bay, we arrive at SGD-borne nutrient fluxes of about 1 t km −1 y −1 of nitrogen and 0.2 t km −1 y −1 of phosphorous. These fluxes are lower for DIN and in the same range for phosphorus as compared to the riverine nutrient supply (DIN: 6.3 t km −1 y −1 , P: 0.2 km −1 y −1 ) to the German Baltic Sea identifying SGD-borne nutrients as a secondary nutrient source to the Baltic Sea.