Evaluation of atmospheric nitrogen inputs into marine ecosystems of the North Sea and Baltic Sea – part B: contribution by shipping and agricultural emissions

Abstract. Atmospheric nitrogen deposition constitutes 20 % to 40 % of the nitrogen input into the North Sea and the Baltic Sea contributing to phytoplankton growth and inducing eutrophication. Major contributors to the atmospheric nitrogen emissions in the vicinity of marine water bodies are shipping and agricultural activities. The contribution of individual emission sectors to the biogeochemical nitrogen cycle needs to be evaluated in order to assess improvement of marine water quality through emission reductions of these sectors. Hence, one focus of this modeling study was to evaluate the contribution of total, shipping-related, and agricultural-related nitrogen deposition to dissolved inorganic nitrogen (DIN), particulate organic nitrogen (PON), chlorophyll-a. A second focus of this study was to compare both water bodies with respect to the accumulation and processing of nitrogen from the mentioned sources. These two research questions were evaluated by a modeling study with the coupled physical-biogeochemical model HBM-ERGOM. The fate of atmospheric deposition in total and of atmospheric nitrogen deposition from two individual sources – shipping and agricultural activities – was traced in the marine ecosystem by a tagging method. In the Baltic Sea it was found that the atmospheric nitrogen deposition contributes up to 50 % to the DIN pool at individual locations during summer. On annual average, 13 % are contributed. Approximately 5 % of DIN originated from agricultural activities and 2 % from the shipping sector. In the western and central Baltic Sea, the shipping sector contributed nearly 5 %. The pattern was similar for the agricultural share indicating that these two sources have a higher relevance in these regions. In the Baltic Sea, the atmospheric nitrogen shares of chlorophyll-a and bioavailable PON were 19 % and 18 %, respectively, and, hence, higher than of DIN. In contrast in offshore regions only, atmospheric nitrogen shares to DIN, PON, and chlorophyll-a were on a similar level compared to each other (20 % to 35 %). This difference is caused by high DIN loads and phosphorus limitation in coastal regions of the Baltic Sea. In the North Sea, the atmospheric contribution to DIN was on a similar level but showed considerable spatial variability caused by a north-south gradient. The shipping contribution to DIN was slightly lower (approximately 1.4 %) and the agricultural contribution higher (6 %) compared to the Baltic Sea. The atmospheric contribution to chlorophyll-a and bioavailable PON was considerably lower than in the Baltic Sea and on the level of the atmospheric DIN shares, which is a result of short residence times of nutrients in the North Sea compared to the Baltic Sea. The shipping and agricultural contributions to PON and chlorophyll-a were also lower.