Evaluation of atmospheric nitrogen inputs into marine ecosystems of the North Sea and Baltic Sea – part A: validation and time scales of nutrient accumulation

Abstract. The North Sea and the Baltic Sea are impacted by several anthropogenic activities, which put pressure onto the marine ecosystem. One of these pressures is the input of nitrogen compounds, which act as nutrients for phytoplankton growth and induce eutrophication. Atmospheric deposition is a relevant contributor to the marine nitrogen budget, making up 20 % to 40 % of the nitrogen input of the North Sea and Baltic Sea. But the concentrations of dissolved and particulate nitrogen in the sea are not only determined by the input, but also by the residence time of nitrogen in the system before it is removed by biogeochemical processes or physical advection. Our study aims to estimate the contribution of atmospherically deposited nitrogen to the nitrogen pools of North Sea and Baltic Sea. The contribution of atmospheric nitrogen deposition to dissolved inorganic nitrogen and to particulate organic nitrogen in the surface water was evaluated for both Seas in this study showing the relevance of deposition. Both seas differ significantly with respect to the residence time of water and nutrients. Hence, both water bodies were compared with respect to the accumulation of atmospheric nitrogen. Model simulations with the coupled physical biogeochemical model HBM-ERGOM were performed for this purpose. The fate of atmospheric nitrogen deposition was traced in the marine ecosystem. The model-predicted relevant nutrient concentrations in the surface layer compared well to measurements. Nutrient and oxygen concentrations in deep parts of the Baltic Sea were not properly reproduced but did not impact the simulation quality of surface layer concentrations. The denitrification in the Wadden Sea was underestimated by the model. Tagged dissolved inorganic nitrogen (DIN) with nitrogen from atmospheric deposition reaches a steady-state in the southern North Sea after two years of simulation. This is consistent with the published residence time of nutrients in this region. In contrast, in the Baltic Sea region, the atmospheric nitrogen shares increased year-by-year reaching a steady-state not before the fifth year. This is also consistent with published studies on the residence time of riverine nitrogen in the Baltic Sea. Atmospheric nitrogen shares were evaluated in detail in the second part of this study.