Nitrogen cycling in the German Bight (SE North Sea) — Clues from modelling stable nitrogen isotopes |
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| Authors: | Johannes Pätsch Alexandra Serna Kirstin Dähnke Tim Schlarbaum Astrid Johannsen Kay-Christian Emeis |
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| Institution: | 1. Institute of Oceanography, University of Hamburg, Germany;2. Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, Germany;3. Institute for Coastal Research, GKSS-Forschungszentrum Geesthacht, Germany |
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| Abstract: | Nitrogen isotope values (δ15N) of surface sediments in the German Bight of the North Sea exhibit a significant gradient from values of 5–6‰ of the open shelf sea to values above 11‰ in the German Bight. This signal has been attributed to high reactive N (Nr) loading enriched in 15N from rivers and the atmosphere. To better understand the processes that determine the intensity and spatial distribution of δ15N anomalies in surface sediments, and to explore their usefulness for reconstructions of pristine N-input from rivers, we modeled the cycling of the stable isotopes 14N and 15N in reactive nitrogen through the ecosystem of the central and southern North Sea (50.9–57.3°N, 3.4°W−9.2°E) for the year 1995. The 3D-ecosystem model ECOHAM amended with an isotope-tracking module was validated by δ15N data of surface sediments within the model domain. A typical marine value (δ15Nnitrate=5‰) was prescribed for nitrate advected into the model domain at the seaside boundaries, whereas δ15Nnitrate of river inputs were those measured bi-monthly over 1 year; δ15N values of atmospheric deposition were set to 6‰ and 7‰ for NOx and NHy, respectively. The simulated δ15N values of different nitrogen compounds in the German Bight strongly depend on the mass transfers in the ecosystem. These fluxes, summarized in a nitrogen budget for 1995, give an estimate of the impacts of hydrodynamical and hydrological boundary conditions, and internal biogeochemical transformations on the nitrogen budget of the bight. |
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| Keywords: | Stable isotopes Nitrogen Sediments North Sea German Bight River input modelling |
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