Nitrogen cycling in the German Bight (SE North Sea) — Clues from modelling stable nitrogen isotopes

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. ivity tests suggest that the most relevant parameters to reproduce observed sediment δ15N are the 15N/14N ratios in Nr-sources (e.g. river, atmosphere), and the fractionation factors associated with Nr turnover processes, in particular nitrate uptake by phytoplankton and nitrogen burial. In accord with observations, the modeled surface sediments of the inner German Bight are enriched in 15N (δ15N>9.5‰). The general gradient of decreasing δ15N in sediments from the coast to the open shelf primarily reflects the amount of 15N-enriched reactive nitrogen discharged by the German rivers into the North Sea. Smaller patterns are created by different conditions of the nitrogen pools in combination with corresponding isotope fractionation processes in the course of the year. These conditions can be caused by a heterogeneous topography or by varying sediment properties, most prominently porosity variations. Both simulation results and observational data show that maximum δ15N values do not occur directly in front of riverine discharge areas, but along the North Frisian coast due to incomplete nitrate assimilation near the river mouths and as a consequence of the prevailing current pattern. In a scenario run with reduced nitrogen river loads, this maximum migrates towards the river mouth. This shift is a consequence of the lower nitrogen loads and a faster complete consumption of river-borne nitrogen by phytoplankton.