科研之友
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摘要
Nitrogen isotope values (delta(15)N) of surface sediments in the German Bight of the North Sea exhibit a significant gradient from values of 5-6 parts per thousand of the open shelf sea to values above 11 parts per thousand in the German Bight. This signal has been attributed to high reactive N (N(r)) loading enriched in (15)N from rivers and the atmosphere. To better understand the processes that determine the intensity and spatial distribution of delta(15)N 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 (14)N and (15)N in reactive nitrogen through the ecosystem of the central and southern North Sea (50.9-57.3 degrees N, 3.4 degrees W-9.2 degrees E) for the year 1995. The 3D-ecosystem model ECOHAM amended with an isotope-tracking module was validated by delta(15)N data of surface sediments within the model domain. A typical marine value (delta(15)N(nitrate)=5 parts per thousand) was prescribed for nitrate advected into the model domain at the seaside boundaries, whereas delta(15)N(nitrate) of river inputs were those measured bi-monthly over 1 year: delta(15)N values of atmospheric deposition were set to 6 parts per thousand and 7 parts per thousand for NO(x) and NH(y), respectively. The simulated delta(15)N 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. Sensitivity tests suggest that the most relevant parameters to reproduce observed sediment delta(15)N are the (15)N/(14)N ratios in N(r)-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 (15)N (delta(15)N > 9.5 parts per thousand). The general gradient of decreasing delta(15)N in sediments from the coast to the open shelf primarily reflects the amount of (15)N-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 delta(15)N 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.
- 出版日期2010-2-15
