Nitrogen isotope values (δ
15N) of surface sediments in t
he German Big
ht of t
he Nort
h Sea ex
hibit a significant gradient from values of 5–6‰ of t
he open s
helf sea to values above 11‰ in t
he German Big
ht. T
his signal
has been attributed to
hig
h reactive N (N
r) loading enric
hed in
15N from rivers and t
he atmosp
here. To better understand t
he processes t
hat determine t
he intensity and spatial distribution of δ
15N anomalies in surface sediments, and to explore t
heir usefulness for reconstructions of pristine N-input from rivers, we modeled t
he cycling of t
he stable isotopes
14N and
15N in reactive nitrogen t
hroug
h t
he ecosystem of t
he central and sout
hern Nort
h Sea (50.9–57.3°N, 3.4°W−9.2°E) for t
he year 1995. T
he 3D-ecosystem model ECOHAM amended wit
h an isotope-tracking module was validated by δ
15N data of surface sediments wit
hin t
he model domain. A typical marine value (δ
15N
nitrate=5‰) was prescribed for nitrate advected into t
he model domain at t
he seaside boundaries, w
hereas δ
15N
nitrate of river inputs were t
hose measured bi-mont
hly over 1 year; δ
15N values of atmosp
heric deposition were set to 6‰ and 7‰ for NO
x and NH
y, respectively. T
he simulated δ
15N values of different nitrogen compounds in t
he German Big
ht strongly depend on t
he mass transfers in t
he ecosystem. T
hese flu
xes, summarized in a nitrogen budget for 1995, give an estimate of t
he impacts of
hydrodynamical and
hydrological boundary conditions, and internal biogeoc
hemical transformations on t
he nitrogen budget of t
he big
ht.
Sensitivity 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.