Nitroge
n isotope values (δ
15N) of surface sedime
nts i
n the Germa
n Bight of the North Sea exhibit a sig
nifica
nt gradie
nt from values of 5–6‰ of the ope
n shelf sea to values above 11‰ i
n the Germa
n Bight. This sig
nal has bee
n attributed to high reactive N (N
r) loadi
ng e
nriched i
n 15N from rivers a
nd the atmosphere. To better u
ndersta
nd the processes that determi
ne the i
nte
nsity a
nd spatial distributio
n of δ
15N a
nomalies i
n surface sedime
nts, a
nd to explore their useful
ness for reco
nstructio
ns of pristi
ne N-i
nput from rivers, we modeled the cycli
ng of the stable isotopes
14N a
nd
15N i
n reactive
nitroge
n through the ecosystem of the ce
ntral a
nd souther
n North Sea (50.9–57.3°N, 3.4°W−9.2°E) for the year 1995. The 3D-ecosystem model ECOHAM ame
nded with a
n isotope-tracki
ng module was validated by δ
15N data of surface sedime
nts withi
n the model domai
n. A typical mari
ne value (δ
15N
nitrate=5‰) was prescribed for
nitrate advected i
nto the model domai
n at the seaside bou
ndaries, whereas δ
15N
nitrate of river i
nputs were those measured bi-mo
nthly over 1 year; δ
15N values of atmospheric depositio
n were set to 6‰ a
nd 7‰ for NO
x a
nd NH
y, respectively. The simulated δ
15N values of differe
nt
nitroge
n compou
nds i
n the Germa
n Bight stro
ngly depe
nd o
n the mass tra
nsfers i
n the ecosystem. These flu
xes, summarized i
n a
nitroge
n budget for 1995, give a
n estimate of the impacts of hydrody
namical a
nd hydrological bou
ndary co
nditio
ns, a
nd i
nter
nal biogeochemical tra
nsformatio
ns o
n the
nitroge
n budget of the bight.
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.