TY - JOUR

T1 - Isotopic composition of nitrate in five German rivers discharging into the North Sea

AU - Johannsen, Astrid

AU - Dähnke, Kirstin

AU - Emeis, Kay

PY - 2008/12

Y1 - 2008/12

N2 - We determined concentrations and isotopic composition of nitrate in five German rivers (Rhine, Elbe, Weser, Ems, and Eider) that discharge into the North Sea. Samples were obtained on a biweekly to monthly basis and chemical and isotopic analyses were conducted for the period January 2006 to March 2007 at sampling stations situated before estuarine mixing with North Sea water. We observed maximum nitrate loads in winter and fall, when both discharge and concentration of nitrate are highest. Mean annual isotope values in nitrate ranged from 8.2‰ to 11.3‰ for δ 15 N NO3- and 0.4‰ to 2.2‰ for δ 18 O NO3-. The ranges of isotope values suggest that nitrate in these rivers derives from soil nitrification, sewage, and/or manure. These and published data on other rivers in northern Europe and northern America reveal a correlation between agricultural land use (>60% in the catchment areas of rivers examined) and δ 15 N NO3- values. The rivers Rhine, Elbe, Weser and Ems show similar seasonal patterns of the isotopic fractionation of nitrate with increasing δ 15 N NO3- values and simultaneously decreasing NO3- concentrations during summer months, indicating that assimilation of nitrate is the main fractionation process of riverine nitrate. Isotopic signals in winter are more depleted than the mean summer isotope values, attributed to less microbial activity and assimilative processes. Load weighted nitrate δ15N of the riverine input to the German Bight Coastal Water mass before estuarine mixing and processing is between 8‰ and 12‰. The high δ15N value of river nitrate is matched by high δ15N of nitrate in surface sediments in the German Bight.

AB - We determined concentrations and isotopic composition of nitrate in five German rivers (Rhine, Elbe, Weser, Ems, and Eider) that discharge into the North Sea. Samples were obtained on a biweekly to monthly basis and chemical and isotopic analyses were conducted for the period January 2006 to March 2007 at sampling stations situated before estuarine mixing with North Sea water. We observed maximum nitrate loads in winter and fall, when both discharge and concentration of nitrate are highest. Mean annual isotope values in nitrate ranged from 8.2‰ to 11.3‰ for δ 15 N NO3- and 0.4‰ to 2.2‰ for δ 18 O NO3-. The ranges of isotope values suggest that nitrate in these rivers derives from soil nitrification, sewage, and/or manure. These and published data on other rivers in northern Europe and northern America reveal a correlation between agricultural land use (>60% in the catchment areas of rivers examined) and δ 15 N NO3- values. The rivers Rhine, Elbe, Weser and Ems show similar seasonal patterns of the isotopic fractionation of nitrate with increasing δ 15 N NO3- values and simultaneously decreasing NO3- concentrations during summer months, indicating that assimilation of nitrate is the main fractionation process of riverine nitrate. Isotopic signals in winter are more depleted than the mean summer isotope values, attributed to less microbial activity and assimilative processes. Load weighted nitrate δ15N of the riverine input to the German Bight Coastal Water mass before estuarine mixing and processing is between 8‰ and 12‰. The high δ15N value of river nitrate is matched by high δ15N of nitrate in surface sediments in the German Bight.

KW - Chemistry

UR - http://www.scopus.com/inward/record.url?scp=56349151897&partnerID=8YFLogxK

U2 - 10.1016/j.orggeochem.2008.03.004

DO - 10.1016/j.orggeochem.2008.03.004

M3 - Journal articles

AN - SCOPUS:56349151897

VL - 39

SP - 1678

EP - 1689

JO - Organic Geochemistry

JF - Organic Geochemistry

SN - 0146-6380

IS - 12

ER -