Seasonal and interannual variation of the isotopic composition of nitrate in five German rivers discharging into the North Sea

N-isotope signatures and concentrations of nitrate and particulate nitrogen in five German rivers that discharge into the North Sea were determined on a biweekly to monthly basis from 2006 to 2008. The rivers under study (Rhine, Elbe, Weser, Ems and Eider) varied considerably in their monthly nitrate loads, and in values of δ15N-NO3¯ and δ18O-NO3¯. Seasonal variations can be attributed to biological turnover processes in the water column and sources of reactive nitrogen in the catchment area. A highly significant inverse correlation exists between increasing δ15N-NO3¯ values and decreasing NO3¯concentrations; this inverse relationship is observed in each seasonal cycle, as well as in interannual trends, when comparing the datasets of the two years. For both years, load-weighted δ15N-NO3¯ of the river-borne input to the German Bight coastal water mass (before estuarine mixing and processing) was between 8‰ and 12‰, representing the source signature of nitrate from rivers entering the German Bight.