AbstractDissolved organic nitrogen (DON) is often the dominant form of reactive nitrogen transported from land to sea by rivers, but is considered to be largely recalcitrant and behaves conservatively in many estuaries. We measured the concentration and the isotope ratio δ15N of combined DON and ammonium (δ15DON + NH4+) in the Elbe River estuary (SE North Sea, NW Europe) by a combination of a modified persulfate digestion and the denitrifier method. Measurements were made on samples taken along the salinity gradient from 1 to 32 during different seasons, in order to gauge the effects of internal biological processes and external signatures (such as pollution). Combined DON and ammonium concentrations ranged from 20 to 60 µM, and δ15DON + NH4+ from 0 to 11‰. The results show that DON + NH4+ contributes < 20% to total reactive nitrogen in the river end member and rises to 50% in the outer estuary. By comparison with older data, the DON load in the Elbe River did not change since the 1980s, when nitrate and phosphate pollution was maximal. We find evidence that DON and/or ammonium or reactive components in DON are both consumed and produced in the estuary, indicated by changing isotope ratios and non-conservative mixing gradients. The estuarine turbidity maximum zone (TMZ) at salinities < 5, which today is a significant source of nitrate from nitrification, coincides with significantly decreased DON + NH4+ concentrations and δ15DON + NH4+ in all seasons sampled. Whether this is due to selective absorption/desorption of 15N enriched moieties onto particle surfaces, or to selective heterotrophic assimilation and nitrification is yet unclear, and the loss of DON + NH4+ does not balance the added nitrate. Because DON + NH4+ concentrations and δ15DON + NH4+ rise sharply seaward of the TMZ, we consider adsorption/desorption processes most likely. In the salinity gradient 5 to 30, DON + NH4+ behaves conservatively in both concentration and isotopic composition.