Rivers collect and transport reactive nitrogen to coastal seas as nitrate, ammonium, dissolved organic nitrogen (DON), or particulate nitrogen. DON is an important component of reactive nitrogen in rivers and is suspected to contribute to coastal eutrophication, but little is known about seasonality of DON loads and turnover within rivers. We measured the concentrations and the isotope ra- 3 ) and particulate nitrogen (δ 15 PN) in the non-tidal Elbe River (SE North Sea, NW Europe) over a period

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... of 2 yr (June 2005 to December 2007) at monthly resolution. Combined DON + NH + 4 concentrations ranged from 22 to 75 µM and comprised nearly 23 % of total dissolved nitrogen in the Elbe River in annual mean; PN and nitrate concentrations ranged from 11 to 127 µM, and 33 to 422 µM, respectively. Combined PN and DON + NH + 4 concentrations were, to a first approximation, inversely correlated to nitrate concentrations. δ 15 DON + NH + 4 , which varied between from 0.8 ‰ to 11.5 ‰, changed in parallel to δ 15 PN (range 6 to 10 ‰), and both were anti-correlated to δ 15 N − NO − 3 (range 6 to 23 ‰). Seasonal patterns of DON + NH + 4 concentrations and δ 15 DON + NH + 4 diverge from those expected from biological DON + NH + 4 production in the river alone and suggest that the elution of organic fertilisers significantly affects the DON + NH + 4 pool in the Elbe River. 4 , nitrate, or PN, and in the Published by Copernicus Publications on behalf of the European Geosciences Union.