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 ratios 15N/14N of combined DON + NH4+ (δ15DON + NH4+), nitrate (δ15N − NO3−) and particulate nitrogen (δ15PN) in the non-tidal Elbe River (SE North Sea, NW Europe) over a period of 2 yr (June 2005 to December 2007) at monthly resolution. Combined DON + NH4+ 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 + NH4+ concentrations were, to a first approximation, inversely correlated to nitrate concentrations. δ15DON + NH4+, which varied between from 0.8‰ to 11.5‰, changed in parallel to δ15PN (range 6 to 10‰), and both were anti-correlated to δ15N − NO3− (range 6 to 23‰). Seasonal patterns of DON + NH4+ concentrations and δ15DON + NH4+ diverge from those expected from biological DON + NH4+ production in the river alone and suggest that the elution of organic fertilisers significantly affects the DON + NH4+ pool in the Elbe River.
Schlarbaum, T., Dähnke, K., & Emeis, K. (2011). Dissolved and particulate reactive nitrogen in the Elbe River/NW Europe: A 2-yr N-isotope study. Biogeosciences, 8(12), 3519–3530. https://doi.org/10.5194/bg-8-3519-2011