Dissolved and particulate reactive nitrogen in the Elbe River/NW Europe: a 2-yrN-isotope study

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 N-15/N-14 of combined DON+NH4+ (delta(DON)-D-15+NH4+), nitrate (delta N-15-NO3-) and particulate nitrogen (delta(PN)-P-15) 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 mu M and comprised nearly 23% of total dissolved nitrogen in the Elbe River in annual mean; PN and nitrate concentrations ranged from 11 to 127 mu M, and 33 to 422 mu M, respectively. Combined PN and DON+NH4+ concentrations were, to a first approximation, inversely correlated to nitrate concentrations. delta(DON)-D-15+NH4+, which varied between from 0.8 parts per thousand to 11.5 parts per thousand, changed in parallel to delta(PN)-P-15 (range 6 to 10 parts per thousand), and both were anti-correlated to delta N-15-NO3- (range 6 to 23 parts per thousand). Seasonal patterns of DON+ NH4+ concentrations and delta(DON)-D-15+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