Abstract
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.
