Abstract
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 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.
Highlights
Dissolved organic nitrogen (DON) is a major contributor to total dissolved nitrogen (TDN, the sum of nitrate, nitrite, ammonium and dissolved organic nitrogen (DON)) discharged from land to the coastal ocean
Throughout the entire sampling period covered in our current study, nitrate displays a clear seasonal trend in concentrations and isotopic composition. δ15N − NO−3 and δ18O − NO−3 were both enriched during summer months and were both depleted during winter times (Fig. 2a) and are anti-correlated (r2 = 0.84, r = −0.92, α ≤ 0.01) (Table 1) to nitrate concentrations. δ15N − NO−3 and δ18O − NO−3 varied almost parallel and are strongly correlated (r2 = 0.96, r = 0.98, α ≤ 0.01, Table 1)
DON + NH+4 concentrations showed a distinct seasonality: concentrations varied between 22 μM and 76 μM, δ15DON + NH+4 ranged from 1 ‰ in November 2006 to 12 ‰ in March 2006 (Fig. 2b)
Summary
Dissolved organic nitrogen (DON) is a major contributor to total dissolved nitrogen (TDN, the sum of nitrate, nitrite, ammonium and DON) discharged from land to the coastal ocean. Meybeck (1993) estimated that nearly 70 % of the nitrogen entering coastal regions via rivers is in the form of DON, and highest relative DON contributions characterize TDN loads of unpolluted rivers (Wiegner et al, 2006). DON comprises a smaller fraction than nitrate of the total N load in many eutrophied rivers, anthropogenic sources may significantly increase the natural DON background (Agedah et al, 2009; Howarth, 2004) up to a point where anthropogenic sources exceed wetland-derived DON in low-N streams (Stanley and Maxted, 2008). When discharged to coastal seas, DON may substitute for dissolved inorganic nitrogen (DIN) as a substrate for phytoplankton assimilation. In seasons when the inorganic N pools are exhausted DON may in particular promote harmful algal blooms (Bronk, 2002)
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