Measurements to date have shown that both bulk and high molecular weight marine dissolved organic nitrogen (DON) have a 15N/ 14N that is substantially higher than the 15N/ 14N of suspended particulate organic nitrogen (PN susp) found in the same surface waters (with δ 15N of ∼4 to 5‰ and ∼-1 to 1‰, respectively). Moreover, the concentration and 15N/ 14N of DON are much less dynamic than those of PN susp. These observations raise questions regarding the role of DON in the upper ocean nitrogen (N) cycle. In this study, the concentration and 15N/ 14N of nitrate and DON was measured in the upper 300 m of the oligotrophic North Atlantic and North Pacific Oceans. Comparing these two regions, the average DON concentration in the upper 100 m is similar, between 4.5 and 5.0 M, but the average δ 15N of DON is significantly different, 3.9‰ versus air in the North Atlantic and 4.7‰ in the North Pacific. This difference parallels a similar isotopic difference between shallow nitrate in these two regions; at 200 m in the North Atlantic, the δ 15N of nitrate is 2.6‰, while it is 4.0‰ in the North Pacific. This isotopic correlation between surface DON and subsurface nitrate indicates that DON is actively participating in the upper ocean N cycle of each region. We describe a conceptual model that explains the elevation of the 15N/ 14N of DON relative to surface ocean PN susp as well as the interbasin difference in the 15N/ 14N of DON. In this model, DON is produced from PN susp without isotopic fractionation but DON is removed by fractionating processes. The ammonium and simple organic N compounds released by DON decomposition reactions are reassimilated by algae into the PN susp pool, as an integral part of the ammonium-centered cycle that lowers the 15N/ 14N of PN susp relative to the nitrate supply from below. This interpretation is consistent with the understanding of the chemical controls on isotope fractionation and is analogous to the previously posed explanation for the 15N/ 14N elevation of herbivorous zooplankton. In addition, it explains a lack of correlation between in situ N 2 fixation rates and DON concentration and 15N/ 14N on short time scales.
All Science Journal Classification (ASJC) codes
- Global and Planetary Change
- Environmental Chemistry
- General Environmental Science
- Atmospheric Science