The ocean as a source of nitrogen to the marine atmosphere
Global models indicate that human-derived nitrogen emissions reaching the ocean through atmospheric transport and deposition directly impact biology and the oceanic CO2 sink. However, there is little data from the marine environment to validate this. In particular, there is a paucity of atmospheric organic N data, even though organic nitrogen can represent 20-80% of total nitrogen deposition. On a global basis, organic nitrogen is typically parameterized as averaging 30% of total nitrogen deposition, with 50-80% of that assumed to derive from anthropogenic sources.
Using 18 months of rain and aerosol data collected on the island of Bermuda along with oceanographic data from BATS, we find that the organic nitrogen in marine aerosols derives predominantly from biological primary production in the surface ocean rather than from pollution on land (figure 1). Previous work by Altieri, Hastings, Peters, and Sigman (Altieri JGR 2013, Altieri GBC 2014) has shown a significant anthropogenic influence on the nitrate deposited to the North Atlantic whereas ammonium appears to cycle dynamically between the upper ocean and lower atmosphere. Collectively, these findings suggest that the ocean is not a passive recipient of anthropogenic nitrogen deposition, contrary to how it is generally viewed. One implication is that the contribution of atmospheric nitrogen deposition to ocean fertility, oceanic CO2 removal, and nitrous oxide emissions has been overestimated.
The depth distribution of nitrate assimilation and nitrification in the Sargasso Sea
The fraction of phytoplankton growth that leads to the rain of organic carbon out of the euphotic zone (“export production”) is central to the ocean’s sequestration of atmospheric CO2. Nitrate assimilation has long been taken as a measure of export production; however, this has recently been questioned by suggestions that much of the nitrate in the euphotic zone originates from biological N already in surface waters. The implications of this are that export production (and thus CO2 sequestration) in much of the modern ocean has been overestimated since some fraction of the nitrate being assimilated was produced in the euphotic zone by in situ nitrification (the oxidation of recycled ammonium to nitrite and then nitrate).
We measured the δ15N and δ18O of seawater nitrate for samples collected on 18 cruises in the Sargasso Sea, and plotted the data from 0-300 m in δ18O vs. δ15N space (figure 2). Culture studies have shown that nitrate assimilation by phytoplankton causes a nearly equivalent rise in the δ15N and δ18O of residual nitrate; thus, if the only biological process acting upon nitrate in the upper water column at BATS were assimilation, we would expect the nitrate samples to fall along a 1:1 line in δ18O vs. δ15N space. Our data show a distinct deviation from 1:1, with a significantly greater rise in nitrate δ18O than δ15N (figure 2A). Previous observations of such a decoupling of the N and O isotopes have been interpreted as indicating co-occurring euphotic zone nitrate assimilation and nitrification. In our dataset, however, the extent of the decoupling appears to be correlated with the proportion of nitrite (relative to nitrate) in the samples. For example, the springtime samples that deviate most from the 1:1 line in δ18O vs. δ15N space (triangles in the dashed red box; figure 2A) are characterized by ≥30% nitrite, whereas summertime samples containing almost no nitrite (circles in the dashed blue ellipse) lie along a roughly 1:1 upwards trajectory from the thermocline.
We thus removed nitrite from all samples, which dramatically changes our interpretation of the data. Regardless of season, nitrate-only δ15N and δ18O increase in unison from below the base of the euphotic zone towards the surface (figure 1B), consistent with nitrate assimilation as the dominant biological process acting on euphotic zone nitrate. In the twilight zone below the euphotic zone, nitrite removal does not remove the increase in nitrate δ18O that occurs in the absence of an equal rise in δ15N (purple arrow in figure 2B). This deviation from 1:1 argues for nitrification co-occurring with nitrate assimilation. Nitrification, therefore, does not appear to occur in the euphotic zone, and overlaps with nitrate assimilation only in the ~150 m-thick layer below it. In net, the data argue for a simpler upper ocean Sargasso Sea N cycle than has recently been suggested, with the rate of euphotic zone nitrate assimilation approximating that of organic carbon export to the deep ocean.
Fawcett, S.E., Johnson, K.S., Riser, S.C., Van Oostende, N., Sigman, D.M. Low-nutrient organic matter in the Sargasso Sea thermocline: A hypothesis for its role, identity, and carbon cycle implications. Marine Chemistry doi:10.1016/j.marchem.2018.10.008 (2018). Fawcett MarChem 2018.
Smart, S.M., Ren, H., Fawcett, S.E., Schiebel, R., Conte, M., Rafter, P.A., Ellis, K.K., Weigand, M.A., Sigman, D.M., Haug, G.H. Ground-truthing the planktic foraminifer-bound nitrogen isotope paleo-proxy in the Sargasso Sea. Geochimica et Cosmochimica Acta 235, 463-482 (2018). Smart GCA 2018
Marconi, D., Sigman, D.M., Casciotti, K.L., Campbell, E.C., Weigand, M.A., Fawcett, S.E., Knapp, A.N., Ward, B.B., Haug, G.H. Tropical dominance of N2 fixation in the North Atlantic Ocean. Global Biogeochemical Cycles 31, 1608-1623 (2017). Marconi GBC 2017
Altieri, K.E., Fawcett, S.E., Peters, A.J., Sigman, D.M., Hastings, M.G. Marine biogenic source of atmospheric organic nitrogen in the subtropical North Atlantic. PNAS 113: 925-930 (2016). Altieri PNAS 2016. (Check out the news stories about it here).
Fawcett, S.E., Ward, B.B., Lomas, M.W., Sigman, D.M. Vertical decoupling of nitrate assimilation and nitrification in the Sargasso Sea. Deep-Sea Research I 103: 64-72 (2015). Fawcett DSR 2015
Collaborators: Katye Altieri, University of Cape Town; Meredith Hastings, Brown University; Ken Johnson, MBARI; Michael Lomas, Bigelow Laboratory for Ocean Sciences; Daniel Sigman, Princeton University; Nicolas Van Oostende, Princeton University; Bess Ward, Princeton University