TY - JOUR
T1 - Vertical decoupling of nitrate assimilation and nitrification in the Sargasso Sea
AU - Fawcett, Sarah E.
AU - Ward, Bess B.
AU - Lomas, Michael W.
AU - Sigman, Daniel M.
N1 - Funding Information:
We thank all those who have assisted with sample collection at sea over the years: K. Altieri, A. Babbin, D. Chien, O. Coyle, M. Drake, S. Dresner, J. Granger, M. Higgins, J. Lueders-Dumont, D. Marconi, P. Rafter, H. Ren, T. Serwatka, K. Swart, T. Tamasi, L. Treibergs, and N. Van Oostende, as well as S. Oleynik and M. Weigand for laboratory support. We are grateful to BATS, the captains and crew and of the R/V Atlantic Explorer , and the faculty and staff of the Bermuda Institute of Ocean Sciences, particularly S. Bell, D. Evans, R. Johnson, J. Sadler, and D. Lomas. We thank M. McIlvin and K. Casciotti for the GEOTRACES samples. This work was supported by the Charrock Foundation , the Grand Challenges Program of Princeton University , and the US NSF through grants OCE-1136345 (BBW and DMS), OCE-1060947 (DMS), OCE-1030149 (BATS), and OCE-0752366 (BATS).
Publisher Copyright:
© 2015 Elsevier Ltd.
PY - 2015/9/1
Y1 - 2015/9/1
N2 - The fraction of phytoplankton growth that leads to the rain of organic carbon out of the sunlit surface ocean ("export production") is central to the ocean's sequestration of atmospheric carbon dioxide. 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 sunlit layer (the "euphotic zone") originates from biological nitrogen (N) already in surface waters. This view has been supported by recent observations of euphotic zone nitrate elevated in δ18O relative to its δ15N, taken as indicative of nitrification (the oxidation of recycled ammonium to nitrite and then nitrate). To evaluate the potential importance of nitrification in the Sargasso Sea euphotic zone, we measured the δ15N and δ18O of seawater nitrate for samples with ≥0.2μM nitrate collected on 18 cruises in the Sargasso Sea, and here we present the first large data set to correct for the low but often measurable concentrations of nitrite. Regardless of season, nitrate (i.e., nitrate-only rather than nitrate+nitrite as is commonly reported) δ15N and δ18O increase in unison from below the base of the euphotic zone toward the surface. This pattern derives from nitrate assimilation by phytoplankton, implying that nitrification is much slower than the upward transport of nitrate into the lower Sargasso Sea euphotic zone. In the twilight zone below the euphotic zone, we observe a rise in nitrate δ18O (relative to deeper waters) that is not accompanied by a rise in δ15N, suggesting nitrification co-occurring with nitrate assimilation. Nitrification, therefore, does not appear to occur in euphotic zone waters overlapping with nitrate assimilation only in the ~150m-thick twilight zone layer below it. In net, the data argue for a simpler N cycle for the Sargasso Sea euphotic zone than has recently been suggested, with the rate of euphotic zone nitrate assimilation approximating that of organic carbon export to the deep ocean.
AB - The fraction of phytoplankton growth that leads to the rain of organic carbon out of the sunlit surface ocean ("export production") is central to the ocean's sequestration of atmospheric carbon dioxide. 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 sunlit layer (the "euphotic zone") originates from biological nitrogen (N) already in surface waters. This view has been supported by recent observations of euphotic zone nitrate elevated in δ18O relative to its δ15N, taken as indicative of nitrification (the oxidation of recycled ammonium to nitrite and then nitrate). To evaluate the potential importance of nitrification in the Sargasso Sea euphotic zone, we measured the δ15N and δ18O of seawater nitrate for samples with ≥0.2μM nitrate collected on 18 cruises in the Sargasso Sea, and here we present the first large data set to correct for the low but often measurable concentrations of nitrite. Regardless of season, nitrate (i.e., nitrate-only rather than nitrate+nitrite as is commonly reported) δ15N and δ18O increase in unison from below the base of the euphotic zone toward the surface. This pattern derives from nitrate assimilation by phytoplankton, implying that nitrification is much slower than the upward transport of nitrate into the lower Sargasso Sea euphotic zone. In the twilight zone below the euphotic zone, we observe a rise in nitrate δ18O (relative to deeper waters) that is not accompanied by a rise in δ15N, suggesting nitrification co-occurring with nitrate assimilation. Nitrification, therefore, does not appear to occur in euphotic zone waters overlapping with nitrate assimilation only in the ~150m-thick twilight zone layer below it. In net, the data argue for a simpler N cycle for the Sargasso Sea euphotic zone than has recently been suggested, with the rate of euphotic zone nitrate assimilation approximating that of organic carbon export to the deep ocean.
KW - Dual isotopes of nitrate
KW - Nitrate assimilation
KW - Nitrification
KW - Nitrite removal
KW - Organic carbon export
KW - Sargasso Sea
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U2 - 10.1016/j.dsr.2015.05.004
DO - 10.1016/j.dsr.2015.05.004
M3 - Article
AN - SCOPUS:84934942217
SN - 0967-0637
VL - 103
SP - 64
EP - 72
JO - Deep-Sea Research Part I: Oceanographic Research Papers
JF - Deep-Sea Research Part I: Oceanographic Research Papers
ER -