TY - JOUR
T1 - The Seasonal Cycle of Nitrogen Uptake and Nitrification in the Atlantic Sector of the Southern Ocean
AU - Mdutyana, Mhlangabezi
AU - Thomalla, Sandy J.
AU - Philibert, Raissa
AU - Ward, Bess B.
AU - Fawcett, Sarah E.
N1 - Funding Information:
We thank the Captain and crew of the R/V S.A. Agulhas II for their professional support throughout the cruises, as well as the Marine Biogeochemistry Lab team at the University of Cape Town (UCT) for their assistance (R. Flynn, H. Little, R. Parrott, L. Stirnimann, and S. Smith), R. Roman for the onboard analysis of nutrients during the winter cruise, C. Karriem for extensive administrative support, and I. Newton from the Stable Light Isotope Laboratory at UCT for PM analyses. The nitrification measurements were made possible through the Princeton University Visiting Student Research Collaborator program; we are grateful to X. Sun, J. Qixing, and S. Oleynik in the Department of Geosciences for expert assistance during the first author's visits. This work was supported by the South African National Research Foundation through an Innovation Doctoral Scholarship to M. M. (112380), an Equipment-related Travel and Training Grant to S. E. F. (105177), South African National Antarctic Programme Grants to S. E. F. (105539 and 110735) and S.J.T. (93076), and a SANCOR-NRF postdoctoral fellowship to R. P. (94915); the Department of Science and Innovation Biogeochemistry Research Infrastructure Platform (BIOGRIP); UCT through a Harry Crossley Foundation Research Fellowship to M. M., a Postgraduate Travel Grant to M. M., a University Research Council Launching Grant to S. E. F. and a College of Fellows Emerging Researcher Award to S. E. F.; and by the CSIR's Parliamentary Grant.
Funding Information:
We thank the Captain and crew of the R/V for their professional support throughout the cruises, as well as the Marine Biogeochemistry Lab team at the University of Cape Town (UCT) for their assistance (R. Flynn, H. Little, R. Parrott, L. Stirnimann, and S. Smith), R. Roman for the onboard analysis of nutrients during the winter cruise, C. Karriem for extensive administrative support, and I. Newton from the Stable Light Isotope Laboratory at UCT for PM analyses. The nitrification measurements were made possible through the Princeton University Visiting Student Research Collaborator program; we are grateful to X. Sun, J. Qixing, and S. Oleynik in the Department of Geosciences for expert assistance during the first author's visits. This work was supported by the South African National Research Foundation through an Innovation Doctoral Scholarship to M. M. (112380), an Equipment‐related Travel and Training Grant to S. E. F. (105177), South African National Antarctic Programme Grants to S. E. F. (105539 and 110735) and S.J.T. (93076), and a SANCOR‐NRF postdoctoral fellowship to R. P. (94915); the Department of Science and Innovation Biogeochemistry Research Infrastructure Platform (BIOGRIP); UCT through a Harry Crossley Foundation Research Fellowship to M. M., a Postgraduate Travel Grant to M. M., a University Research Council Launching Grant to S. E. F. and a College of Fellows Emerging Researcher Award to S. E. F.; and by the CSIR's Parliamentary Grant. S.A. Agulhas II
Publisher Copyright:
©2020. American Geophysical Union. All Rights Reserved.
PY - 2020/7/1
Y1 - 2020/7/1
N2 - Net primary production (NPP) fueled by nitrate is often equated with carbon export, providing a metric for CO2 removal to the deep ocean. This “new production paradigm” assumes that nitrification, the oxidation of regenerated ammonium to nitrate, is negligible in the sunlit upper ocean. While surface layer nitrification has been measured in other oceanic regions, very few data exist for the Southern Ocean. We measured NPP, nitrogen (N) uptake, and nitrification in the upper 200 m across the Atlantic Southern Ocean in winter and summer. Rates of winter mixed-layer nitrate uptake were low, while ammonium uptake was surprisingly high. NPP was also low, such that NPP and total N (nitrate+ammonium) uptake were decoupled; we attribute this to ammonium consumption by heterotrophic bacteria. By contrast, NPP and total N uptake were strongly coupled in summer except at two stations where an additional regenerated N source, likely dissolved organic N, apparently supported 30–45% of NPP. Summertime nitrate uptake rates were fairly high and nitrate fueled >50% of NPP, indicating the potential for significant carbon export. Nitrification supplied <10% of the nitrate consumed in summertime surface waters, while in winter, mixed-layer nitrification was on average 16 times higher than nitrate uptake. Despite the near-zero nitrification rates measured in the summer mixed layer, the classically defined f ratio does not well-represent Southern Ocean carbon export potential annually. This is because some fraction of the nitrate regenerated in the winter mixed layer is likely supplied to phytoplankton in summer; its consumption cannot, therefore, be equated with export.
AB - Net primary production (NPP) fueled by nitrate is often equated with carbon export, providing a metric for CO2 removal to the deep ocean. This “new production paradigm” assumes that nitrification, the oxidation of regenerated ammonium to nitrate, is negligible in the sunlit upper ocean. While surface layer nitrification has been measured in other oceanic regions, very few data exist for the Southern Ocean. We measured NPP, nitrogen (N) uptake, and nitrification in the upper 200 m across the Atlantic Southern Ocean in winter and summer. Rates of winter mixed-layer nitrate uptake were low, while ammonium uptake was surprisingly high. NPP was also low, such that NPP and total N (nitrate+ammonium) uptake were decoupled; we attribute this to ammonium consumption by heterotrophic bacteria. By contrast, NPP and total N uptake were strongly coupled in summer except at two stations where an additional regenerated N source, likely dissolved organic N, apparently supported 30–45% of NPP. Summertime nitrate uptake rates were fairly high and nitrate fueled >50% of NPP, indicating the potential for significant carbon export. Nitrification supplied <10% of the nitrate consumed in summertime surface waters, while in winter, mixed-layer nitrification was on average 16 times higher than nitrate uptake. Despite the near-zero nitrification rates measured in the summer mixed layer, the classically defined f ratio does not well-represent Southern Ocean carbon export potential annually. This is because some fraction of the nitrate regenerated in the winter mixed layer is likely supplied to phytoplankton in summer; its consumption cannot, therefore, be equated with export.
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U2 - 10.1029/2019GB006363
DO - 10.1029/2019GB006363
M3 - Article
AN - SCOPUS:85088594762
SN - 0886-6236
VL - 34
JO - Global Biogeochemical Cycles
JF - Global Biogeochemical Cycles
IS - 7
M1 - e2019GB006363
ER -