10 Jun 2021

10 Jun 2021

Review status: a revised version of this preprint is currently under review for the journal BG.

Biogeochemical controls on wintertime ammonium accumulation in the surface layer of the Southern Ocean

Shantelle Smith1, Katye E. Altieri1, Mhlangabezi Mdutyana1,2, David R. Walker3, Ruan G. Parrott1, Kurt A. M. Spence1, Jessica M. Burger1, and Sarah E. Fawcett1,4 Shantelle Smith et al.
  • 1Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, Cape Town, South Africa
  • 2Southern Ocean Carbon and Climate Observatory (SOCCO), CSIR, Rosebank, Cape Town, South Africa
  • 3Department of Conservation and Marine Sciences, Cape Peninsula University of Technology, Cape Town, South Africa
  • 4Marine and Antarctic Research centre for Innovation and Sustainability (MARIS), University of Cape Town, Cape Town, South Africa

Abstract. The production and consumption of ammonium (NH4+) are essential upper-ocean nitrogen cycle pathways, yet in the Southern Ocean where NH4+ has been observed to accumulate in surface waters, its mixed-layer cycling remains poorly understood. For surface samples collected between Cape Town and the marginal ice zone (MIZ) in winter 2017, we found that NH4+ concentrations were five-fold higher than is typical for summer, and lower north than south of the Subantarctic Front (SAF; 0.01–0.26 µM versus 0.19–0.70 µM). Our observations confirm that NH4+ accumulates in the Southern Ocean’s winter mixed layer, particularly in polar waters. NH4+ uptake rates were highest near the Polar Front (PF; 12.9 ± 0.4 nM day−1) and in the Subantarctic Zone (10.0 ± 1.5 nM day−1), decreasing towards the MIZ (3.0 ± 0.8 nM day−1) despite high ambient NH4+ concentrations, likely due to low sea surface temperatures and light availability. By contrast, rates of NH4+ oxidation were higher south than north of the PF (16.0 ± 0.8 versus 11.1 ± 0.5 nM day−1), perhaps due to the lower light and higher iron conditions characteristic of polar waters. Augmenting our dataset with NH4+ concentration measurements spanning the 2018/2019 annual cycle reveals that mixed-layer NH4+ accumulation south of the SAF likely derives from sustained heterotrophic NH4+ production in late summer through winter that outpaces NH4+ consumption by temperature-, light, and iron-limited microorganisms. Our observations thus imply that the Southern Ocean becomes a biological source of CO2 to the atmosphere for half the year not only because nitrate drawdown is weak, but also because the ambient conditions favour net heterotrophy and NH4+ accumulation.

Shantelle Smith et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-149', Anonymous Referee #1, 11 Jun 2021
    • AC1: 'Reply on RC1, RC2, and RC3', Shantelle Smith, 14 Sep 2021
  • RC2: 'Comment on bg-2021-149', Anonymous Referee #2, 12 Jul 2021
    • AC1: 'Reply on RC1, RC2, and RC3', Shantelle Smith, 14 Sep 2021
  • RC3: 'Comment on bg-2021-149', Anonymous Referee #3, 16 Jul 2021
    • AC1: 'Reply on RC1, RC2, and RC3', Shantelle Smith, 14 Sep 2021

Shantelle Smith et al.

Data sets

Biogeochemical data - 2017 Winter Cruise Atlantic-Indian Southern Ocean Shantelle Smith

Shantelle Smith et al.


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Short summary
Ammonium is a crucial yet poorly-understood component of the Southern Ocean nitrogen cycle. We attribute our finding of consistently high ammonium concentrations in the winter mixed layer to limited ammonium consumption and sustained ammonium production, conditions under which the Southern Ocean becomes a source of carbon dioxide to the atmosphere. From similar data collected over an annual cycle, we propose a seasonal cycle for ammonium in shallow polar waters – a first for the Southern Ocean.