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

• Main Authors: Smith, Shantelle, Altieri, Katye E., Mdutyana, Mhlangabezi, Walker, David R., Parrott, Ruan G., Spence, Kurt A. M., Burger, Jessica M., Fawcett, Sarah E.
• Format: Text
• Language: English
• Published: 2021
• Subjects: Southern Ocean
• Online Access: https://doi.org/10.5194/bg-2021-149; https://bg.copernicus.org/preprints/bg-2021-149/

The production and consumption of ammonium (NH 4 + ) are essential upper-ocean nitrogen cycle pathways, yet in the Southern Ocean where NH 4 + 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 NH 4 + 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 NH 4 + accumulates in the Southern Ocean’s winter mixed layer, particularly in polar waters. NH 4 + 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 NH 4 + concentrations, likely due to low sea surface temperatures and light availability. By contrast, rates of NH 4 + 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 NH 4 + concentration measurements spanning the 2018/2019 annual cycle reveals that mixed-layer NH 4 + accumulation south of the SAF likely derives from sustained heterotrophic NH 4 + production in late summer through winter that outpaces NH 4 + consumption by temperature-, light, and iron-limited microorganisms. Our observations thus imply that the Southern Ocean becomes a biological source of CO 2 to the atmosphere for half the year not only because nitrate drawdown is weak, but also because the ambient conditions favour net heterotrophy and NH 4 + accumulation.