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

Abstract

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\u2009µM versus 0.19–0.70\u2009µM). 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\u2009±\u20090.4\u2009nM\u2009day−1) and in the Subantarctic Zone (10.0\u2009±\u20091.5\u2009nM\u2009day−1), decreasing towards the MIZ (3.0\u2009±\u20090.8\u2009nM\u2009day−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\u2009±\u20090.8 versus 11.1\u2009±\u20090.5\u2009nM\u2009day−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.\n