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

Abstract. The production and removal 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 seawater samples collected between Cape Town

and the Marginal Ice Zone in winter 2017, we found that NH4+ concentrations

were 5-fold higher than is typical for summer and lower north than south

of the Subantarctic Front (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+ assimilation rates

were highest near the Polar Front (12.9 ± 0.4 nM d−1) and in

the Subantarctic Zone (10.0 ± 1.5 nM d−1), decreasing towards

the Marginal Ice Zone (3.0 ± 0.8 nM d−1) despite the high

ambient NH4+ concentrations in these southernmost waters, likely due to the

low temperatures and limited light availability. By contrast, rates of NH4+

oxidation were higher south than north of the Polar Front (16.0 ± 0.8

versus 11.1 ± 0.5 nM d−1), perhaps due to the lower-light and

higher-iron conditions characteristic of polar waters. NH4+ concentrations

were also measured along five transects of the Southern Ocean (Subtropical Zone to

Marginal Ice Zone) spanning the 2018/19 annual cycle. These measurements

reveal that mixed-layer NH4+ accumulation south of the Subantarctic Front

derives from sustained heterotrophic NH4+ production in late summer through

winter that, in net, outpaces NH4+ removal by temperature-, light-, and

iron-limited microorganisms. Our observations thus imply that the Southern

Ocean becomes a biological source of CO2 to the atmosphere in autumn

and winter not only because nitrate drawdown is weak but also because the

ambient conditions favour net heterotrophy and NH4+ accumulation.