Comment on bg-2021-149

The production and removal 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 seawater samples collected between Cape Town and the Marginal Ice Zone in winter 2017, we found that NH 4 + concentrations were 5-fold higher than is typical for summer and lower north than south of the Subantarctic Front (0.01–0.26 µ M versus 0.19–0.70 µ M). Our observations confirm that NH 4 + accumulates in the Southern Ocean's winter mixed layer, particularly in polar waters. NH 4 + 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 NH 4 + concentrations in these southernmost waters, likely due to the low temperatures and limited light availability. By contrast, rates of NH 4 + 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. NH 4 + 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 NH 4 + accumulation south of the Subantarctic Front derives from sustained heterotrophic NH 4 + production in late summer through winter that, in net, outpaces NH 4 + removal 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 in autumn and winter not only because nitrate drawdown is weak but also because the ambient conditions favour net heterotrophy and NH 4 + accumulation.