Abstract
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.
Highlights
The Southern Ocean impacts the Earth system through its role in global thermohaline circulation, which drives the exchange of heat and nutrients among ocean basins (Frölicher et al, 2015; Sarmiento et al, 2004)
Latitudinal variations in each parameter are assessed by comparing the various Southern Ocean zones – the Subtropical Zone (STZ) north of the Subtropical Front (STF); the Subantarctic Zone (SAZ) between the STF and the Subantarctic Front (SAF), the Polar Frontal Zone (PFZ) between the SAF and the Polar Front (PF); and south of the PF, the Open Antarctic Zone and Polar Antarctic Zone (OAZ and PAZ, which are divided by the Southern Antarctic Circumpolar Current Front (SACCF) and collectively termed the Antarctic Zone (AZ); see Sect
We attribute the elevated NH+4 concentrations that persist in the winter mixed layer south of the SAF to sustained heterotrophic NH+4 production in excess of NH+4 removal, driven by the temperature, light, and possibly iron limitation of phytoplankton and nitrifiers
Summary
The Southern Ocean impacts the Earth system through its role in global thermohaline circulation, which drives the exchange of heat and nutrients among ocean basins (Frölicher et al, 2015; Sarmiento et al, 2004). Upper Southern Ocean circulation is dominated by the eastward-flowing Antarctic Circumpolar Current (ACC) that consists of a series of broad circumpolar bands (“zones”) separated by oceanic fronts These fronts can drive water mass formation (Ito et al, 2010) and nutrient upwelling that supports elevated productivity (Sokolov and Rintoul, 2007). Smith et al.: Ammonium accumulation in the shallow Southern Ocean ocean Assimilation of these nutrients, and primary productivity, is limited in the Southern Ocean by numerous overlapping factors, including temperature, light, micronutrient concentrations, and grazing pressure Once the mixed layer shoals in spring and summer, phytoplankton consume the available nutrients until some form of limitation (usually iron; Nelson et al, 2001; Nicholson et al, 2019) sets in This balance between wintertime nutrient recharge and summertime nutrient drawdown is central to the Southern Ocean’s role in setting atmospheric CO2 (Sarmiento and Toggweiler, 1984)
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