The kinetics of ammonium uptake and oxidation during winter across the Indian sector of the Southern Ocean

Mhlangabezi Mdutyana1, Xin Sun2, Jessica Burger1, Sandy Jane Thomalla3, Bess B Ward4 and Sarah Fawcett5, (1)University of Cape Town, Oceanography, Cape Town, South Africa, (2)Princeton University, Princeton, NJ, United States, (3)CSIR, Southern Ocean Carbon and Climate Observatory, South Africa, (4)Princeton University, Department of Geosciences, Princeton, NJ, United States, (5)University of Cape Town, Oceanography Department, Cape Town, South Africa
The alternation between summertime nitrate drawdown and wintertime nitrate recharge is central to the role of the Southern Ocean (SO) in setting atmospheric CO2. However, active cycling of nitrogen (N) within the seasonally-varying mixed layer – including the release of ammonium (NH4+) and its subsequent removal via phytoplankton uptake and nitrification (i.e., NH4+ oxidation to nitrate) – remains poorly understood. Our previous work shows high rates of NH4+ uptake and oxidation in the winter mixed layer across the SO. Although both processes remove NH4+, winter mixed layer NH4+ concentrations ([NH4+]) remain fairly high (up to 700 nM), particularly south of the Polar Front (PF; 50-55°S). To assess the role of organism (phytoplankton and nitrifier) physiology in SO NH4+ cycling, we conducted NH4+ uptake and oxidation experiments across the Indian sector (from 37-55°S at 20-30°E) in winter. Throughout the transect NH4+ uptake was well-described by the hyperbolic Michaelis-Menten (MM) equation. Maximum uptake rates (Vmax) varied with latitude, from 38 nM/d at 37°S to 3 nM/d at 55°S. This suggests that light and/or temperature exert a stronger control than [NH4+] on NH4+ uptake in winter. The half saturation constant (Km) ranged from 61 to 339 nM and varied with ambient [NH4+] ([NH4+]amb). Phytoplankton north of the PF ([NH4+]amb ≤50 nM) showed a high affinity for NH4+, while the affinity of those to the south ([NH4+]amb = 250-700 nM) was low. This observation implies that high mixed layer [NH4+] is not unusual in the polar SO in winter, in contrast to more northern SO surface waters where the [NH4+] is likely always low. NH4+ oxidation was not well described by the MM equation. Instead, maximum NH4+ oxidation rates (8-17 nM/d) occurred at substrate concentrations similar to [NH4+]amb, and decreased at higher [NH4+]. The apparent inhibition of NH4+ oxidation by NH4+ has been observed previously, but at much higher [NH4+].