The kinetics of ammonium uptake and oxidation across the Southern Ocean

Abstract

Central to the Southern Ocean's role in setting atmospheric CO 2 is the seasonal alternation between upward mixing of nutrients and their subsequent consumption by phytoplankton. Active nutrient cycling within the mixed layer, including the release of ammonium (NH 4 + ) and its removal by phytoplankton and nitrifiers, also affects Southern Ocean CO 2 drawdown, yet remains poorly understood. We conducted kinetics experiments across the Southern Ocean south of Africa to investigate the dependence of NH 4 + uptake (summer, winter) and oxidation (winter) on NH 4 + concentration. NH 4 + uptake followed a Michaelis–Menten function in both seasons, with the maximum rate ( V max ) decreasing poleward, apparently controlled mainly by light in winter and temperature in summer. The half‐saturation constant ( K m ) increased poleward with increasing ambient NH 4 + ([NH 4 + ] amb ) and was threefold higher in winter (150–405 nM) than in summer (41–115 nM), suggesting that summertime phytoplankton are adapted to low‐NH 4 + conditions while winter communities typically receive a higher NH 4 + supply. NH 4 + oxidation showed a high affinity for NH 4 + ( K m = 28–137 nM), suggesting a dominant role for ammonia‐oxidizing archaea, and followed a Michaelis–Menten curve only when [NH 4 + ] amb was ≤ 90 nM. V max was near‐constant across the region regardless of [NH 4 + ] amb , temperature, or light. From coincident mixed‐layer NH 4 + oxidation and iron measurements, we hypothesize that iron availability may (co‐)limit the V max of NH 4 + oxidation. If verified, this suggestion has implications for models that parameterize nitrification as a linear function of [NH 4 + ] amb . Additionally, iron depletion may limit the role of mixed‐layer nitrification, which is dominant in the winter Southern Ocean, in offsetting phytoplankton CO 2 drawdown annually.

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