Mixed layer nitrogen cycling in the Southern Ocean: seasonality, kinetics, and biogeochemical implications

• Main Author: Mdutyana, Mhlangabezi
• Other Authors: Fawcett, Sarah, Thomalla, Sandy J
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Faculty of Science 2021
• Subjects: Oceanography; Southern Ocean
• Online Access: http://hdl.handle.net/11427/35931; https://open.uct.ac.za/bitstream/11427/35931/1/thesis_sci_2021_mdutyana%20mhlangabezi.pdf

The alternation between summertime nitrate drawdown and wintertime nitrate recharge is central to the role of the Southern Ocean in setting atmospheric CO2. However, active cycling of nitrogen (N) in the seasonally-varying mixed layer – including the production of ammonium and its subsequent removal via phytoplankton uptake and nitrification (i.e., the oxidation of ammonium to nitrite and then nitrate) – remains poorly understood. Following the “new production paradigm”, phytoplankton production fueled by ammonium (“regenerated production”) results in no net drawdown of CO2 to the deep ocean, while growth supported by nitrate (“new production”) can be equated to CO2 removal provided that mixed-layer nitrification is negligible. While non-zero mixed-layer nitrification has been measured in many ocean regions, very few data exist for the Southern Ocean. This thesis presents new N cycle data collected across the Southern Ocean south of Africa in winter and summer that emphasize the integral role of mixed-layer N transformations in Southern Ocean productivity and biological CO2 drawdown. To evaluate the new production paradigm as a framework for quantifying Southern Ocean carbon export potential, rates of net primary production (NPP), N uptake (as ammonium and nitrate) and nitrification (ammonium and nitrite oxidation) were measured across the Atlantic sector in winter and summer. Winter mixed-layer NPP and total N (i.e., ammonium + nitrate) uptake were strongly decoupled, likely due to elevated heterotrophic bacterial consumption of ammonium. In summer, NPP and total N were generally well-coupled, although dissolved organic N apparently supported more than a third of NPP at some stations. Nitrification accounted for >100% of the nitrate consumed by phytoplankton in winter, rendering the new production paradigm ill-suited for quantifying carbon export in this season. By contrast, of the >50% of summertime NPP fueled by nitrate, < 4% on average derived from mixed-layer nitrification. While the near-zero ...