Nitrogen Losses and Nutrient Regeneration in Oxygen Minimum Zones

In the tropical oceans, coastal upwelling of nutrient-rich deep waters fuels high surface productivity. The decomposition of sinking algal biomass results in the formation of large oxygen-deficient water bodies at mid depths (~100 1,000 m). Although, these oxygen minimum zones (OMZs) amount to <1% of the global ocean volume, they account for ~30-50% of total oceanic nitrogen (N) loss. Anammox, the anaerobic oxidation of NH4 with NO2- to gaseous N2 is the major N-loss pathway in OMZs. The recirculation of N-deficient waters to the surface limits phytoplankton growth and thus carbon sequestration in large parts of the tropical oceans. Continuing ocean de-oxygenation is expected to result in significantly increasing N-losses, thereby reducing the ocean s capacity to attenuate rising atmospheric CO2. This thesis aimed to determine regulatory effects of O2 and organic matter availability on anammox and N-linked processes in OMZs to facilitate model-based assessments of future ocean changes.