Oxygen Minimum Zones and the Marine Nitrogen Cycle: a Modeling Approach

Abstract:

In the oxygen minimum zones (OMZ) of the global ocean fixed nitrogen is lost primarily by heterotrophic denitrification and anaerobic ammonium oxidation by nitrite (anammox). As it is projected that the OMZ expand in future due to global warming there is an urgent need to understand the controlling mechanisms of nitrogen loss within these regions. However, most Earth system models (ESM) rather have a poor representation of nitrogen related processes, primarily focusing only on nitrate. This simplification of the nitrogen cycle could contribute to the common model bias of too large and too intense OMZ in most ESM.

We set up a more comprehensive marine N-cycle including interactions between nitrate, nitrite, ammonium, and organic material in the water column and the sediments in the framework of the global biogeochemical ocean model of the Max Planck Institute for Meteorology. We present a global marine nitrogen budget and discuss changes in the subsurface oxygen distributions. We find that the representation of a stepwise dissimilatory nitrate reduction to nitrite and ammonium improves the simulated OMZs and captures observed features like the secondary nitrite maximum.