Anaerobic microbial activity in the ocean causes losses of bioavailable
nitrogen and emission of nitrous oxide to the atmosphere, but its
predictability at global scales remains limited. Resource ratio theory
suggests that anaerobic activity becomes sustainable when the ratio of
oxygen to organic matter supply is below the ratio required by aerobic
metabolisms. Here, we demonstrate the relevance of this framework at the
global scale using three-dimensional ocean datasets, providing a new
interpretation of existing observations. Evaluations of the location and
extent of anoxic zones and a diagnostic rate of pelagic nitrogen loss
are consistent with previous estimates. However, we demonstrate that the
flux-based threshold is qualitatively different from a threshold based
solely on the ambient oxygen concentration. Since the framework is
feasible for application in global biogeochemical models, it represents
a way forward for more dynamic, mechanistic predictions of anaerobic
activity and nitrogen loss.