The regulation of oxygen to low concentrations in marine oxygen-minimum zones

The Bay of Bengal hosts persistent, measurable, but sub-micromolar, concentrations of oxygenin its oxygen-minimum zone (OMZ). Such low-oxygen conditions are not necessarily rare in theglobal ocean and seem also to characterize the OMZ of the Pescadero Basin in the Gulf of California,as well as the outer edges of otherwise anoxic OMZs, such as can be found, for example, in theEastern Tropical North Pacific. We show here that biological controls on oxygen consumption arerequired to allow the semistable persistence of low-oxygen conditions in OMZ settings; otherwise,only small changes in physical mixing or rates of primary production would drive the OMZ betweenanoxic and oxic states with potentially large swings in oxygen concentration. We propose that twocontrols are active: an oxygen-dependent control on oxygen respiration and an oxygen inhibition ofdenitrification. These controls, working alone and together, can generate low-oxygen concentrationsover a wide variability in ocean mixing parameters. More broadly, we discuss the oxygen regulationof organic matter cycling and N2 production in OMZ settings. Modern biogeochemical models ofnitrogen and oxygen cycling in OMZ settings do contain some of the parameterizations that weexplore here. However, these models have not been applied to understanding the persistence of low,but measurable, concentrations of oxygen in settings like the Bay of Bengal, nor have they been appliedto understanding what biological/physical processes control the transition from a weakly oxygenatedstate to a “functionally” anoxic state with implications for nitrogen cycling. Therefore, we believethat the approach here illuminates the relationship between oxygen and the biogeochemical cycling ofcarbon and nitrogen in settings like the Bay of Bengal. Furthermore, we believe that our results couldfurther inform large-scale ocean models seeking to explore how global warming might influence thespread of low-oxygen waters, influencing the cycles of oxygen, carbon, and nitrogen in OMZ settings.Keywords: oxygen-minimum zone, OMZ, denitrification, aerobic respiration, climate change,model, feedback, Bay of Bengal