Oxygen Minimum Zones

Oxygen Minimum Zones (OMZ) are the places in the world ocean where oxygen saturation in the water column is at its lowest.  This zone typically occurs at depths of about 200 to 1,000 meters.  The AOG lab is interested in OMZs because of their importance in controlling carbon and nitrogen cycling in the oceans.  OMZ water is exposed to the rain of sinking organic matter, which we evaluate using our drifting net traps and in situ incubators.  Bacteria and archea feed on this organic matter and oxygen is used.  Thus, the concentration of oxygen in deep water is dependent on the amount of oxygen it had when it was at the surface minus depletion by deep sea organisms.

In many OMZ regions oxygen actually reaches zero, in which case the OMZ can be called an ODZ (oxygen deficient zone).  ODZs provide appropriate conditions to enable substantial nitrogen loss because in the absence of oxygen, nitrate represents the ‘next best’ electron acceptor available for respiration.  Starting with organic nitrogen (primarily amines and amides), the nitrogen cycle in suboxic waters consists of a series of remineralization and nitrification reactions producing ammonium and nitrate, respectively.  While nitrification is typically assumed to be an aerobic process, substantial suboxic nitrification has been reported in many o the world ocean’s major suboxc zones.  Nitrogen loss is attributed both to hetrotrophic denitrification and to anammox.

Many microorganisms inhabiting the OMZs are capable of multiple functions in the nitrogen cycle.  Versatile metabolic potentials versus actual activities present a challenge, as the two are not the same but currently potential is ‘easier’ to measure than activity.  In their review, Lam and Kuypers (2011) note that “these challenge of the ODZ needs to be tackled before we can realistically predict how N-cycling in OMZs, and thus oceanic N-balance, will respond to future global perturbations.”

AOG studies ODZ regions such as the anoxic fjords of Vancouver Island Canada, the Arabian Sea, the Eastern Tropical North Pacific and the Eastern Tropical South Pacific.