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The Oxygen Minimum Zone (OMZ) is the naturally occurring ocean layer where dissolved oxygen concentrations in seawater are at a minimum relative to the water above and below it. OMZs occupy 8% (30.4 ± 3·106 km2) of the ocean's total area and are located primarily in the Pacific and Indian Oceans. Hypoxia and anoxia are terms commonly used to describe conditions within OMZs and refer to a general state of oxygen concentration in the water, rather than a specific area. All OMZs are hypoxic, as the generally accepted threshold for hypoxia is 62.5 µM kg-1, but others can experience periodic anoxia when concentrations reach 0 µM kg-1. A combination of oxygen consumption through respiration, weak oxygen exchange at the surface, and low deep water oxygen concentration leads to OMZ formation. OMZs are permanent ocean features. OMZs are primarily found along the western margin of continents in eastern boundary current systems, which are characterized by these processes. OMZs play an important role in several biogeochemical cycles, particularly the nitrogen, sulfur, and carbon cycles, and their unique chemistry leads to the presence of anaerobic bacteria. OMZs are also home to other organisms that have adapted to low oxygen conditions, primarily small invertebrates and bottom-dwelling fish species, but many organisms cannot tolerate OMZ conditions. As OMZs expand, animals that are not able to survive in hypoxic conditions are driven out of a habitat that is no longer suitable.

Characteristics
At the center of the OMZ is the core, which has less than 20 μM (units: kg-1???) oxygen concentration and is the least oxygenated portion. The top of the OMZ is defined as the oxycline, which is where oxygen concentrations decrease by about 1.6 μM m-1 (micromole per meter) and the oxygen concentration is around 65 μM kg-1, while the bottom oxygen gradient is around 100 μM kg-1 with a more gradual increase of about 0.4 μM m-1 in oxygen concentration.

Formation
Surface ocean waters generally have oxygen concentrations close to equilibrium with the atmosphere's oxygen concentration. The atmosphere affects ocean surface waters through processes like wind-driven mixing (circulation), gas exchange and heat exchange; it creates the ocean mixed layer, which transports dissolved oxygen throughout the surface waters. Dissolved oxygen is present in high concentrations in the mixed layer due to atmospheric contact, as well as photosynthesis. As water moves out of the mixed layer and below the thermocline, organic matter created by photosynthetic organisms sinks as well. Aerobic bacteria feed on this organic matter and consume oxygen as part of their metabolic processes, which leads to lower oxygen concentrations in deeper waters (100 to 1,500 meters deep). The transport of water, dissolved oxygen, and organic material from the mixed layer to the ocean interior is known as ventilation. In much of the ocean, ventilation (via seasonal mixing with the surface layer or horizontal movement of oxygen from regions of high concentration to low concentration by ocean currents) enables resupply of oxygen to mid-depth waters. For example, water that is part of a subtropical gyre circulation experiences rapid exchange with the atmosphere and never acquires a large oxygen deficit. But if ventilation weakens, then the oxygen consumed by biological processes in mid-depth waters cannot be replenished, resulting in an OMZ.