Oxygen minimum zone

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The Oxygen minimum zone is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 500 to 1,000 metres, depending on local circumstances.

In the deep ocean, the presence of oxygen in the seawater is a remnant of when that water mass was located at or near the surface of the ocean. Most if not all deep water masses have their origin in the seas around Antarctica or in the Arctic Ocean, where oxygen rich cold water sinks down and flows north or south. The oxygen in this water is no longer replenished after sinking down, since no photosynthesis can take place at these depths due to lack of sunlight, and no exchange with the Earth's atmosphere is possible.

Therefore, the amount of saturation of oxygen in deep water is dependent on the amount of oxygen it had when it reached that depth minus depletion by deep sea organisms. In the oxygen minimum zone, this saturation reaches a minimum.

The reason this zone is located at 500 to 1,000 metres deep, is because above 500 metres, replenishment by air and from photosynthesis is still possible, while below about 1,000 metres, there are too few organisms present to exhaust the supply.

For those organisms, like the Vampire Squid, who live in the oxygen minimum zone, special adaptations are needed to either make do with lesser amounts of oxygen or to extract oxygen from the water more efficiently.