Oxygen minimum zone

Aerobic bacteria feed on this organic matter; oxygen is used as part of the bacterial metabolic process, lowering its concentration within the water.

A distribution of the open-ocean oxygen minimum zones is controlled by the large-scale ocean circulation as well as local physical as well as biological processes.

For example, wind blowing parallel to the coast causes Ekman transport that upwells nutrients from deep water.

The increased nutrients support phytoplankton blooms, zooplankton grazing, and an overall productive food web at the surface.

The byproducts of these blooms and the subsequent grazing sink in the form of particulate and dissolved nutrients (from phytodetritus, dead organisms, fecal pellets, excretions, shed shells, scales, and other parts).

This "rain" of organic matter (see the biological pump) feeds the microbial loop and may lead to bacterial blooms in water below the euphotic zone due to the influx of nutrients.

Horizontal mixing is constrained by bathymetry and boundaries formed by interactions with sub-tropical gyres and other major current systems.

[4][5][6] Low oxygen water may spread (by advection) from under areas of high productivity up to these physical boundaries to create a stagnant pool of water with no direct connection to the ocean surface even though (as in the Eastern Tropical North Pacific) there may be relatively little organic matter falling from the surface.

In OMZs oxygen concentration drops to levels <10 nM at the base of the oxycline and can remain anoxic for over 700 m depth.

Another fraction of organic matter sinks to the deep ocean forming aggregates called marine snow.

As the organism reaches maturity, metabolic demands switch from growth and development to maintenance, which requires far fewer resources.

[17] However, organisms have ways of altering their oxygen intake and carbon dioxide release, so the strict stoichiometric equation is not necessarily accurate.

The oxygen minimum zones thus play an important role in regulating the productivity and ecological community structure of the global ocean.

[28][31] In addition to affecting their vital functions, zooplankton alter their distribution in response to hypoxic or anoxic zones.

[43] Throughout the history of Earth's oceans, OMZs have fluctuated on long time scales, becoming larger or smaller depending on multiple variables.

[44] While oxygen minimum zones (OMZs) occur naturally, they can be exacerbated by human impacts like climate change and land-based pollution from agriculture and sewage.

[51] Research has attempted to model potential changes to OMZs as a result of rising global temperatures and human impact.

[54][53][55] Existing Earth system models project considerable reductions in oxygen and other physical-chemical variables in the ocean due to climate change, with potential ramifications for ecosystems and humans.

The global decrease in oceanic oxygen content is statistically significant and emerging beyond the envelope of natural fluctuations.