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Zone in which oxygen saturation in seawater in the ocean is at its lowest
a natural phenomenon in the ocean at particular depths

The oxygen minimum zone (OMZ), sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about , depending on local circumstances. OMZs are found worldwide, typically along the western coast of continents, in areas where an interplay of physical and biological processes concurrently lower the oxygen concentration (biological processes) and restrict the water from mixing with surrounding waters (physical processes), creating a "pool" of water where oxygen concentrations fall from the normal range of 4–6 mg/L to below 2 mg/L.

Surface ocean waters generally have oxygen concentrations close to equilibrium with the Earth's atmosphere. In general, colder waters hold more oxygen than warmer waters. As water moves out of the mixed layer into the thermocline, it is exposed to a rain of organic matter from above. Aerobic bacteria feed on this organic matter; oxygen is used as part of the bacterial metabolic process, lowering its concentration within the water. Therefore, 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. The downward flux of organic matter decreases sharply with depth, with 80–90% being consumed in the top The deep ocean thus has higher oxygen because rates of oxygen consumption are low compared with the supply of cold, oxygen rich deep waters from polar regions. In the surface layers, oxygen is supplied by photosynthesis of phytoplankton. Depths in between, however, have higher rates of oxygen consumption and lower rates of advective supply of oxygen rich waters. In much of the ocean, mixing processes enable the resupply of oxygen to these waters (see upwelling). 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. Since oxygen is not being produced as a byproduct of photosynthesis below the euphotic zone, these microbes use up what oxygen is in the water as they break down the falling organic matter thus creating the lower oxygen conditions. 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. This lack of oxygen can be reinforced or increased due to physical processes changing oxygen supply such as eddy driven advection, increases in ocean stratification, and increases in ocean temperature which reduces oxygen solubility. At a microscopic scale the processes causing ocean deoxygenation rely on microbial aerobic respiration. A large fraction of the organic matter from photosynthesis becomes dissolved organic matter (DOM) that is consumed by bacteria during aerobic respiration in sunlit waters. Another fraction of organic matter sinks to the deep ocean forming aggregates called marine snow. These sinking aggregates are consumed via degradation of organic matter and respiration at depth. Different species have different basal metabolic rates and therefore different oxygen demands. Life stages of organisms also have different metabolic demands. In general, younger stages tend to grow in size and advance in developmental complexity quickly. As the organism reaches maturity, metabolic demands switch from growth and development to maintenance, which requires far fewer resources. Smaller organisms have higher metabolisms per unit of mass, so smaller organisms will require more oxygen per unit mass, while larger organisms generally require more total oxygen. Higher activity levels also require more oxygen. This is why bioavailability is important in deoxygenated systems: an oxygen quantity which is dangerously low for one species might be more than enough for another species.

Several indices to measure bioavailability have been suggested: Respiration Index, Oxygen Supply Index, and the Metabolic Index. The Respiration Index describes oxygen availability based on the free energy available in the reactants and products of the stoichiometric equation for respiration. The Oxygen Supply Index accounts for oxygen solubility and partial pressure, along with the Q10 of the organism, but does not account for behavioral or physiological changes in organisms to compensate for reduced oxygen availability. Since bioavailability is specific to each organism and temperature, calculation of these thresholds is done experimentally by measuring activity and respiration rates under different temperature and oxygen conditions, or by collecting data from separate studies.

Despite the low oxygen conditions, organisms have evolved to live in and around OMZs. For those organisms, like the vampire squid, special adaptations are needed to either make do with lesser amounts of oxygen or to extract oxygen from the water more efficiently. For example, the giant red mysid (Gnathophausia ingens) continues to live aerobically (using oxygen) in OMZs. They have highly developed gills with large surface area and thin blood to water diffusion distance that enables effective removal of oxygen from the water (up to 90% O2 removal from inhaled water) and an efficient circulatory system with high capacity and high blood concentration of a protein (hemocyanin) that readily binds oxygen. Another strategy used by some classes of bacteria in the oxygen minimum zones is to use nitrate rather than oxygen, thus drawing down the concentrations of this important nutrient. This process is called denitrification. The oxygen minimum zones thus play an important role in regulating the productivity and ecological community structure of the global ocean. For example, giant bacterial mats floating in the oxygen minimum zone off the west coast of South America may play a key role in the region's extremely rich fisheries, as bacterial mats the size of Uruguay have been found there.

Decreased oxygen availability results in decreases in many zooplankton species’ egg production, food intake, respiration, and metabolic rates. Temperature and salinity in areas of decreased oxygen concentrations also affect oxygen availability. Higher temperatures and salinity lower oxygen solubility decrease the partial pressure of oxygen. This decreased partial pressure increases organisms’ respiration rates, causing the oxygen demand of the organism to increase. while others take advantage of their predators’ low tolerance for hypoxia and use these areas as a refuge. the habitable ranges for phytoplankton, zooplankton, and nekton increasingly overlap, increasing their susceptibility to predation and human exploitation. To assess these changes, scientists utilize climate models and sediment samples to understand changes to dissolved oxygen in OMZs. Many recent studies of OMZs have focused on their fluctuations over time and how they may be currently changing as a result of climate change.

Some research has aimed to understand how OMZs have changed over geological time scales. The factors that change OMZs are the amount of oceanic primary production resulting in increased respiration at greater depths, changes in the oxygen supply due to poor ventilation, and amount of oxygen supplied through thermohaline circulation.