Also known as the shadow zone, the oxygen minimum zone [OMZ] is the lowest zone of oxygen concentration in the ocean's seawater. Depending on local circumstances OMZs happens at depths of approximately 200 to 1,500m [660-4920ft]. OMZs are normally found along the western coast of continents hence found worldwide. Biological and physical processes interplay concurrently. Biological processes lower the oxygen concentration whereas physical processes restrict the mixing of this water with the surrounding waters. This creates a pool of water in which the concentration of oxygen falls to below 2mg/l from the normal range of 4-6m/l.
Physical and Biological Processes
Generally warmer waters concentrate less oxygen than colder waters whereas surface ocean waters concentrate oxygen close to equilibrium with the earth’s atmosphere. Water is normally exposed to a rain of organic matter from above as water moves out of the mixed layer into the thermocline. Oxygen is used up by aerobic bacteria to feed hence lowers its concentration in water. Thus the concentration of oxygen in deep waters depends on its concentration while it was on the surface to subtract the one depleted by sea organisms.
The oxygen concentration is higher in the deep ocean, due to the polar region supply of cold, deep waters rich in oxygen. This is also due to low oxygen consumption by sea organisms which are rarely found in such zones. There is, however, a lower rate of oxygen-rich water supply and elevated rates of oxygen consumption in depths in between. Mixing of the waters of the wind-driven circulations of the subtropical gyre and surface water enable the resupply of oxygen in these waters. Open-ocean oxygen minimum zones distribution is normally controlled by local biological and physical processes and also large-scale ocean circulation.
Life in Oxygen Minimum Zone
Organisms have evolved to live around and in OMZs despite the fact that there is low oxygen concentration. They include the vampire squid (Vampyroteuthis infernalis) and the giant red mysid (Gnathophausia ingens). These organisms have special adaptive features to cope with the lesser amount of oxygen or rather extract oxygen from water more efficiently. For instance, some develop gills with a huge surface area and narrow blood-to-water distance of diffusion. This is effective in elimination of oxygen from water and for an efficient circulatory system with a higher capacity and blood that is highly concentrated of a protein (hemocyanin) that readily pickles oxygen. Some bacteria use denitrification process where they use nitrate in place of oxygen. This pulls down the concentration of nitrate as an important nutrient.
Role of Oxygen Minimum Zones
These zones regulate the productivity and the ecological community structure of the global oxygen. For instance, in the OMZ off the west coast of South America, giant bacterial mats play an important role in the regions immensely rich fisheries as bacterial mats the size of Uruguay are found there. Due to continuing climate change with potential divarifications for ecosystems and people, Earth system models are projecting considerable reductions in oxygen and other physicochemical variables in the ocean.