The minimum zone of Oxygen ( OMZ ), sometimes referred to as shadow zone , is the zone where oxygen saturation in seawater in the oceans is at the lowest point. This zone occurs at depths of about 200 to 1,500 m (660-4,920 ft), depending on local circumstances. OMZ is found worldwide, usually along the western coast of the continent, in areas where physical and biological process interactions simultaneously decrease the concentration of oxygen (biological processes) and limit water from mixing with surrounding waters (physical processes), creating "ponds" "water in which the oxygen concentration falls from the normal range of 4-6 mg/l to below 2 mg/l.
Video Oxygen minimum zone
Physical and biological processes
The seawater surface generally has an oxygen concentration that is close to equilibrium with Earth's atmosphere. In general, cooler waters hold more oxygen than warm water. When the water moves out of the mixed layer into the thermocline, it is exposed to rain of organic matter from above. Aerobic bacteria feed on this organic material; oxygen is used as part of the bacterial metabolism process lowering its concentration in water. Therefore, the concentration of oxygen in the water depends on the amount of oxygen it has on the surface less depletion by deep-sea organisms.
The decreased flow of organic matter decreases sharply with depth, with 80-90% consumed at 1,000 over m (3,300 feet). Deep oceans have higher oxygen because of low oxygen consumption levels compared to the cold and oxygen-rich deep water supply of the polar regions. In the surface layer, oxygen is supplied by the exchange with the atmosphere. The depth between, however, has higher levels of oxygen consumption and lower levels of oxygen-rich water supply. In most oceans, the mixing process allows the supply of oxygen to these waters (ie waters that are part of a wind-driven, subtropical spinning circulation quickly switched to the surface and never get a strong oxygen deficit).
The distribution of open oxygen minimum zones is controlled by large-scale ocean circulation as well as local physical and biological processes. For example, the winds blowing parallel to the shore cause Ekman's transport that flows the nutrients from the deep water. Increased nutrients support phytoplankton flowers, grazing zooplankton, and whole productive food tissues on the surface. This by-product of these flowers and subsequent grazing sinks in the form of particulate and dissolved nutrients (from phytodetritus, dead organisms, pellets, excretions, shells, scales, and other parts). The "rain" of this organic material (see biological pump) feeds the microbial circle and can cause bacterial blooms in the water below the euphotic zone due to nutrient entry. Because oxygen is not produced as a byproduct of photosynthesis under the euphotic zone, these microbes use oxygen present in the water when they break down fallen organic matter thus creating a lower oxygen state.
The physical process then limits the mixing and isolates this low oxygen water from the outside water. The vertical mixing is limited by the separation of the mix layer by depth. Horizontal mixing is limited by bathymetry and boundaries formed by interactions with sub-tropical gyre and other mainstream systems. Low oxygen water can be spread (by advection) from below the high productivity area to the physical boundaries to create stagnant waterlogging without direct connection to the sea surface although (as in the North Eastern Tropical North) there may be relatively small organic matter falling from the surface.
Maps Oxygen minimum zone
Life in OMZ
Despite the low oxygen conditions, organisms have evolved to live in and around OMZ. For such organisms, such as vampire squid, special adaptations are required to perform oxygen with fewer amounts or to extract oxygen from water more efficiently. For example, the giant red mysid ( Gnathophausia ingens ) continues to live aerobically (using oxygen) in OMZ. They have highly developed gills with large surface area and distances from blood to water which permit effective removal of oxygen from water (up to 90% O 2 removal from water inhaled) and efficient circulation. systems with high capacity and high protein concentration (hemosanin) ready to bind oxygen.
Another strategy used by some bacterial classes in the minimum oxygen zone is to use nitrates rather than oxygen, thereby reducing the concentration of these essential nutrients. This process is called denitrification. Minimum oxygen zones play an important role in regulating the productivity and structure of global marine ecological communities. For example, a giant bacterial mat that floats in the oxygen minimum zone off South America's west coast might play a key role in the region's very rich fishery as a Uruguay-sized bacteria mat has been found there. Existing Earth system models project a substantial reduction in oxygen and other physico-chemical variables in the oceans due to ongoing climate change, with potential consequences for ecosystems and humans.
See also
- Dead zone (ecology), a local area that dramatically reduces oxygen levels, often because of human impact.
- Hypoxia (environment) for a number of articles relating to environmental oxygen depletion.
References
Source of the article : Wikipedia
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