Oxygen minimum zone
Encyclopedia
The Oxygen minimum zone 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 200 to 1,000 metres, depending on local circumstances.
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 this 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 1000m. 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 exchange with the atmosphere. Depths in between, however, have higher rates of oxygen consumption and (as discussed below) lower rates of advective supply of oxygen-rich waters.
The distribution of the open-ocean oxygen minimum zones is controlled by the large-scale ocean circulation. Essentially, waters that are part of the wind-driven subtropical gyre circulations are rapidly exchanged with the surface and never acquire a strong oxygen deficit. However, along the equatorial edge of the gyres, one finds a stagnant pool of water which has no direct connection to the ocean surface. As a result these "shadow zones" have very low oxygen concentrations-even though in regions such as the Eastern Tropical North Pacific there may be relatively little organic matter falling from the surface.
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. One 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. The oxygen minimum zones thus play an important role in regulating the productivity and ecological community structure of the global ocean (Deutsch et al., 2007). For example, giant bacterial mats floating in the oxygen minimum zone off the west coast of South America
may may play a key role in the region's extremely rich fisheries as bacterial mats the size of Uruguay
have been found there.
Oxygen saturation
Oxygen saturation or dissolved oxygen is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water.It has particular significance in medicine and...
in seawater in the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
is at its lowest. This zone occurs at depths of about 200 to 1,000 metres, depending on local circumstances.
Surface ocean waters generally have oxygen concentrations close to equilibrium with the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
. In general, colder waters hold more oxygen than warmer waters. As this water moves out of the mixed layer
Mixed layer
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, cooling, or processes such as evaporation or sea ice formation which result in an increase in salinity...
into the thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
it is exposed to a rain of organic matter from above. Aerobic
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
feed on this organic matter; oxygen is used as part of the bacterial metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
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 1000m. 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 exchange with the atmosphere. Depths in between, however, have higher rates of oxygen consumption and (as discussed below) lower rates of advective supply of oxygen-rich waters.
The distribution of the open-ocean oxygen minimum zones is controlled by the large-scale ocean circulation. Essentially, waters that are part of the wind-driven subtropical gyre circulations are rapidly exchanged with the surface and never acquire a strong oxygen deficit. However, along the equatorial edge of the gyres, one finds a stagnant pool of water which has no direct connection to the ocean surface. As a result these "shadow zones" have very low oxygen concentrations-even though in regions such as the Eastern Tropical North Pacific there may be relatively little organic matter falling from the surface.
For those organisms, like the Vampire Squid
Vampire Squid
The vampire squid is a small, deep-sea cephalopod found throughout the temperate and tropical oceans of the world. Unique retractile sensory filaments justify the Vampire Squid's placement in its own order: Vampyromorphida , which shares similarities with both squid and octopuses...
, 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. One 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. The oxygen minimum zones thus play an important role in regulating the productivity and ecological community structure of the global ocean (Deutsch et al., 2007). For example, giant bacterial mats floating in the oxygen minimum zone off the west coast of South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
may may play a key role in the region's extremely rich fisheries as bacterial mats the size of Uruguay
Uruguay
Uruguay ,officially the Oriental Republic of Uruguay,sometimes the Eastern Republic of Uruguay; ) is a country in the southeastern part of South America. It is home to some 3.5 million people, of whom 1.8 million live in the capital Montevideo and its metropolitan area...
have been found there.
See also
- Hypoxia (environmental)Hypoxia (environmental)Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
for a number of articles related to environmental oxygen depletion