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The Oxygen Minimum Zone (OMZ) is a naturally occurring ocean layer where dissolved oxygen concentrations in seawater are at a minimum relative to the water above and below. OMZs occupy 8% (30.4 ± 3·106 km2) of the ocean's total area and are located primarily in the Pacific and Indian Oceans.[1] At the center of the OMZ is the core, which has an oxygen concentration of less than 20 μM kg-1 (micromole per kilogram).[1] At the top of the OMZ is the oxycline, where oxygen concentrations decrease by about 1.6 μM m-1 and the oxygen concentration is around 65 μM kg-1, while at the bottom of the OMZ oxygen concentration is around 100 μM kg-1 with a more gradual increase of about 0.4 μM m-1.[1] Hypoxia and anoxia are terms commonly used to describe conditions within OMZs and refer to a general state of oxygen concentration in the water, rather than a specific area. All OMZs are hypoxic, as the generally accepted threshold for hypoxia is 62.5 µM kg-1, but others can experience periodic anoxia when concentrations reach 0 µM kg-1. A combination of oxygen consumption through respiration, weak oxygen exchange with surface waters, and low deep water oxygen concentration leads to OMZ formation and they are permanent ocean features. Because of this, they are primarily found along the western margin of continents in eastern boundary current systems, which are characterized by these processes. OMZs play an important role in several biogeochemical cycles, particularly the nitrogen, sulfur, and carbon cycles, and their unique chemistry leads to the presence of anaerobic bacteria. OMZs are also home to other organisms that have adapted to low oxygen conditions, primarily small invertebrates and bottom-dwelling fish species, but many organisms cannot tolerate the low oxygen concentrations found in OMZs. As OMZs expand, animals that are not able to survive in hypoxic conditions are driven out of a habitat that is no longer suitable.

Overview

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Formation

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Surface ocean waters generally have oxygen concentrations close to equilibrium with the atmosphere's oxygen concentration.[2] The atmosphere affects ocean surface waters through processes like wind-driven mixing, gas exchange and heat exchange; it creates the ocean mixed layer, which transports dissolved oxygen throughout surface waters. Dissolved oxygen is present in high concentrations in the mixed layer due to atmospheric contact. As water moves out of the mixed layer and below the thermocline, aerobic bacteria feed on organic matter that sinks from the surface layer and consume oxygen as part of their metabolic processes, which leads to lower oxygen concentrations in mid-depth waters (100 to 1,500 meters deep). The transport of water and dissolved oxygen from the mixed layer to the ocean interior is known as ventilation.[3] Ventilation can occur via seasonal mixing with the surface layer or via horizontal movement of oxygen from regions of high concentration to low concentration by ocean currents. In much of the ocean, ventilation enables the resupply of oxygen to mid-depth waters. For example, mid-depth water that is part of a subtropical gyre circulation experiences rapid exchange with oxygenated surface waters and never acquires a large oxygen deficit. But if ventilation weakens, then the oxygen consumed by biological processes in mid-depth waters cannot be replenished, resulting in an OMZ.

Oxygen in deep waters

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In general, the concentration of dissolved oxygen in the interior ocean depends on three factors: (1) the amount of oxygen it had when it was last at the surface, (2) the consumption of oxygen due to respiration in the mid-water depths (the mesopelagic region), and (3) the presence of a resupply mechanism.[4] In the surface ocean, oxygen is supplied by exchange with the atmosphere, while in the deep ocean (below 2,000 meters) there is a higher oxygen concentration than at mid-water depths due to the subduction of well-oxygenated waters in high latitude regions in the ocean's thermohaline circulation[5] and low amounts of respiration. The amount of oxygen consumption in deep waters is low due to the limited availability of organic matter; 80–90% of the available organic matter is consumed in the upper 1,000 meters.[2] At mid-depth waters, however, the high amount of respiration must be balanced by ventilation to prevent dissolved oxygen from being depleted. It is at poorly ventilated mid-water depths that OMZs are commonly found.

Coastal upwelling contributes to the formation of coastal OMZs.

Effects of upwelling

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The distribution of OMZs is controlled by large-scale ocean circulation as well as local physical and biological processes. For example, in eastern boundary current systems, wind blowing parallel to the coast causes Ekman transport, a physical mechanism that upwells nutrients and deep water with low oxygen concentrations to the surface. The increased supply of nutrients supports phytoplankton blooms, zooplankton grazing, and an overall productive food web at the surface. The byproducts of these blooms and of the subsequent grazing sink in the form of particulate nutrients (from phytodetritus, dead organisms, fecal pellets, excretions, shed shells, scales, and other parts). The sinking byproducts form a "rain" of organic matter (see the biological pump, marine snow) that feeds the microbial loop and may lead to bacterial blooms in water below the euphotic zone.[6] 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 lower oxygen conditions.[7]

Lack of ventilation

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OMZ cores are found at mid-water depths (100 to 1,500 meters) with poor ventilation. A lack of ventilation in these zones means that the transport of dissolved oxygen from the mixed layer to mid-water depths is limited.[8] The factors limiting the ventilation of waters within an OMZ include bathymetric boundaries, stratification of surface waters, and local circulation. The physical processes associated with these factors constrain mixing at mid-water depths: bathymetric boundaries limit horizontal mixing, stratification limits vertical mixing and shifts in local circulation can temporarily limit mixing in any direction. Constrained mixing isolates mid-water ocean layers from the more oxygenated surface waters in constant contact with the atmosphere, and from the more oxygenated deeper waters that have accrued less respiratory O2 demand from the reduced organic matter supply at depth. Vertical mixing in most OMZs is primarily constrained due to the separation from the mixed layer by depth. Horizontal mixing in the OMZ depth range is primarily constrained by boundaries formed by interactions with sub-tropical gyres and other major current systems.[9][10][11] Advection spreads low oxygen waters from under areas of high productivity to surrounding areas, creating a stagnant pool of water with no direct connection to the ocean surface even though there may be relatively little organic matter sinking from the surface. This type of OMZ formation has been noted in the eastern tropical North Pacific in particular.

Observing OMZs

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Water samples are taken at different depths throughout the water column, and oxygen concentrations along with chemical and physical properties of the water are measured in order to determine OMZ size, nutrient composition, and chemistry. Instruments used include remotely operated vehicles (ROVs) and CTDs. CTD casts obtain measurements of salinity, temperature, dissolved oxygen, pH, transmissivity (a measure of suspended particles), and chlorophyll concentration.[5]

Distribution

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Open-ocean OMZs

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OMZs are often found away from large-scale, ocean circulation patterns that would otherwise induce effective ventilation of the interior ocean and are in areas that introduce low oxygen water through upwelling.[12] OMZs are found along the eastern edges of subtropical gyres and on the flanks of the equator in the Pacific Ocean and the northern Indian Ocean. There is an area in the eastern South Atlantic Ocean along the coast of Africa that is often discussed as an OMZs, but does not reach an oxygen concentration of 20 μM kg-1.

Pacific Ocean

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The eastern Pacific Ocean has three distinct OMZs which have been divided by areas of relatively higher dissolved oxygen concentrations. The three OMZs are the eastern subtropical North Pacific (ESTNP), eastern tropical North Pacific (ETNP), and eastern tropical South Pacific (ETSP) zones.[1]  

Spatial distribution of Oxygen Minimum Zones (OMZs) in the global open-oceans at 400 and 800 m depths. Station data at the center of each OMZ is also included on the right with their respective abbreviated names. Note: 0.5 ml L-1 is approximately 20 μM kg-1, but depends on potential density for an exact conversion. Arabian Sea OMZ = AS and Bay of Bengal OMZ = BB. Other abbreviations are defined in the accompanying text.

The ESTNP OMZ exists between about 25 to 50°N and 75 to 180°W and is centered along 40°N, with a core oxygen concentration of 17 ± 1 μM kg-1.[1][13] The top of the OMZ is found at about 850 m and its core extends 230 ± 130 m down to 1080 m.[1] The ESTNP OMZ is found off the coast of California extending north toward British Columbia where the continental shelf causes shoaling of the OMZ.[13] Due to the ESTNP's location in the North Pacific Ocean, it is ventilated by some of the oldest water in the ocean, depleted of oxygen along the Thermohaline Circulation, from great depths.[8]

The ETNP OMZ exists between about 0 to 25°N and 75 to 180°W and the minimum oxygen concentration is centered around 12°N and is limited to about 160°W in its western extent.[13][1] At the core, its oxygen concentrations are typically between 13 to 14 ± 1 μM kg-1.[1] The ETNP OMZ encompasses depths between 320 to 740 m with a mean core thickness of 420 ± 290 m.[1] Additionally, the ETNP OMZ is often considered to be the second thickest, at nearly 1000 m thick in some portions of the OMZ core.[13][14] The ETNP OMZ exists in the far eastern edge of the North Pacific along the flanks of the equatorial region off the coast of Central America. It is isolated from the subtropical gyre and the California Current that drives the ESTNP.[15] The equatorial region acts as a strong divider between the ETNP and ETSP OMZs. The Equatorial Undercurrent ventilates the upper few hundred meters of the interior ocean along the equator.[12][15] The current increases the local oxygen concentration and forms two distinct areas, the ETNP and ETSP OMZs, with an area of thin, OMZ water dividing them.

The ETSP OMZ is found between 0-37°S and 75-120°W and has two distinct regions. These are defined as the Peruvian section of the ETSP and the Chilean section, both found along the continental shelf of South America.[1] Typical core oxygen concentrations in the Peruvian portion are 13 ± 2 μM kg-1 and 15 ± 1 μM kg-1 in the Chilean portion.[1] This OMZ is located at depths between 280 to 470 m and is 190 ± 170 m thick at its core; but, along Peru it generally thicker, closer to 400 m, and both Peru and Chile OMZ sections exist at shallower depths than the overall ETSP OMZ that extend west into the pelagic zone.[1][8][15] The Humboldt Current is a driver of coastal upwelling along the eastern margins of the ETSP OMZ.

Indian Ocean

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The northern Indian Ocean includes two distinct OMZs located in the Bay of Bengal and the Arabian Sea. In the Arabian Sea, the OMZ extends from 7 to 23°N and 55 to 77°E with a core oxygen concentration of 13 ± 4 μM kg-1 while the Bay of Bengal OMZ is bounded by 8 to 20°N and 80 to 100°E and has a core oxygen concentration of 16 ± 2 μM kg-1.[1] The Arabian Sea has the thickest Indian Ocean OMZ with a thickness of 760 ± 340 m at depths of 240 to 1000 m while the Bay of Bengal OMZ's core extends vertically from 180 to 490 m and is 310 ± 160 m thick, approaching 500 m thick in some places.[1][13]

The geography of the these OMZs is different from the rest of the open-ocean OMZs since land surrounds them on three sides. They are separated from each other by the Indian subcontinent and are highly productive. The Arabian Sea has an extensive OMZ due to a combination of poor ventilation from the north, little deep ocean circulation from the south, and high levels of respiration due to upwelling-generated productivity. Input of moderately oxygenated water from the Persian Gulf and Red Sea allows for some weak ventilation.[16][17] Restricted exchange of water between hemispheres also contributes to OMZ formation, as the Arabian Sea is a relatively enclosed body of water.[16][17][18] Therefore, minimum oxygen concentrations generally occur in the northern portion of the Arabian Sea, which is furthest from the sources of water with higher oxygen content. The Bay of Bengal OMZ experiences a but nutrients enter this system through river input.[19]

Atlantic Ocean

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Some literature refers to a tropical North Atlantic OMZ, but it is defined by a higher core oxygen concentration threshold (45 μM kg-1) than that which is commonly used for other open-ocean OMZs (< 20 μM kg-1).[4][12] This region does not reach lower oxygen concentrations due to its proximity to deep water formation in the North Atlantic near southern Greenland, which helps resupply oxygen to the mid-depth ocean layers.[4] This area of low oxygen concentration occurs between 200 to 800 m depth and is located in the Benguela Current.[12]

Other low oxygen regions

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There are locations, such as the Black Sea, Scandinavian and British Columbia fjords, and other isolated water bodies, that experience hypoxic and anoxic conditions but are not generally considered to be OMZs since they are not consistently supplied by ocean water sources.[20] The Black Sea frequently experiences anoxic water conditions between 50 to 125 m depth but it is a basin that does not have large inputs from other bodies of water, making it isolated.[21] Fjords along the western coast of British Columbia, surrounding the Baltic Sea, and on Norwegian coastlines can experience low oxygen minima during certain seasons. Some fjords have a physical barrier in the form of a sill that prevents mixing with other water sources.[22] This physical barrier in conjunction with high productivity at the fjord's surface can lead to oxygen depletion in water below the level of the sill. The deep water below the sill is isolated from other water sources and leads to depleted oxygen in conjunction with phytoplankton blooms that deliver organic matter into the oxygen minima.[23]

Biogeochemical cycles and processes

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Biological and chemical processes within the Earth's oceans contribute to the creation and persistence of OMZs. Organic matter, also known as marine snow, rains down from the surface ocean above OMZs and bacteria respire the matter for energy. This process consumes oxygen, and if oxygen is not replenished then this mid-ocean layer transitions from a moderate concentration of oxygen to hypoxia, thus contributing to the creation of an OMZ. Additionally, anaerobic bacteria will dominate once oxygen concentrations are low, replacing aerobic bacteria that require higher oxygen concentrations to survive. These anaerobic bacteria use specific types of redox reactions that do not require oxygen in order to perform respiration.

Nitrogen cycle

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Nitrogen is an important element for life in the ocean and can limit growth. [24][25] Globally, there is a net loss of fixed inorganic nitrogen estimated to be 195 Tg N yr-1[26] (generally in the form of nitrogen gas, or N2), and 30-50% of that amount can be traced to OMZs.[26] It was originally thought that heterotrophic denitrification was the sole sink of nitrogen that was causing this loss, until a new process called annamox, or anaerobic ammonium oxidation, was discovered to occur in the Black Sea and OMZs.[27][28]

File:Nitrogen cycle OMZ.jpg
Schematic of the nitrogen cycle in Eastern Boundary Currents OMZs and open ocean OMZs

In the areas around OMZs, nitrification occurs, a chemoautotrophic process that converts ammonia, a plankton waste product, to nitrate.[29] These bacteria are considered nitrifiers, and the nitrate produced is then used for denitrification in the OMZ.[29] Heterotrophic denitrification is the dominant redox reaction used by bacteria in OMZs, using electron acceptors containing nitrogen atoms to oxidize organic matter and obtain energy.[28] This process is comprised of a series of half-reactions that reduce nitrate (NO3-) to nitrogen gas (N2).[1][30] The first reaction reduces nitrate to nitrite, and the produced nitrite is then reduced to nitric oxide (NO). This nitric oxide is then reduced to nitrous oxide (N2O), some nitrous oxide is released to the atmosphere, and the rest is reduced to nitrogen gas. The bacteria that perform these reactions are considered denitrifiers and are mainly heterotrophic, though there are some autotrophic denitrifying bacteria that have been discovered.[31][32] Multiple species of bacteria are involved in this process, and there are different enzymes used to complete each half-reaction.[33]

Another process that occurs in certain OMZs is annamox, or anaerobic ammonium oxidation, which is used by chemoautotrophic bacteria.[27] Anammox requires anaerobic conditions and has been found to occur in both low oxygen sediment and in the water column where anammox bacteria oxidize ammonium with nitrite to produce energy and nitrogen gas.[28][27] In particular, anammox is known to play a role in the ETSP and Atlantic Ocean OMZ nitrogen cycles.[34] New research suggests most of the nitrogen loss in OMZs is due to annamox, not denitrification as previously thought.[27][28]

Sulfur cycle

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Thioploca bacteria from a mat collected in the Humboldt Current.

In areas of OMZs with no oxygen, anaerobic bacteria are present that reduce sulfate (SO42-), obtained from the seafloor, to produce hydrogen sulfide (H2S) and elemental sulfur.[35] It is hypothesized that these bacteria complete heterotrophic denitrification and sulfate oxidation in tandem. Hydrogen sulfide is toxic to most life forms but can be used by certain chemolithotrophic filamentous bacteria that oxidize hydrogen sulfide by using nitrate as an electron receptor instead of oxygen.[36] These bacteria include the large proteobacteria Thioploca, Beggiatoa, and Thiomargarita, which form mats on the seafloor and are thought to help prevent the accumulation of hydrogen sulfide.[37][36] This combination of sulfate oxidizers and hydrogen sulfide reducers has been detected in the Humboldt Current off the coast of Peru and off the coast of Namibia in the Benguela Current, both of which are eastern boundary current systems.[35][38] In these areas, hydrogen sulfide can still accumulate and has been known to kill species found in the water column above the bacteria, which live on the seafloor.[37]

Biology

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Thresholds

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The abundance and diverstiy of life in low oxygen is determined by physiological tolerance of organisms. Throughout the ocean in general, a response by benthic marine life will occur at about an 89 μM kg-1 oxygen concentration, at which point organisms will die or migrate from the area.[37] In persistent OMZs this benthic biological response will often not occur until concentrations are less than 22 μM kg-1, slightly above the dissolved oxygen concentration at the core of many OMZs.[37][4] These operational thresholds, however, do not necessarily hold true for species adapted to low oxygen which is the reason most OMZs are not devoid of life. Certain pelagic crustaceans have developed very efficient methods for removing and using scarce oxygen in hypoxic water, while some cephalopods, notably the vampire squid, have evolved to have slow metabolic rates and hemoglobin with a high oxygen affinity.[39][40][41] High gill surface area and special enzymes are seen in some California Current benthic fish species.[42]

Habitat compression

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Expansion of OMZs can cause habitat compression for benthic, demersal, and pelagic species by decreasing the volume of oxygen rich water, which lowers usable space in the ocean. One process that can lead to compression is shoaling, which is the natural seasonal horizontal and vertical movement of low oxygen up to the continental shelf during upwelling.[43] Whether habitat compression due to shoaling occurs depends on each species' tolerance to low oxygen concentrations. Because eastern boundary current continental slope benthic and demersal species often live in hypoxic conditions, many have adaptations to deal with oxygen levels below 22 μ kg-1 and therefore do not easily experience habitat compression due to hypoxia. However, species that live in shallow water on the continental shelf are often not evolved to deal with hypoxia, which they may be exposed to during seasonal shoaling events. Therefore, depending on their physiological threshold for low oxygen, during seasonal shoaling these shelf species must either migrate from the area or perish. In the the California Current, this seasonal expansion of the OMZ typically happens from late spring to early fall, with the southern portion of the current experiencing shoaling earlier than the northern portion, while in the Benguela Current shoaling happens from September to March.[44][45] These seasonal events have become more intense in some regions and this is thought to be caused by increased wind speeds and changes in ocean circulation.[46][47] Some of these strong upwelling events have caused relatively sudden deaths in some populations of demersal and benthic marine species, notably in the 2002 and 2006 events off the Oregon Coast where extreme hypoxic (2002) and anoxic (2006) conditions resulting from upwelling killed large numbers of Dungeness crab (Metacarcinus magister), other invertebrates, and rockfish species (Sebastes spp.).[48][49]

OMZ expansion can happen through mechanisms other than upwelling. As the ocean warms, less oxygen dissolves into the ocean from the atmosphere and there is more stratification in surface waters, leading to less transport of oxygen into deeper waters and expansion of the OMZ toward the surface waters.[46] Circulation changes can alter the way oxygen is transported throughout ocean basins, possibly through increases in strength of low-oxygen currents and decreased oxygen concentrations in the deep ocean.[46] These mechanisms, along with upwelling, are thought to play a role in open-ocean OMZ expansion.[46] As open-ocean OMZs increase in area, some predator species that prefer mesopelagic depths will be forced to occupy shallower surface habitat, which is often also the habitat of their prey. This type of compression of suitable habitat has been seen with large tropical pelagic fish populations, who have begun to more frequently overlap habitat with their prey during tropical open-ocean Atlantic, ETNP, and ETSP OMZ expansion.[50] Habitat compression for these species is exacerbated by temperature, as the hypoxic water in these regions is colder than the temperatures preferred by billfish and tuna, but also benefits these species as they have more opportunities to hunt.[50] As a result of this overlap in the warm oxygen-rich surface water, community composition is altered because new species co-occur when they previously occupied separate niches. Water column expansion can also affect the seafloor. As oxygen poor waters reach the seafloor, benthic and demersal organisms must move if they cannot tolerate low dissolved oxygen, and high mortality is often seen in populations of less mobile species that cannot escape the hypoxic water.[37] With expansion of OMZs to the seafloor, predator-prey overlap has sometimes been observed to decline which may result in movement due to food scarcity.[51] This often occurs when a predator species has a physiological tolerance for low oxygen and its prey species do not.[51]

Plankton

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Plankton play a role in OMZ formation and their distribution within the water column is influenced by oxygen concentrations. Phytoplankton are found above the upper margin of OMZs as they require light for photosynthesis and produce oxygen as a byproduct.[46] Zooplankton are heterotrophic, so they do not require light to produce energy, and their abundance is typically low in low oxygen concentrations.[39]

Bolivina dilatata is a species of foraminifera that can live in low oxygen environments.

Phytoplankton are important in formation of OMZs, particularly coastal OMZs during upwelling events, and contribute to the high levels of productivity seen in eastern boundary current systems.[31] During upwelling large amounts of nutrients are brought up to the surface waters where they can be used by phytoplankton, sometimes resulting in blooms of these organisms.[31][46] When these phytoplankton die, they contribute to low oxygen concentrations in deeper waters by sinking as organic matter which leads to increased respiration from microbes that use these phytoplankton as an energy source.[31] Respiration by these microbes uses dissolved oxygen, and in combination with reduced ventilation, deoxygenation occurs.[31][12] With increased nutrients, phytoplankton populations can increase in size, leading to more organic matter production and expansion of OMZs from the resulting increase in respiration.[46]

Zooplankton are influenced by OMZs, with the majority of species unable to survive at oxygen concentrations below 7 μM kg-1 oxygen concentrations.[39] In some OMZs there are peaks in zooplankton abundance at the margins.[8][39] Within the Humboldt Current, several strategies have been adopted by zooplankton to deal with low oxygen conditions. Some species stay completely out of the OMZ, like the invertebrate larvae that live above it, while others migrate daily or throughout their early life stages, as seen in pelagic copepod species.[52] These copepod species can be found at oxygen concentrations as low as 6 μM kg-1 while the younger stages live in higher concentrations.[53][52] Jellyfish are also inhabitants of OMZs, as many species can tolerate hypoxia, and this allows them to prey on fish weakened by the low oxygen conditions.[54] It is thought that the expansion of OMZs will contribute to increases in jellyfish blooms.[55]

Benthic invertebrates

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Abundant benthic echinoderms: brittle stars and sea urchins are commonly seen on the sea floor at the margin of some OMZs.

There are numerous benthic invertebrates found within OMZs, and smaller invertebrates are typically more common than larger organisms. The diversity of OMZ organisms is lower than other more oxygenated parts of the ocean and assemblages are often dominated by a particular species or group of organisms.[37] Invertebrates in the OMZ benthos can be grouped into meiofauna, macrofauna, and megafauna.

Some of the most prevalent benthic meiofauna found in OMZs are foraminfera and nematodes. Both can be found in high densities within OMZs and nematodes can comprise 95% of OMZ organisms.[53][8] There is also an OMZ endemic nematode species.[8] The dominant foraminifera are calcareous with thin shells, though small numbers of other types can be found as well, and they are thought to live in OMZs in order to take advantage of abundant organic matter.[37][53] In the Pacific, the foraminifera species Virgulinella fragilis is known to tolerate both hypoxia and hydrogen sulfide, and is thought to be an OMZ specialist.[37] Many meiofauna have developed unique physiological strategies for living in OMZs. Some can more efficiently consume oxygen through use of their many spines or filamentous body parts, while others have evolved thin bodies to decrease diffusion distance.[8] Although foraminifera and nematodes dominate the meiofauna, there are other classes of organisms present in smaller numbers. These include some small polychaetes, gastrotrichs, and certain benthic copepods.[8]

There are some benthic macrofaunal invertebrate species that can survive the low oxygen conditions present in OMZs, but they are in lower abundance and densities than the meiofauna. Typically, macrofauna abundance and species richness are low at the core of the OMZ, and their highest densities are at OMZ margins, but this depends on organic matter presence.[56][8] Within the OMZ, polychaete worms are often the most dominant, along with sipunculan, oligochate, and priapulid worms.[37] Other small benthic invertebrates are also found in smaller numbers. Crustaceans include the giant red mysid (Gnathophausia ingens), which has a particularly efficient form of the respiratory pigment hemocyanin in its blood, as well as some Peruvian and Omani amphipods.[39][56][57] Mollusks are a relatively underrepresented macrofauna in OMZs, but some are able to survive by increasing their localized dissolved oxygen concentration through rapid appendage movement, extension of the body into more oxygenated parts of the water column, or high blood flow.[8]

Invertebrate benthic megafauna are found in similar numbers and diversity to the macrofauna in OMZs, and these larger invertebrates are often not present or are extremely uncommon in the OMZ core.[37] In certain OMZs, benthic echinoderms, sponges, and crustaceans will appear in large numbers at the margins, which is likely the threshold for low oxygen tolerance in these organisms.[53][8] Echinoderms and crustaceans are often the most dominant in these aggregations, with brittle stars, sea urchins, shrimp, and crabs appearing at the edge of many OMZs worldwide.[58][37] In lower abundance are sea cucumbers, snails, and tunicates, which are also occasionally found in the higher oxygen OMZ margins.[58]

Fisheries

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Dissolved oxygen is removed from the water through the gills. This oxygen then enters the bloodstream.

Low dissolved oxygen limits habitat for many fish species, so catches for recreational and commercial fisheries can be low in these areas.[59][60] Within OMZs, it has been found that catch for species like benthic greenstriped rockfish (Sebastes elongatus) and Dover sole will be higher than that of other species, such as the demersal spotted ratfish (Hydrolagus colliei) and benthic Petrale sole, likely due to differences in physiology.[61] Pelagic species, such as billfish and tuna, require higher oxygen levels so their habitat is limited when OMZs move up the water column.[50] Expansion of OMZs has the potential to reduce billfish populations, specifically in the Atlantic where suitable habitat in the upper layers of the ocean has been documented to be shrinking.[62] In the California Current, rockfish (Sebastes spp.) are an abundant group of demersal and benthic fishes but have differing responses to low oxygen concentration. It has been found that a number of these species are regularly experiencing dissolved oxygen levels below their survival threshold, leading to habitat loss, and if OMZs in this region continue to expand more rockfish species will likely be displaced.[63] Additionally, while some fish populations decline or relocate following OMZ expansion, others that are better adapted to low oxygen can become more prevalent. In the case of pelagic species in the Humboldt Current, this has meant more anchovy presence than sardines during shoaling events, as well as the use of the OMZ as a refuge for Chilean jack mackerel (Trachurus murphyi) prey.[64][65] In sulfic areas of the Humboldt Current, the giant sulfur-oxidizing bacterial mats off the western coast of South America are thought to be connected to the region's productive hake fishery, indicating that OMZs do not always lead to low catch.[66]

Marine mammals

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Although marine mammals do not breathe dissolved oxygen in seawater, much of their prey does so marine mammals are indirectly impacted by OMZs. Some marine mammals may have restricted prey access as OMZs expand, particularly in the case of those that feed in the surface layer.[62] Others already prey on organisms that occupy OMZs and it is thought that elephant seals sometimes specifically hunt fish in OMZs because these prey targets are larger and are resting, thereby maximizing the energy spent diving to forage.[67] One of the prey species for sperm whales is the jumbo squid, which is known to spend time either recovering from daytime surface activity or preying on mesopelagic fish within OMZs.[68] In the Gulf of Mexico sperm whales will dive into the OMZ at night to target jumbo squid that are more vulnerable due to the low oxygen concentrations.[68]

Changes

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OMZs have changed over time due to effects from numerous global chemical and biological processes.[69] To assess these changes, scientists utilize climate models and sediment samples to understand changes to dissolved oxygen in OMZs.[55] 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.[55][70]

Some research has aimed to understand how OMZs have changed over geological time scales.[70] Throughout the history of Earth’s oceans, OMZs have fluctuated on long time scales, becoming larger or smaller depending on multiple variables.[71] 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.[71] From recent observations, it is evident that the extent of OMZs has expanded in tropical oceans during the past half century.[72][20] Vertical expansion of tropical OMZs has reduced the area between the OMZ and surface where oxygen is used by many organisms.[55] Currently, research aims to better understand how OMZ expansion affects food webs in these areas.[55] Studies on OMZ expansion in the tropical Pacific and Atlantic have observed negative effects on fish populations and commercial fisheries that likely occurred from reduced habitat when OMZs shoal.[72][73]

Other research has attempted to model potential changes to OMZs as a result of rising global temperatures and human impact. This is challenging due to the many factors that could contribute to changes in OMZs.[74] The factors used for modeling change in OMZs are numerous, and in some cases hard to measure or quantify.[72] Some of the processes being studied are changes in oxygen gas solubility as a result of rising ocean temperatures, as well as changes in the amount of respiration and photosynthesis occurring around OMZs.[55] Many studies have concluded that OMZs are expanding in multiple locations, but fluctuations of modern OMZs are still not fully understood.[20][72][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.[75]

See also

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References

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  2. ^ a b WOR 1 Living with the oceans. A report on the state of the world’s oceans | 2010, Oxygen Chemistry 2. "Oxygen « World Ocean Review". Oxygen in the ocean. Retrieved 2019-11-20.{{cite web}}: CS1 maint: numeric names: authors list (link) CS1 maint: url-status (link)
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