Editing Oxygen minimum zone (section)

== Physical and biological processes ==
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 [[metabolism|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.

[[File:WOA09 180E AOU AYool.png|thumb|400px|Annual mean dissolved oxygen (upper panel) and [[apparent oxygen utilisation]] (lower panel) from the [[World Ocean Atlas]].<ref>{{cite web|url=http://www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html |year=2009 |title=World Ocean Atlas 2009 |publisher=[[National Oceanic and Atmospheric Administration]] |access-date=5 December 2012}}</ref> The data plotted show a section running north–south at the [[180th meridian]] (approximately the centre of the Pacific Ocean).  White regions indicate section [[bathymetry]]. In the upper panel a minimum in oxygen content is indicated by light blue shading between [[Equator|0° (equator)]] and [[60th parallel north|60°N]] at an average depth of ca. {{convert|1000|m|ft|abbr=on}}.]]

The downward flux of organic matter decreases sharply with depth, with 80–90% being consumed in the top {{convert|1000|m|ft|abbr=on}}.  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 organic matter|particulate]] and [[Dissolved organic carbon|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 [[Photic zone|euphotic zone]] due to the influx of nutrients.<ref>{{Cite book|title=Dynamics of Marine Ecosystems: Biological-Physical interactions in the oceans.|last1=Mann|first1=K.H.|last2=Lazier|first2=J.R.N.|publisher=Blackwell Scientific Publications|year=1991|isbn=978-1-4051-1118-8}}</ref> 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.<ref name=":0" />

Physical processes then constrain the mixing and isolate this low oxygen water from outside water. Vertical mixing is constrained due to the separation from the mixed layer by depth. Horizontal mixing is constrained by bathymetry and boundaries formed by interactions with sub-tropical gyres and other major current systems.<ref>{{Cite journal|last1=Gnanadesikan|first1=A.|last2=Bianchi|first2=D.|last3=Pradal|first3=M.A.|year=2013|title=Critical role for mesoscale eddy diffusion in supplying oxygen to hypoxic ocean waters|journal=Geophysical Research Letters|volume=40|issue=19|pages=5194–5198|doi=10.1002/grl.50998|bibcode=2013GeoRL..40.5194G|s2cid=3426474 |doi-access=free}}</ref><ref>{{Cite journal|last1=Luyten|first1=J|last2=Pedlosky|first2=J|last3=Stommel|first3=H|year=1983|title=The ventilated thermocline|journal=J Phys Oceanogr|volume=13|issue=2|pages=292–309|doi=10.1175/1520-0485(1983)013<0292:tvt>2.0.co;2|bibcode=1983JPO....13..292L|doi-access=free}}</ref><ref>{{Cite journal|last=Pedlosky|first=J.|year=1990|title=The dynamics of the oceanic subtropical gyres|journal=Science|volume=248|issue=4953|pages=316–322|doi=10.1126/science.248.4953.316|pmid=17784484|bibcode=1990Sci...248..316P|s2cid=37589358}}</ref> 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.

=== Microbes ===
In OMZs oxygen concentration drops to levels <10 nM at the base of the oxycline and can remain anoxic for over 700 m depth.<ref name="Bertagnoli2018">{{cite journal |last1=Bertagnolli |first1=AD |last2=Stewart |first2=FJ |date=2018 |title=Microbial niches in marine oxygen minimum zones |journal=Nature Reviews Microbiology |volume=16 |issue=12 |pages=723–729 |doi=10.1038/s41579-018-0087-z |pmid=30250271 |s2cid=52811177}}</ref> This lack of oxygen can be reinforced or increased due to physical processes changing oxygen supply such as eddy-driven advection,<ref name="Bertagnoli2018" /> sluggish ventilation,<ref name="LK2011">{{cite journal |last1=Lam |first1=P |last2=Kuypers |first2=MM |date=2011 |title=Microbial nitrogen cycling processes in oxygen minimum zones |journal=Annual Review of Marine Science |volume=3 |pages=317–345 |bibcode=2011ARMS....3..317L |doi=10.1146/annurev-marine-120709-142814 |pmid=21329208 |hdl-access=free |hdl=21.11116/0000-0001-CA25-2}}</ref> increases in [[ocean stratification]], and increases in ocean temperature which reduces oxygen solubility.<ref name="Robinson2019">{{cite journal |last1=Robinson |first1=C |date=2019 |title=Microbial respiration, the engine of ocean deoxygenation |journal=Frontiers in Marine Science |volume=5 |page=533 |doi=10.3389/fmars.2018.00533 |s2cid=58010259 |doi-access=free}}</ref>

At a microscopic scale the processes causing ocean deoxygenation rely on microbial aerobic respiration.<ref name="Robinson2019" /> Aerobic respiration is a metabolic process that microorganisms like bacteria or archaea use to obtain energy by degrading organic matter, consuming oxygen, producing CO<sub>2</sub> and obtaining energy in the form of ATP.<ref name="Robinson2019" /> In the ocean surface photosynthetic microorganisms called phytoplankton use solar energy and CO<sub>2</sub> to build organic molecules (organic matter) releasing oxygen in the process.<ref name="SigmanHain2012">{{cite journal |last1=Sigman |first1=DM |last2=Hain |first2=MP |date=2012 |title=The biological productivity of the ocean |journal=Nature Education Knowledge |volume=3 |issue=6 |pages=1–16}}</ref>  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.<ref name="AzamMalfatti2007">{{cite journal |last1=Azam |first1=F |last2=Malfatti |first2=F |date=2007 |title=Microbial structuring of marine ecosystems |journal=Nature Reviews Microbiology |volume=5 |issue=10 |pages=782–791 |doi=10.1038/nrmicro1747 |pmid=17853906 |s2cid=10055219}}</ref> These sinking aggregates are consumed via degradation of organic matter and respiration at depth.<ref name="LK2011" />

At depths in the ocean where no light can reach, aerobic respiration is the dominant process. When the oxygen in a parcel of water is consumed, the oxygen cannot be replaced without the water reaching the surface ocean. When oxygen concentrations drop to below <10 nM, microbial processes that are normally inhibited by oxygen can take place like [[denitrification]] and [[anammox]]. Both processes extract elemental nitrogen from nitrogen compounds and that elemental nitrogen which does not stay in solution escapes as a gas, resulting in a net loss of nitrogen from the ocean.<ref name="LK2011" />

=== Bioavailability of oxygen ===

==== Oxygen demand ====
An organism's demand for oxygen is dependent on its [[metabolic rate]]. Metabolic rates can be affected by external factors such as the temperature of the water, and internal factors such as the species, life stage, size, and activity level of the organism. The body temperature of [[ectotherms]] (such as fishes and [[invertebrates]]) fluctuates with the temperature of the water. As the external temperature increases, ectotherm metabolisms increase as well, increasing their demand for oxygen.<ref name="Schulte2015">{{cite journal |last1=Schulte |first1=PM |date=2015 |title=The effects of temperature on aerobic metabolism: towards a mechanistic understanding of the responses of ectotherms to a changing environment |journal=Journal of Experimental Biology |volume=218 |issue=12 |pages=1856–1866 |doi=10.1242/jeb.118851 |pmid=26085663 |s2cid=24578826 |doi-access=free}}</ref> Different species have different basal metabolic rates and therefore different oxygen demands.<ref>{{cite journal |last1=Makarieva |first1=AM |last2=Gorshkov |first2=VG |last3=Li |first3=BA |last4=Chown |first4=SL |last5=Reich |first5=PB |last6=Gavrilov |first6=VM |date=2008 |title=The effects of temperature on aerobic metabolism: towards a mechanistic understanding of the responses of ectotherms to a changing environment |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=44 |pages=16994–16999 |doi=10.1073/pnas.0802148105 |pmc=2572558 |pmid=18952839 |doi-access=free}}</ref><ref>{{cite book |last1=Balmer |first1=RT |title=Modern Engineering Dynamics |date=2011 |publisher=Academic Press}}</ref>

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.<ref>{{cite journal |last1=Rosenfeld |first1=J |last2=Van Leeuwen |first2=T |last3=Richards |first3=J |last4=Allen |first4=D |date=2015 |title=Relationship between growth and standard metabolic rate: measurement artefacts and implications for habitat use and life-history adaptation in salmonids |journal=Journal of Animal Ecology |volume=84 |issue=1 |pages=4–20 |doi=10.1111/1365-2656.12260 |pmid=24930825 |doi-access=free|bibcode=2015JAnEc..84....4R }}</ref> 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.<ref>{{cite journal |last1=Singer |first1=D |date=2004 |title=Metabolic adaptation to hypoxia: cost and benefit of being small |journal=Respiratory Physiology & Neurobiology |volume=141 |issue=3 |pages=215–228 |doi=10.1016/j.resp.2004.02.009 |pmid=15288595 |s2cid=34768843}}</ref> 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.

==== Indices and calculations ====
Several indices to measure bioavailability have been suggested: Respiration Index,<ref name="BP2009">{{cite journal |last1=Brewer |first1=PG |last2=Peltzer |first2=ET |date=2009 |title=Limits to Marine Life |journal=Science |volume=324 |issue=5925 |pages=347–348 |doi=10.1126/science.1170756 |pmid=19372421 |s2cid=206518536}}</ref> Oxygen Supply Index,<ref name="Verberk2011">{{cite journal |last1=Verberk |first1=WCEP |last2=Bilton |first2=DT |last3=Calosi |first3=P |last4=Spicer |first4=JI |date=2011 |title=Oxygen supply in aquatic ectotherms: partial pressure and solubility together explain biodiversity and size patterns. |journal=Ecology |volume=92 |issue=8 |pages=1565–1572 |doi=10.1890/10-2369.1 |pmid=21905423 |bibcode=2011Ecol...92.1565V |s2cid=299377 |hdl-access=free |hdl=2066/111573}}</ref> and the Metabolic Index.<ref name="Deutsch2015">{{cite journal |last1=Deutsch |first1=C |last2=Ferrel |first2=A |last3=Seibel |first3=B |last4=Pörtner |first4=HO |last5=Huey |first5=R |date=2015 |title=Climate change tightens a metabolic constraint on marine habitats |journal=Science |volume=348 |issue=6239 |pages=1132–1135 |bibcode=2015Sci...348.1132D |doi=10.1126/science.aaa1605 |pmid=26045435 |s2cid=206633086 |doi-access=free}}</ref> The Respiration Index describes oxygen availability based on the [[Thermodynamic free energy|free energy]] available in the [[wiktionary:reactant|reactants]] and [[Product (chemistry)|products]] of the [[Stoichiometry|stoichiometric]] equation for respiration.<ref name="BP2009" /> However, organisms have ways of altering their oxygen intake and carbon dioxide release, so the strict stoichiometric equation is not necessarily accurate.<ref name="SC2013">{{cite journal |last1=Seibel |first1=BA |last2=Childress |first2=JJ |date=2013 |title=The real limits to marine life : a further critique of the Respiration Index |journal=Biogeosciences |volume=10 |issue=5 |page=2815 |bibcode=2013BGeo...10.2815S |doi=10.5194/bg-10-2815-2013 |doi-access=free}}</ref> The Oxygen Supply Index accounts for oxygen solubility and partial pressure, along with the [[Q10 (temperature coefficient)|Q<sub>10</sub>]] of the organism, but does not account for behavioral or physiological changes in organisms to compensate for reduced oxygen availability.<ref name="Verberk2011" /> The Metabolic Index accounts for the supply of oxygen in terms of solubility, partial pressure, and diffusivity of oxygen in water, and the organism's metabolic rate.<ref name="Deutsch2015" /> The metabolic index is generally viewed as a closer approximation of oxygen bioavailability than the other indices.

There are two thresholds of oxygen required by organisms:
[[File:Respiration-_Pcrit_and_Pleth.png|thumb|Respiration- Pcrit and Pleth]]

* ''P<sub>crit</sub>'' (critical partial pressure)- the oxygen level below which an organism cannot support a normal [[Cellular respiration|respiration]] rate
* ''P<sub>leth</sub>'' (lethal partial pressure)- the oxygen level below which an organism cannot support the minimum respiration rate necessary for survival.<ref name="Portner2010">{{cite journal |last1=Pörtner |first1=HO |date=2010 |title=Oxygen- And capacity-limitation of thermal tolerance: A matrix for integrating climate-related stressor effects in marine ecosystems |journal=Journal of Experimental Biology |volume=213 |issue=6 |pages=881–893 |doi=10.1242/jeb.037523 |pmid=20190113 |s2cid=14695028 |doi-access=free}}</ref><ref name="Elliott20132">{{cite journal |last1=Elliott |first1=DT |last2=Pierson |first2=JJ |last3=Roman |first3=MR |date=2013 |title=Elliott, D.T., Pierson, J.J. and Roman, M.R., 2013. Predicting the effects of coastal hypoxia on vital rates of the planktonic copepod Acartia tonsa Dana |journal=PLOS ONE |volume=8 |issue=5 |page=e63987 |doi=10.1371/journal.pone.0063987 |pmc=3656935 |pmid=23691134 |doi-access=free}}</ref>

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.