Editing Oxygen minimum zone (section)

=== 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.