View source for Seawater

{{short description|Water from a sea or an ocean}}
{{Use dmy dates|date=October 2016}}
{{redirect|Ocean water|the standard for isotope composition of pure water|Vienna Standard Mean Ocean Water}}
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{{multiple image
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 | width = 220
 | image1 = Mar de Siete Colores - West (8331578284).jpg
 | caption1 = Seawater off [[San Andrés (island)|San Andrés]]
 | image2 = T-S_diagram.pdf
 | caption2 = Temperature-salinity diagram of changes in density of water
 | image3 = WaterDepthSalinity.png
 | caption3 = Ocean salinity at different latitudes in the Atlantic and Pacific
}}
{{Water salinity}}
}}
'''Seawater''', or '''sea water''', is [[water]] from a [[sea]] or [[ocean]]. On average, seawater in the world's oceans has a [[salinity]] of about 3.5% (35&nbsp;g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximately {{convert|35|g|oz}} of [[sea salt|dissolved salts]] (predominantly [[sodium]] ({{chem2|Na+}}) and [[chloride]] ({{chem2|Cl-}}) [[ions]]). The average density at the surface is 1.025&nbsp;kg/L. Seawater is [[density|denser]] than both [[fresh water]] and pure water (density 1.0&nbsp;kg/L at {{convert|4|C|F}}) because the dissolved salts increase the mass by a larger proportion than the volume.{{cn |date=November 2025}} The freezing point of seawater decreases as salt concentration increases. At typical salinity, it [[Freezing|freezes]] at about {{convert|-2|C|F}}.<ref>{{cite web |url=http://www.onr.navy.mil/Focus/ocean/water/temp3.htm |title=Ocean Water: Temperature |website=Science and Technology Focus |publisher=[[U.S. Office of Naval Research]] |url-status=dead |archive-url=https://web.archive.org/web/20071212151229/http://www.onr.navy.mil/Focus/ocean/water/temp3.htm |archive-date=12 December 2007 |df=dmy-all }}</ref> The coldest seawater still in the liquid state ever recorded was found in 2010, in a stream under an [[Antarctic]] [[glacier]]: the measured temperature was {{convert|-2.6|C|F}}.<ref>{{Cite web |last= Sylte |first=Gudrun Urd |title= Den aller kaldaste havstraumen |url= http://www.forskning.no/artikler/2010/mai/250690 |language= no |date= 24 May 2010 |work= [[Forskning.no]] |access-date= 24 May 2010 |archive-url= https://web.archive.org/web/20120306040502/http://www.forskning.no/artikler/2010/mai/250690 |archive-date= 6 March 2012 |url-status= dead |df= dmy-all }}</ref> 

Seawater [[pH]] is typically limited to a range between 7.5 and 8.4.<ref name="Chester Marine Geochem">{{cite book |last1=Chester |first1=Jickells |last2=Roy |first2=Tim |year=2012 |title=Marine Geochemistry |publisher=Blackwell Publishing |isbn=978-1-118-34907-6}}</ref>{{pn |date=November 2025}} However, there is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.<ref name=":0">{{cite book |last1=Stumm |first1= W. |last2=Morgan |first2=J. J. |year=1981 |title=Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters |publisher=John Wiley & Sons |pages=414–416 |ISBN=0471048313}}.</ref>

==Properties==
===Salinity===
{{Further|Salinity#Seawater|Ocean#Salinity}}
[[File:WOA09 sea-surf SAL AYool.png|thumb|left|Annual mean sea surface salinity expressed in the [[Practical Salinity Scale]] for the [[World Ocean]]. Data from the [[World Ocean Atlas]]<ref>{{cite web |url=http://www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html |title=World Ocean Atlas 2009 |publisher=[[NOAA]] | access-date=5 December 2012 }}</ref>]]

Although the vast majority of seawater has a salinity of between 31 and 38&nbsp;g/kg, that is 3.1–3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with freshwater runoff from river mouths, near melting glaciers or vast amounts of precipitation (e.g. [[monsoon]]), seawater can be substantially less saline. The most saline open sea is the [[Red Sea]], where high rates of [[evaporation]], low [[Precipitation (meteorology)|precipitation]] and low river run-off, and confined circulation result in unusually salty water. The salinity in isolated bodies of water can be considerably greater still{{snd}} about ten times higher in the case of the [[Dead Sea]]. Historically, several salinity scales were used to approximate the absolute salinity of seawater. A popular scale was the "Practical Salinity Scale" where salinity was measured in "practical salinity units (PSU)". The current standard for salinity is the "Reference Salinity" scale <ref>{{cite journal |last1=Millero |first1=Frank J. |last2=Feistel |first2=Rainer |last3=Wright |first3=Daniel G. |last4=McDougall |first4=Trevor J. |title=The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale |journal=Deep Sea Research Part I: Oceanographic Research Papers |date=January 2008 |volume=55 |issue=1 |pages=50–72 |doi=10.1016/j.dsr.2007.10.001 |bibcode=2008DSRI...55...50M}}</ref> with the salinity expressed in units of "g/kg".

===Density===
The [[density]] of surface seawater ranges from about 1020 to 1029&nbsp;kg/m<sup>3</sup>, depending on the temperature and salinity. At a temperature of 25&nbsp;°C, the salinity of 35&nbsp;g/kg and 1&nbsp;atm pressure, the density of seawater is 1023.6&nbsp;kg/m<sup>3</sup>.<ref name=Nayar>{{cite journal |last1=Nayar |first1=Kishor G. |last2=Sharqawy |first2=Mostafa H. |last3=Banchik |first3=Leonardo D. |last4=Lienhard V |first4=John H. |title=Thermophysical properties of seawater: A review and new correlations that include pressure dependence |journal=Desalination |date=July 2016 |volume=390 |pages=1–24 |doi=10.1016/j.desal.2016.02.024 |doi-access=free|bibcode=2016Desal.390....1N |hdl=1721.1/106794 |hdl-access=free }}</ref><ref name=MIT>{{cite web |title=Thermophysical properties of seawater |url=http://web.mit.edu/seawater |publisher=Department of Mechanical Engineering, [[Massachusetts Institute of Technology]] |access-date=February 24, 2017 |language=en}}</ref> Deep in the ocean, under high pressure, seawater can reach a density of 1050&nbsp;kg/m<sup>3</sup> or higher. The density of seawater also changes with salinity. Brines generated by seawater desalination plants can have salinities up to 120&nbsp;g/kg. The density of typical seawater brine of 120&nbsp;g/kg salinity at 25&nbsp;°C and atmospheric pressure is 1088&nbsp;kg/m<sup>3</sup>.<ref name=Nayar/><ref name=MIT/>

=== pH value ===
{{Further|pH#Seawater|Ocean acidification}}

The [[pH value]] at the surface of oceans in pre-industrial time (before 1850) was around 8.2.<ref name=":82">Arias, P.A., N. Bellouin, E. Coppola, R.G. Jones, G. Krinner, J. Marotzke, V. Naik, M.D. Palmer, G.-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P.W. Thorne, B. Trewin, K. Achuta Rao, B. Adhikary, R.P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J.G. Canadell, C. Cassou, A. Cherchi, W. Collins, W.D. Collins, S.L. Connors, S. Corti, F. Cruz, F.J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F.J. Doblas-Reyes, A. Dosio, H. Douville, F. Engelbrecht, V.&nbsp; Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J.S. Fuglestvedt, J.C. Fyfe, et al., 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary] {{Webarchive|url=https://web.archive.org/web/20220721021347/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf|date=21 July 2022}}. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20210809131444/https://www.ipcc.ch/report/ar6/wg1/|date=9 August 2021}} [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 33−144.</ref> Since then, it has been decreasing due to a human-caused process called [[ocean acidification]] that is related to [[carbon dioxide emissions]]: Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.<ref>{{Cite journal |last1=Terhaar |first1=Jens |last2=Frölicher |first2=Thomas L. |last3=Joos |first3=Fortunat |date=2023 |title=Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-{{CO2}} greenhouse gases |journal=Environmental Research Letters |language=en |volume=18 |issue=2 |pages=024033 |bibcode=2023ERL....18b4033T |doi=10.1088/1748-9326/acaf91 |issn=1748-9326 |s2cid=255431338|doi-access=free }}</ref>

The pH value of seawater is naturally as low as 7.8 in deep ocean waters as a result of the degradation of organic matter in these waters.<ref>{{Cite book |last1=Emerson |first1=Steven |url=https://www.cambridge.org/core/product/identifier/9780511793202/type/book |title=Chemical Oceanography and the Marine Carbon Cycle |last2=Hedges |first2=John |date=2008-04-24 |publisher=Cambridge University Press |isbn=978-0-521-83313-4 |edition=1 |chapter=Chapter 4: Carbonate chemistry |doi=10.1017/cbo9780511793202}}</ref> It can be as high as 8.4 in surface waters in areas of high [[biological productivity]].<ref name=":5">{{Cite book |last1=Chester |first1=R. |url=https://www.wiley.com/en-us/Marine+Geochemistry%2C+3rd+Edition-p-9781118349090 |title=Marine geochemistry |last2=Jickells |first2=Tim |date=2012 |publisher=Wiley/Blackwell |isbn=978-1-118-34909-0 |edition=3rd |location=Chichester, West Sussex, UK |chapter=Chapter 9: Nutrients, oxygen, organic carbon and the carbon cycle in seawater |oclc=781078031}}</ref>  

Measurement of pH is complicated by the [[Chemical property|chemical properties]] of seawater, and several distinct pH scales exist in [[chemical oceanography]].<ref name="zeebe">Zeebe, R. E. and Wolf-Gladrow, D. (2001) ''CO<sub>2</sub> in seawater: equilibrium, kinetics, isotopes'', Elsevier Science B.V., Amsterdam, Netherlands {{ISBN|0-444-50946-1}}</ref> There is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.<ref name=":0" /> 

===Chemical composition===
[[File:Sea salt-e-dp hg.svg|thumb|Diagram showing concentrations of various salt ions in seawater. The composition of the total salt component is: {{chem2|Cl-}} 55%, {{chem2|Na+}} 30.6%, {{chem2|SO4(2-)}} 7.7%, {{chem2|Mg(2+)}} 3.7%, {{chem2|Ca(2+)}} 1.2%, {{chem2|K+}} 1.1%, other 0.7%, by mass.]]

Seawater contains more dissolved [[ion]]s than all types of freshwater.<ref name=Gale>{{cite web |url=http://www.waterencyclopedia.com/Mi-Oc/Ocean-Chemical-Processes.html |last=Gale |first=Thomson |title=Ocean Chemical Processes |access-date=2 December 2006 }}</ref> However, the ratios of solutes differ dramatically. For instance, although seawater contains about 2.8 times more [[bicarbonate]] than river water, the [[percentage]] of bicarbonate in seawater as a ratio of ''all'' dissolved [[ion]]s is far lower than in river water. Bicarbonate ions constitute 48% of river water solutes but only 0.14% for seawater.<ref name=Gale/><ref name=Pinet>{{cite book |last=Pinet |first=Paul R. |year=1996 |title=Invitation to Oceanography |location=St. Paul |pages=126, 134–135 |publisher=West Publishing Company |isbn=978-0-314-06339-7 }}</ref> Differences like these are due to the varying [[Residence time (fluid dynamics)|residence time]]s of seawater solutes; [[sodium]] and [[chlorine|chloride]] have very long residence times, while [[calcium]] (vital for [[carbonate]] formation) tends to precipitate much more quickly.<ref name=Pinet/> The most abundant dissolved ions in seawater are sodium, chloride, [[magnesium]], [[sulfate]] and calcium.<ref>Hogan, C. Michael (2010). [http://www.eoearth.org/article/Calcium?topic=49557 "Calcium"], eds. A. Jorgensen, C. Cleveland. ''Encyclopedia of Earth''. Some evidence shows the potential for fairly regular ratios of elements maintained across surface oceans in a phenomenon known as the [[Redfield ratio|Redfield Ratio]]. National Council for Science and the Environment.</ref> Its [[osmotic concentration|osmolarity]] is about 1000 mOsm/L.<ref>{{Cite web|url=https://bionumbers.hms.harvard.edu/bionumber.aspx?&id=100802&ver=0|title=Osmolarity of sea water - Biosphere - BNID 100802|website=bionumbers.hms.harvard.edu}}</ref>

Small amounts of other substances are found, including [[amino acids]] at concentrations of up to 2 micrograms of nitrogen atoms per liter,<ref>{{Cite journal| doi = 10.1007/BF02742615| url = http://www.terrapub.co.jp/journals/JO/pdf/5404/54040313.pdf| title = Dissolved free amino acids in coastal seawater using a modified fluorometric method| journal = Journal of Oceanography| volume = 54| issue = 4| pages = 313–321| year = 1998| last1 = Tada| first1 = K.| last2 = Tada| first2 = M.| last3 = Maita| first3 = Y.| bibcode = 1998JOce...54..313T| s2cid = 26231863| access-date = 28 August 2015| archive-date = 21 January 2021| archive-url = https://web.archive.org/web/20210121231018/http://www.terrapub.co.jp/journals/JO/pdf/5404/54040313.pdf| url-status = dead}}</ref> which are thought to have played a key role in the [[abiogenesis|origin of life]].

{|class="wikitable sortable" style="float: left; margin: 0 0 1em 1em;"
|+ Seawater elemental composition<br>(salinity = 3.5%) {{citation needed|date=June 2019}}
|-
!Element
!Percent by mass
|-
|[[Oxygen]]
|85.84
|-
|[[Hydrogen]]
|10.82
|-
|[[Chlorine]]
|1.94
|-
|[[Sodium]]
|1.08
|-
|[[Magnesium]]
|0.1292
|-
|[[Sulfur]]
|0.091
|-
|[[Calcium]]
|0.04
|-
|[[Potassium]]
|0.04
|-
|[[Bromine]]
|0.0067
|-
|[[Carbon]]
|0.0028
|}

{|class="wikitable sortable" style="float: left; margin: 0 0 1em 1em;"
|+ Total molar composition of seawater (salinity = 35)<ref>{{cite book
|author=DOE
|year=1994
|chapter-url=http://cdiac.esd.ornl.gov/ftp/cdiac74/chapter5.pdf
|title=Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water
|publisher=ORNL/CDIAC-74
|version=2
|editor1=A. G. Dickson
|editor2=C. Goyet
|chapter=5
|access-date=18 May 2006
|archive-date=25 May 2011
|archive-url=https://web.archive.org/web/20110525155035/http://cdiac.esd.ornl.gov/ftp/cdiac74/chapter5.pdf
|url-status=dead
}}</ref>
|-
!Component !! Concentration (mol/kg)
|-
| {{H2O}} || 53.6
|-
| {{chem2|Cl-|link=Chloride}} || 0.546
|-
| {{chem2|Na+}} || 0.469
|-
| {{chem2|Mg(2+)}} || 0.0528
|-
| {{chem2|SO4(2-)|link=Sulfate}} || 0.0282
|-
| {{chem2|Ca(2+)}} || 0.0103
|-
| {{chem2|K+}} || 0.0102
|-
| [[Total inorganic carbon|C<sub>T</sub>]] || 0.00206
|-
| {{chem2|Br-|link=Bromide}} || 0.000844
|-
| [[Total boron|B<sub>T</sub>]] || 0.000416
|-
| {{chem2|Sr(2+)|link=Strontium}} || 0.000091
|-
| {{chem2|F-|link=Fluoride}} || 0.000068
|}
{{-}}

===Microbial components===
Research in 1957 by the [[Scripps Institution of Oceanography]] sampled water in both [[pelagic]] and [[neritic]] locations in the Pacific Ocean. Direct microscopic counts and cultures were used, the direct counts in some cases showing up to 10 000 times that obtained from cultures. These differences were attributed to the occurrence of bacteria in aggregates, selective effects of the culture media, and the presence of inactive cells. A marked reduction in bacterial culture numbers was noted below the [[thermocline]], but not by direct microscopic observation. Large numbers of [[spirilli]]-like forms were seen by microscope but not under cultivation. The disparity in numbers obtained by the two methods is well known in this and other fields.<ref>{{cite journal | title= Bacterial Populations in Sea Water as Determined by Different Methods of Enumeration | first1= Holger W. | last1= Jannasch | first2= Galen E. | last2= Jones | volume= 4 | number= 2 | journal= Limnology and Oceanography | pages= 128–139 | doi= 10.4319/lo.1959.4.2.0128 | df= dmy-all | year= 1959 | bibcode= 1959LimOc...4..128J | doi-access= free }}</ref> In the 1990s, improved techniques of detection and identification of microbes by probing just small snippets of [[DNA]], enabled researchers taking part in the [[Census of Marine Life]] to identify thousands of previously unknown microbes usually present only in small numbers. This revealed a far greater diversity than previously suspected, so that a litre of seawater may hold more than 20,000 species. [[Mitchell Sogin]] from the [[Marine Biological Laboratory]] feels that "the number of different kinds of bacteria in the oceans could eclipse five to 10 million."<ref>{{cite news| url= https://www.sciencedaily.com/releases/2006/08/060829081744.htm| work= ScienceDaily | title= Ocean Microbe Census Discovers Diverse World of Rare Bacteria | date= 2 September 2006 | access-date= 13 May 2013}}</ref>

Bacteria are found at all depths in the [[water column]], as well as in the sediments, some being aerobic, others anaerobic. Most are free-swimming, but some exist as [[symbionts]] within other organisms – examples of these being bioluminescent bacteria. [[Cyanobacteria]] played an important role in the evolution of ocean processes, enabling the development of [[stromatolites]] and oxygen in the atmosphere.

Some bacteria interact with [[diatoms]], and form a critical link in the cycling of silicon in the ocean. One anaerobic species, ''[[Thiomargarita namibiensis]]'', plays an important part in the breakdown of [[hydrogen sulfide]] eruptions from diatomaceous sediments off the Namibian coast, and generated by high rates of [[phytoplankton]] growth in the [[Benguela Current]] upwelling zone, eventually falling to the seafloor.

Bacteria-like [[Archaea]] surprised marine microbiologists by their survival and thriving in extreme environments, such as the [[hydrothermal vents]] on the ocean floor. Alkalotolerant [[marine bacteria]] such as ''[[Pseudomonas]]'' and ''[[Vibrio]]'' spp. survive in a [[pH]] range of 7.3 to 10.6, while some species will grow only at pH 10 to 10.6.<ref>{{cite journal|title= Alkalotolerant and Alkalophilic Bacteria in Seawater |first1= M. |last1= Maeda |first2= N. |last2= Taga | journal= Marine Ecology Progress Series|volume= 2 | pages= 105–108 | date= 31 March 1980 | doi=10.3354/meps002105|bibcode= 1980MEPS....2..105M |doi-access= free }}</ref> Archaea also exist in pelagic waters and may constitute as much as half the ocean's [[Biomass (ecology)|biomass]], clearly playing an important part in oceanic processes.<ref>{{cite news| url= http://news.bbc.co.uk/2/hi/science/nature/5232928.stm | date= 31 July 2006 | title= Thousands of microbes in one gulp | first= Louisa |last= Cheung | work= BBC News | access-date= 13 May 2013}}</ref> In 2000 sediments from the ocean floor revealed a species of Archaea that breaks down [[methane]], an important [[greenhouse]] gas and a major contributor to atmospheric warming.<ref>{{cite news | url= http://news.sciencemag.org/sciencenow/2000/10/05-01.html | title= The Case of the Missing Methane | first= Mitchell | last= Leslie | date= 5 October 2000 | work= ScienceNOW | publisher= American Association for the Advancement of Science | access-date= 13 May 2013 | url-status= dead | archive-url= https://web.archive.org/web/20130526182906/http://news.sciencemag.org/sciencenow/2000/10/05-01.html | archive-date= 26 May 2013 | df= dmy-all }}</ref> Some bacteria break down the rocks of the sea floor, influencing seawater chemistry. Oil spills, and runoff containing human sewage and chemical pollutants have a marked effect on microbial life in the vicinity, as well as harbouring pathogens and toxins affecting all forms of [[marine life]]. The protist [[dinoflagellates]] may at certain times undergo population explosions called blooms or [[red tide]]s, often after human-caused pollution. The process may produce [[metabolites]] known as biotoxins, which move along the ocean food chain, tainting higher-order animal consumers.

''[[Pandoravirus salinus]]'', a species of very large virus, with a genome much larger than that of any other virus species, was discovered in 2013. Like the other very large viruses ''[[Mimivirus]]'' and ''[[Megavirus]]'', ''Pandoravirus'' infects amoebas, but its genome, containing 1.9 to 2.5 megabases of DNA, is twice as large as that of ''Megavirus'', and it differs greatly from the other large viruses in appearance and in genome structure.

In 2013, researchers from [[Aberdeen University]] announced that they were starting a hunt for undiscovered chemicals in organisms that have evolved in deep sea trenches, hoping to find "the next generation" of antibiotics, anticipating an "antibiotic apocalypse" with a dearth of new infection-fighting drugs. The EU-funded research will start in the [[Atacama Trench]] and then move on to search trenches off New Zealand and Antarctica.<ref>{{cite news| url= https://www.bbc.co.uk/news/health-21457149 |title= Antibiotics search to focus on sea bed | date= 14 February 2013 | work= BBC News | access-date= 13 May 2013}}</ref>

The ocean has a long history of human waste disposal on the assumption that its vast size makes it capable of absorbing and diluting all noxious material.<ref>{{cite book |last1=Panel On Radioactivity In The Marine Environment |first1=National Research Council (U.S.) |url=https://archive.org/details/bub_gb_nKkrAAAAYAAJ |title=Radioactivity in the marine environment - National Academies, 1971 |publisher=National Academies |year=1971 |isbn=9780309018654 |page=[https://archive.org/details/bub_gb_nKkrAAAAYAAJ/page/n47 36]}}</ref>
While this may be true on a small scale, the large amounts of sewage routinely dumped has damaged many coastal ecosystems, and rendered them life-threatening. Pathogenic viruses and bacteria occur in such waters, such as ''[[Escherichia coli]]'', ''[[Vibrio cholerae]]'' the cause of [[cholera]], [[hepatitis A]], [[hepatitis E]] and [[polio]], along with protozoans causing [[giardiasis]] and [[cryptosporidiosis]]. These pathogens are routinely present in the ballast water of large vessels, and are widely spread when the ballast is discharged.<ref>{{cite encyclopedia| url= http://www.waterencyclopedia.com/Mi-Oc/Microbes-in-the-Ocean.html |title= Microbes in the Ocean |first1= Brian D. |last1= Hoyle | first2= Richard |last2= Robinson | encyclopedia= Water Encyclopedia }}</ref>

=== Other parameters ===
The [[speed of sound]] in seawater is about 1,500&nbsp;m/s (whereas the speed of sound is usually around 330&nbsp;m/s in air at roughly 101.3&nbsp;kPa pressure, 1&nbsp;atmosphere), and varies with water temperature, salinity, and pressure. The [[thermal conductivity]] of seawater is 0.6&nbsp;W/mK at 25&nbsp;°C and a salinity of 35&nbsp;g/kg.<ref>{{cite journal |last1=Sharqawy |first1=Mostafa H. |last2=Lienhard V |first2=John H. |last3=Zubair |first3=Syed M. |date=April 2010 |title=The thermophysical properties of seawater: A review of existing correlations and data |url=http://web.mit.edu/lienhard/www/Thermophysical_properties_of_seawater-DWT-16-354-2010.pdf |journal=Desalination and Water Treatment |volume=16 |issue=1–3 |pages=354–380 |doi=10.5004/dwt.2010.1079 |bibcode=2010DWatT..16..354S |hdl-access=free |hdl=1721.1/69157|s2cid=93362418 }}</ref>
The thermal conductivity decreases with increasing salinity and increases with increasing temperature.<ref>{{cite web |title=Thermal conductivity of seawater and its concentrates |url=http://twt.mpei.ac.ru/tthb/2/Tab-5-5-13-2-Ther-Cond-Seawater.html |access-date=17 October 2010 |archive-date=29 September 2018 |archive-url=https://web.archive.org/web/20180929063305/http://twt.mpei.ac.ru/tthb/2/Tab-5-5-13-2-Ther-Cond-Seawater.html |url-status=dead }}</ref>

==Origin and history==
{{See also|Origin of water on Earth}}

The water in the sea was thought to come from the Earth's [[volcano]]es, starting 4 billion years ago, released by degassing from molten rock.<ref name=Stow>{{cite book|ref=Stow | title=Encyclopedia of the Oceans | publisher=Oxford University Press | author=Stow, Dorrik | year=2004 | isbn=978-0-19-860687-1}}</ref>{{rp|pages=24–25}} More recent work suggests much of the Earth's water may come from [[comet]]s.<ref>{{cite journal | url=http://www.nature.com/news/2011/111005/full/news.2011.579.html | title=Comets take pole position as water bearers | journal=Nature | date=5 October 2011 | access-date=10 September 2013 | author=Cowen, Ron| doi=10.1038/news.2011.579 | doi-access=free | url-access=subscription }}</ref>

[[Scientific theory|Scientific theories]] behind the origins of sea salt started with Sir [[Edmond Halley]] in 1715, who proposed that salt and other minerals were carried into the sea by rivers after rainfall washed it out of the ground. Upon reaching the ocean, these salts concentrated as more salt arrived over time (see [[Water cycle|Hydrologic cycle]]). Halley noted that most lakes that do not have ocean outlets (such as the [[Dead Sea]] and the [[Caspian Sea]], see [[endorheic basin]]), have high salt content. Halley termed this process "continental weathering".

Halley's theory was partly correct. In addition, sodium leached out of the ocean floor when the ocean formed. The presence of salt's other dominant ion, chloride, results from [[outgassing]] of chloride (as [[hydrochloric acid]]) with other gases from Earth's interior via [[volcano]]s and [[hydrothermal vent]]s. The sodium and chloride ions subsequently became the most abundant constituents of sea salt.

Ocean salinity has been stable for billions of years, most likely as a consequence of a chemical/[[plate tectonics|tectonic]] system which removes as much salt as is deposited; for instance, sodium and chloride sinks include [[evaporite]] deposits, pore-water burial, and reactions with seafloor [[basalt]]s.<ref name=Pinet/>{{rp|133}}

==Human impacts==
{{For|the increase in the Earth's volume of seawater|Sea level rise}}
[[Climate change]], rising levels of [[carbon dioxide in Earth's atmosphere]], excess nutrients, and pollution in many forms are altering global oceanic [[geochemistry]]. Rates of change for some aspects greatly exceed those in the historical and recent geological record. Major trends include an increasing [[ocean acidification|acidity]], reduced subsurface oxygen in both near-shore and pelagic waters, rising coastal nitrogen levels, and widespread increases in [[mercury (element)|mercury]] and persistent organic pollutants. Most of these perturbations are tied either directly or indirectly to human fossil fuel combustion, fertilizer, and industrial activity. Concentrations are projected to grow in coming decades, with negative impacts on ocean biota and other marine resources.<ref>{{cite journal|last=Doney|first=Scott C.|author-link1=Scott Doney|title=The Growing Human Footprint on Coastal and Open-Ocean Biogeochemistry|journal=[[Science (journal)|Science]]|date=18 June 2010|volume=328|issue=5985|pages=1512–1516|doi=10.1126/science.1185198|pmid=20558706|bibcode=2010Sci...328.1512D|s2cid=8792396}}</ref>

One of the most striking features of this is [[ocean acidification]], resulting from increased {{CO2}} uptake of the oceans related to higher atmospheric concentration of {{CO2}} and higher temperatures,<ref>{{Cite journal|date=2009-01-01|title=Ocean Acidification: The Other CO2 Problem|journal=Annual Review of Marine Science|volume=1|issue=1|pages=169–192|doi=10.1146/annurev.marine.010908.163834|pmid=21141034|last1=Doney|first1=Scott C.|last2=Fabry|first2=Victoria J.|last3=Feely|first3=Richard A.|last4=Kleypas|first4=Joan A.|author-link4=Joan Kleypas| bibcode=2009ARMS....1..169D|s2cid=402398}}</ref> because it severely affects [[coral reef]]s, [[mollusk]]s, [[echinoderm]]s and [[crustacean]]s (see [[coral bleaching]]).

Seawater is a means of transportation throughout the world. Every day plenty of ships cross the ocean to deliver goods to various locations around the world. Seawater is a tool for countries to efficiently participate in international commercial trade and transportation, but each ship exhausts emissions that can harm marine life, air quality of coastal areas. Seawater transportation is one of the fastest growing human generated greenhouse gas emissions.<ref>{{Cite journal |last=Vaishnav |first=Parth |date=2014 |title=Greenhouse Gas Emissions from International Transport |url=https://www.jstor.org/stable/43315842 |journal=Issues in Science and Technology |volume=30 |issue=2 |pages=25–28 |jstor=43315842 |issn=0748-5492}}</ref> The emissions released from ships pose significant risks to human health in nearing areas as the [[oil]] and [[Gasoline|gas]] released from the operation of merchant ships decreases the air quality and causes more [[pollution]] both in the seawater and surrounding areas.<ref>{{Cite journal |last1=Iodice |first1=Paolo |last2=Langella |first2=Giuseppe |last3=Amoresano |first3=Amedeo |date=2017 |title=A numerical approach to assess air pollution by ship engines in manoeuvring mode and fuel switch conditions |url=https://www.jstor.org/stable/90015687 |journal=Energy & Environment |volume=28 |issue=8 |pages=827–845 |doi=10.1177/0958305X17734050 |jstor=90015687 |bibcode=2017EnEnv..28..827I |issn=0958-305X|url-access=subscription }}</ref>

Another human use of seawater that has been considered is the use of seawater for [[Agriculture|agricultural]] purposes. In areas with higher regions of [[Dune|sand dunes]], such as [[Israel]], the use of seawater for [[irrigation]] of plants would eliminate substantial costs associated with fresh water when it is not easily accessible.<ref name=":04">{{Cite journal |last=Boyko |first=Hugo |date=1967 |title=Salt-Water Agriculture |url=https://www.jstor.org/stable/24931436 |journal=Scientific American |volume=216 |issue=3 |pages=89–101 |doi=10.1038/scientificamerican0367-89 |jstor=24931436 |bibcode=1967SciAm.216c..89B |issn=0036-8733|url-access=subscription }}</ref> Although it is not typical to use [[Saline water|salt water]] as a means to grow plants as the salt gathers and ruins the surrounding soil, it has been proven to be successful in sand and gravel soils.<ref name=":04" /> Large-scale desalination of seawater is another factor that would contribute to the success of agriculture farming in dry, [[desert]] environments.<ref name=":04" /> One of the most successful plants in salt water agriculture is the [[halophyte]]. The halophyte is a salt tolerant plant whose cells are resistant to the typically detrimental effects of salt in soil.<ref name=":1">{{Cite journal |last1=Glenn |first1=Edward P. |last2=Brown |first2=J. Jed |last3=O’Leary |first3=James W. |date=1998 |title=Irrigating Crops with Seawater |url=https://www.jstor.org/stable/26070601 |journal=Scientific American |volume=279 |issue=2 |pages=76–81 |doi=10.1038/scientificamerican0898-76 |jstor=26070601 |bibcode=1998SciAm.279b..76G |issn=0036-8733|url-access=subscription }}</ref> The [[endodermis]] forces a higher level of salt filtration throughout the plant as it allows for the circulation of more water through the cells.<ref name=":1" /> The cultivation of halophytes irrigated with salt water were used to grow [[animal feed]] for [[livestock]]; however, the animals that were fed these plants consumed more water than those that did not.<ref name=":1" /> Although agriculture from use of saltwater is still not recognized and used on a large scale, initial research has shown that there could be an opportunity to provide more crops in regions where agricultural farming is not usually feasible.

==Human consumption==
{{main|Salt poisoning}}
{{See also|Desalination}}
Accidentally consuming small quantities of clean seawater is not harmful, especially if the seawater is taken along with a larger quantity of fresh water. However, drinking seawater to maintain hydration is counterproductive; more water must be excreted to eliminate the salt (via [[urine]]) than the amount of water obtained from the seawater itself.<ref>{{cite web |url=http://oceanservice.noaa.gov/facts/drinksw.html |title=Can humans drink seawater? |publisher=[[National Ocean Service]] ([[National Oceanic and Atmospheric Administration|NOAA]])|date = 26 February 2021}}</ref> In normal circumstances, it would be considered ill-advised to consume large amounts of unfiltered seawater.

The [[Urinary system|renal system]] actively regulates the levels of sodium and chloride in the blood within a very narrow range around 9&nbsp;g/L (0.9% by mass).

In most open waters, concentrations vary somewhat around typical values of about 3.5%, far higher than the body can tolerate and most beyond what the kidney can process. A point frequently overlooked in claims that the kidney can excrete NaCl in [[Baltic Sea|Baltic]] concentrations of 2% (in arguments to the contrary) is that the gut cannot absorb water at such concentrations, so that there is no benefit in drinking such water. The salinity of Baltic surface water, however, is never 2%; it is 0.9% or less, and thus never higher than that of bodily fluids. Drinking seawater temporarily increases blood's NaCl concentration. This signals the [[kidney]] to excrete sodium, but seawater's sodium concentration is above the kidney's maximum concentrating ability. Eventually, the blood's sodium concentration rises to toxic levels, removing water from cells and interfering with [[nerve]] conduction, ultimately producing fatal [[Non-epileptic seizure|seizure]] and [[cardiac arrhythmia]].{{Citation needed|date=December 2011}}

[[Survival skills|Survival manuals]] consistently advise against drinking seawater.<ref>{{cite book |chapter-url=http://www.bordeninstitute.army.mil/published_volumes/harshEnv2/HE2ch29.pdf |chapter=29 |title=Shipboard Medicine |access-date=17 October 2010 |archive-date=22 June 2007 |archive-url=https://web.archive.org/web/20070622163335/http://www.bordeninstitute.army.mil/published_volumes/harshEnv2/HE2ch29.pdf |url-status=dead }}</ref> A summary of 163 [[Lifeboat (shipboard)|life raft]] voyages estimated the risk of death at 39% for those who drank seawater, compared to 3% for those who did not. The effect of seawater intake on rats confirmed the negative effects of drinking seawater when dehydrated.<ref>{{cite journal
| doi=10.1016/0300-9629(87)90275-1
| title=Metabolic effects in rats drinking increasing concentrations of seawater
| journal= Comp Biochem Physiol A
| date=1987
| pages=49–55
| pmid=2881655
| author=Etzion, Z. |author2=Yagil, R.
| volume=86
| issue=1
}}</ref>

The temptation to drink seawater was greatest for sailors who had expended their supply of fresh water and were unable to capture enough rainwater for drinking. This frustration was described famously by a line from [[Samuel Taylor Coleridge]]'s ''[[The Rime of the Ancient Mariner]]'':

{{poemquote|Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink.}}

Although humans cannot survive on seawater in place of normal drinking water, some people claim that up to two cups a day, mixed with fresh water in a 2:3 ratio, produces no ill effect. The French physician [[Alain Bombard]] survived an ocean crossing in a small Zodiak rubber boat using mainly raw fish meat, which contains about 40% water (like most living tissues), as well as small amounts of seawater and other provisions harvested from the ocean. His findings were challenged, but an alternative explanation could not be given. In his 1948 book ''[[The Kon-Tiki Expedition]]'', [[Thor Heyerdahl]] reported drinking seawater mixed with fresh in a 2:3 ratio during the 1947 expedition.<ref>Heyerdahl, Thor; Lyon, F. H. (translator) (1950). ''Kon-Tiki: Across the Pacific by Raft''. Rand McNally & Company, Chicago, Ill.</ref> A few years later, another adventurer, [[William Willis (sailor)|William Willis]], claimed to have drunk two cups of seawater and one cup of fresh per day for 70 days without ill effect when he lost part of his water supply.<ref>{{cite book
| last=King
| first=Dean
| year=2004
| title=Skeletons on the Zahara: a true story of survival
| publisher=Back Bay Books
| location=New York
| isbn= 978-0-316-15935-7
| page=74
}}</ref>

During the 18th century, [[Richard Russell (doctor)|Richard Russell]] advocated the medical use of this practice in the UK,<ref>{{Cite web|url=https://drinkingseawater.com/benefits/history-sea-water-medical-use-uk-18th-century.html|archive-url=https://web.archive.org/web/20210125093152/http://www.drinkingseawater.com/benefits/history-sea-water-medical-use-uk-18th-century.html|url-status=usurped|archive-date=25 January 2021|title=History of the 18th century medical use of sea water in Britain|website=drinkingseawater.com}}</ref> and [[René Quinton]] expanded the advocation of this practice to other countries, notably France, in the 20th century. Currently, it is widely practiced in Nicaragua and other countries, supposedly taking advantage of the latest medical discoveries.<ref>{{cite book
| last=Martin
| first=Francisco
| year=2020
| chapter=chapter 12: Medical use of sea water in Nicaragua 
| title=Drinking Sea Water
| publisher=F. Martín
| isbn=979-8666741658
}}</ref><ref>{{Cite web|url=https://drinkingseawater.com/benefits/nicaragua-sea-water-medical-use.html|archive-url=https://web.archive.org/web/20210125112445/http://www.drinkingseawater.com/benefits/nicaragua-sea-water-medical-use.html|url-status=usurped|archive-date=25 January 2021|title=Medical use of sea water in Nicaragua|website=drinkingseawater.com}}</ref>{{verify inline|date=August 2024}}

===Purification===
Like any other type of [[raw water|raw or contaminated water]], seawater can be evaporated or filtered to eliminate salt, germs, and other contaminants that would otherwise prevent it from being considered [[Drinking water|potable]]. Most oceangoing vessels [[desalination|desalinate]] potable water from seawater using processes such as [[vacuum distillation]] or [[multi-stage flash distillation]] in an [[evaporator (marine)|evaporator]], or, more recently, [[reverse osmosis]]. These energy-intensive processes were not usually available during the [[Age of Sail]]. Larger sailing warships with large crews, such as [[Horatio Nelson, 1st Viscount Nelson|Nelson]]'s {{HMS|Victory}}, were fitted with distilling apparatus in their [[galley (kitchen)|galleys]].<ref name="Rippon, Vol 1, 78-79" >{{Cite book
| title=The evolution of engineering in the Royal Navy
| volume=1: 1827–1939
| last= Rippon
| first= P. M., Commander, RN
| year=1998
| publisher=Spellmount
| isbn=978-0-946771-55-4
| pages=78–79
}}</ref>
The natural sea salt obtained by evaporating seawater can also be collected and sold as [[table salt]], typically sold separately owing to its unique mineral make-up compared to [[rock salt]] or other sources.

A number of regional cuisines across the world traditionally incorporate seawater directly as an ingredient, cooking other ingredients in a diluted solution of filtered seawater as a substitute for conventional dry [[seasoning]]s. Proponents include world-renowned chefs [[Ferran Adrià]] and [[Quique Dacosta]], whose home country of Spain has six different companies sourcing filtered seawater for culinary use.<ref name=Baker>Baker, Trevor. [https://www.theguardian.com/lifeandstyle/wordofmouth/2015/apr/21/cooking-with-seawater-season-food-spain-la-sal-perfecta "Cooking with seawater – is it the best way to season food?"] ''Guardian'', 21 April 2015.</ref> The water is marketed as {{lang|es|la sal perfecta}}, "the perfect salt", containing less sodium with what is considered a superior taste. A restaurant run by [[Joaquín Baeza]] sources as much as 60,000 litres a month from supplier Mediterranea<ref name=Baker/>

Animals such as fish, whales, [[sea turtle]]s, and [[seabird]]s, such as penguins and [[albatross]]es, have adapted to living in a high-saline habitat. For example, sea turtles and saltwater crocodiles remove excess salt from their bodies through their [[tear ducts]].<ref>{{Cite book |url=https://books.google.com/books?id=z9-eBAAAQBAJ&q=Animals+such+as+seabirds+can+adapt+to+high+saline+habitat&pg=PT158 |title=The Bird in the Waterfall: Exploring the Wonders of Water |last=Dennis |first=Jerry |date=2014-09-23 |publisher=Diversion Books |isbn=9781940941547 |language=en}}</ref>

==Mineral extraction==
Minerals have been extracted from seawater since ancient times. Currently the four most concentrated metals – [[Sodium|Na]], [[Magnesium|Mg]], [[Calcium|Ca]] and [[Potassium|K]] – are commercially extracted from seawater.<ref>{{Cite journal|last1=Loganathan|first1=Paripurnanda|last2=Naidu|first2=Gayathri|last3=Vigneswaran|first3=Saravanamuthu|date=2017|title=Mining valuable minerals from seawater: a critical review|url=https://pubs.rsc.org/en/content/articlelanding/2017/ew/c6ew00268d|journal=Environmental Science: Water Research & Technology|language=en|volume=3|issue=1|pages=37–53|doi=10.1039/C6EW00268D|hdl=10453/121701|hdl-access=free}}</ref> During 2015 in the [[US]] 63% of [[magnesium]] production came from seawater and brines.<ref>{{Cite web|last=Campbell|first=Keith|title=Over 40 minerals and metals contained in seawater, their extraction likely to increase in the future|url=https://www.miningweekly.com/article/over-40-minerals-and-metals-contained-in-seawater-their-extraction-likely-to-increase-in-the-future-2016-04-01|access-date=2023-02-08|website=Mining Weekly|language=en}}</ref> [[Bromine]] is also produced from seawater in [[China]] and Japan.<ref>{{Cite web|url=http://www.bromine.chem.yamaguchi-u.ac.jp/library/L02_Global%20Bromine%20Industry.pdf|title=Global Bromine Industry And Its Outlook}}</ref> [[Lithium]]  extraction from seawater was tried in the 1970s, but the tests were soon abandoned. The idea of extracting [[uranium]]  from seawater has been considered at least from the 1960s, but only a few grams of uranium were extracted in [[Japan]] in the late 1990s.<ref>{{Cite web|title= Mining the Oceans: Can We Extract Minerals from Seawater?
| author= Ugo Bardi
| year= 2008
|url=http://theoildrum.com/node/4558|access-date=2023-02-08|website=theoildrum.com}}</ref> The main issue is not one of technological feasibility but that current prices on the [[uranium market]] for uranium from other sources are about three to five times lower than the lowest price achieved by seawater extraction.<ref>{{Cite web|url=http://large.stanford.edu/courses/2018/ph241/voigt1/|title = Viability of Uranium Extraction from Sea Water}}</ref><ref>{{Cite web|url=https://newatlas.com/nuclear-uranium-seawater-fibers/55033/|title = Cost-effective method of extracting uranium from seawater promises limitless nuclear power|date = 14 June 2018}}</ref> Similar issues hamper the use of [[reprocessed uranium]] and are often brought forth against [[nuclear reprocessing]] and the manufacturing of [[MOX fuel]] as economically unviable.

=== The future of mineral and element extractions ===
In order for seawater mineral and element extractions to take place while taking close consideration of sustainable practices, it is necessary for monitored management systems to be put in place. This requires management of ocean areas and their conditions, [[environmental planning]], structured guidelines to ensure that extractions are controlled, regular assessments of the condition of the sea post-extraction, and constant monitoring.<ref name=":02">{{Cite journal |last=Levin |first=Lisa A. |date=2019 |title=SUSTAINABILITY IN DEEP WATER: The Challenges of Climate Change, Human Pressures, and Biodiversity Conservation |journal=Oceanography |volume=32 |issue=2 |pages=170–180 |doi=10.5670/oceanog.2019.224 |jstor=26651193 |bibcode=2019Ocgpy..32b.170L |issn=1042-8275|doi-access=free }}</ref> The use of technology, such as [[Unmanned underwater vehicle|underwater drones]], can facilitate sustainable extractions.<ref name=":03">{{Cite journal |last=Santos |first=Eleonora |date=2024-04-16 |title=Innovative solutions for coastal and offshore infrastructure in seawater mining: Enhancing efficiency and environmental performance |url=https://www.sciencedirect.com/science/article/pii/S0011916423009141 |journal=Desalination |volume=575 |article-number=117282 |doi=10.1016/j.desal.2023.117282 |bibcode=2024Desal.57517282S |issn=0011-9164|url-access=subscription }}</ref> The use of low-carbon infrastructure would also allow for more sustainable extraction processes while reducing the carbon footprint from mineral extractions.<ref name=":03" />
[[File:Aerial_view_of_Victorian_Desalination_Plant.jpg|thumb|Desalination plant]]
Another practice that is being considered closely is the process of [[desalination]] in order to achieve a more sustainable water supply from seawater. Although desalination also comes with environmental concerns, such as costs and resources, researchers are working closely to determine more sustainable practices, such as creating more productive water plants that can deal with larger water supplies in areas where these plans weren't always available.<ref>{{Cite journal |last1=Ayaz |first1=Muhammad |last2=Namazi |first2=M. A. |last3=Din |first3=M. Ammad ud |last4=Ershath |first4=M. I. Mohamed |last5=Mansour |first5=Ali |last6=Aggoune |first6=el-Hadi M. |date=2022-10-15 |title=Sustainable seawater desalination: Current status, environmental implications and future expectations |url=https://www.sciencedirect.com/science/article/pii/S0011916422004775 |journal=Desalination |volume=540 |article-number=116022 |doi=10.1016/j.desal.2022.116022 |bibcode=2022Desal.54016022A |issn=0011-9164|url-access=subscription }}</ref> Although seawater extractions can benefit society greatly, it is crucial to consider the environmental impact and to ensure that all extractions are conducted in a way that acknowledges and considers the associated risks to the sustainability of seawater ecosystems.

== Standard ==
[[ASTM International]] has an international standard for [[artificial seawater]]: ASTM D1141-98 (Original Standard ASTM D1141-52). It is used in many research testing labs as a reproducible solution for seawater such as tests on corrosion, oil contamination, and detergency evaluation.<ref>{{cite web
 | url = http://www.astm.org/Standards/D1141.htm
 | title = ASTM D1141-98(2013)
 | publisher = ASTM
 | access-date = 17 August 2013
}}</ref>

== Ecosystems ==
{{AI-generated|date=August 2025|section}}
The minerals found in seawater can also play an important role in the ocean and its ecosystem's food cycle. For example, the [[Southern Ocean]] contributes greatly to the environmental [[carbon cycle]]. Given that this body of water does not contain high levels of [[iron]], the deficiency impacts the marine life living in its waters. As a result, this ocean is not able to produce as much [[phytoplankton]] which hinders the first source of the marine food chain.<ref name=":05">{{Cite journal |last1=Nicol |first1=Stephen |last2=Bowie |first2=Andrew |last3=Jarman |first3=Simon |last4=Lannuzel |first4=Delphine |last5=Meiners |first5=Klaus M |last6=Van Der Merwe |first6=Pier |date=2010-05-13 |title=Southern Ocean iron fertilization by baleen whales and Antarctic krill |url=http://dx.doi.org/10.1111/j.1467-2979.2010.00356.x |journal=Fish and Fisheries |volume=11 |issue=2 |pages=203–209 |doi=10.1111/j.1467-2979.2010.00356.x |bibcode=2010AqFF...11..203N |issn=1467-2960|url-access=subscription }}</ref> One of the main types of phytoplankton are [[Diatom|diatoms]] which is the primary food source of [[Antarctic krill]]. As the cycle continues, various larger sea animals feed off of Antarctic krill, but since there is a shortage of iron from the initial phytoplankton/diatoms, then these larger species also lack iron. The larger sea animals include [[Baleen whale|Baleen Whales]] such as the [[Blue whale|Blue Whale]] and [[Fin whale|Fin Whale]].<ref name=":05" /> These whales not only rely on iron for a balance of minerals within their diet, but it also impacts the amount of iron that is regenerated back into the ocean. The whale's excretions also contain the absorbed iron which would allow iron to be reinserted into the ocean’s ecosystem. Overall, one mineral deficiency such as iron in the Southern Ocean can spark a significant chain of disturbances within the marine ecosystems which demonstrates the important role that seawater plays in the [[food chain]].

Upon further analysis of the dynamic relationship between diatoms, krill, and baleen whales, fecal samples of baleen whales were examined in Antarctic seawater.<ref name=":05" /> The findings included that iron concentrations were 10 million times higher than those found in Antarctic seawater, and krill was found consistently throughout their feces which is an indicator that krill is in whale diets.<ref name=":05" /> Antarctic krill had an average iron level of 174.3mg/kg dry weight, but the iron in the krill varied from 12 to 174 mg/kg dry weight.<ref name=":05" /> The average iron concentration of the muscular tissue of blue whales and fin whales was 173 mg/kg dry weight, which demonstrates that the large marine mammals are important to marine ecosystems such as they are to the Southern Ocean.<ref name=":05" /> In fact, to have more whales in the ocean could heighten the amount of iron in seawater through their excretions which would promote a better ecosystem.

Krill and baleen whales act as large iron reservoirs in seawater in the Southern Ocean. Krill can retain up to 24% of iron found on surface waters within its range.<ref name=":05" />The process of krill feeding on diatoms releases iron into seawater, highlighting them as an important part of the ocean's [[iron cycle]]. The advantageous relationship between krill and baleen whales increases the amount of iron that can be recycled and stored in seawater.<ref name=":05" /> A [[Positive feedback|positive feedback loop]] is created, increasing the overall productivity of marine life in the Southern Ocean.

Organisms of all sizes play a significant role in the balance of marine ecosystems with both the largest and smallest inhabitants contributing equally to recycling nutrients in seawater. Prioritizing the recovery of whale populations because they boost the overall productivity in marine ecosystems as well as increasing iron levels in seawater would allow for a balanced and productive system for the ocean. However, a more in depth study is required to understand the benefits of whale feces as a fertilizer and to provide further insight in iron recycling in the Southern Ocean.<ref name=":05" /> Projects on the management of ecosystems and conservation are vital for advancing knowledge of marine ecology.

== Environmental impact and sustainability ==
Like any mineral extraction practices, there are environmental advantages and disadvantages. [[Cobalt]] and [[Lithium]] are two key metals that can be used for aiding with more environmentally friendly technologies above ground, such as powering batteries that energize [[Electric vehicle|electric vehicles]] or creating [[wind power]].<ref>{{Cite journal |last=McCarthy |first=Rebecca |date=2020 |title=Deep Sea Rush: With valuable metals on the ocean floor, speculators are circling |url=https://www.jstor.org/stable/26975674 |journal=The Baffler |issue=54 |pages=114–124 |jstor=26975674 |issn=1059-9789}}</ref> An environmentally friendly approach to mining that allows for more sustainability would be to extract these metals from the seafloor. [[Lithium mining]] from the seafloor at mass quantities could provide a substantial amount of renewable metals to promote more environmentally friendly practices in society to reduce humans' [[carbon footprint]]. Lithium mining from the seafloor could be successful, but its success would be dependent on more productive [[recycling]] practices above ground.<ref>{{Cite journal |last=Bardi |first=Ugo |date=April 2010 |title=Extracting Minerals from Seawater: An Energy Analysis |journal=Sustainability |language=en |volume=2 |issue=4 |pages=980–992 |doi=10.3390/su2040980 |doi-access=free |bibcode=2010Sust....2..980B |issn=2071-1050|hdl=2158/779042 |hdl-access=free }}</ref>
[[File:Ocean_Floor_V_(33712023355).jpg|thumb|Marine life flourishing on the seafloor]]
There are also risks that come with extracting from the seafloor. Many biodiverse species have long lifespans on the seafloor, which means that their reproduction takes more time.<ref name=":02"/> Similarly to fish harvesting from the seafloor, the extraction of minerals in large amounts, too quickly, without proper protocols, can result in a disruption of the underwater ecosystems.<ref name=":02" /> Contrarily, this would have the opposite effect and prevent mineral extractions from being a long-term sustainable practice, and would result in a shortage of required metals. Any seawater mineral extractions also risk disrupting the habitat of the underwater life that is dependent on the uninterrupted ecosystem within their environment as disturbances can have significant disturbances on animal communities.<ref name=":02" />

==See also==
{{Portal|Oceans}}
* {{annotated link|Artificial seawater}}
* {{annotated link|Brackish water}}
* {{annotated link|Brine}}
* {{annotated link|Brine mining}}
* {{annotated link|CORA dataset}} global ocean salinity
* {{annotated link|Fresh water}}
* {{annotated link|Ocean color}}
* {{annotated link|Saline water}}
* {{annotated link|Sea ice}}
* {{annotated link|PH#Seawater|Seawater pH}}
* {{annotated link|Surface tension#Surface tension of seawater|Surface tension of seawater}}
* {{annotated link|Thalassotherapy}}
* {{annotated link|Thermohaline circulation}}

==References==
{{Reflist|30em}}

==External links==
{{Spoken Wikipedia|En-seawater-article.ogg|date=2014-08-16}}

* [http://ioc-unesco.org/index.php?option=com_oe&task=viewDocumentRecord&docID=3939UNESCO Technical Papers in Marine Science 44, Algorithms for computation of fundamental properties of seawater, ioc-unesco.org, UNESCO 1983]
Tables
* [http://web.mit.edu/seawater/ Tables and software for thermophysical properties of seawater], MIT
* {{cite book|url=http://www.kayelaby.npl.co.uk/general_physics/2_7/2_7_9.html|chapter=Physical properties of sea water|archive-url=https://web.archive.org/web/20190508003610/http://www.kayelaby.npl.co.uk/general_physics/2_7/2_7_9.html|archive-date=2019-05-08|title=Tables of physical and chemical constants|author=G. W. C Kaye, T. H. Laby|edition=16th|year=1995|bibcode=1995tpcc.book.....K }}

{{physical oceanography|expanded=other}}

{{Authority control}}

[[Category:Aquatic ecology]]
[[Category:Chemical oceanography]]
[[Category:Liquid water]]
[[Category:Physical oceanography]]
[[Category:Oceanographical terminology]]