Editing Salinity (section)

==Definitions==
Salinity in rivers, lakes, and the ocean is conceptually simple, but technically challenging to define and measure precisely. Conceptually the salinity is the quantity of dissolved salt content of the water. Salts are compounds like [[sodium chloride]], [[magnesium sulfate]], [[potassium nitrate]], and [[sodium bicarbonate]] which dissolve into ions. The concentration of dissolved chloride ions is sometimes referred to as chlorinity. Operationally, dissolved matter is defined as that which can pass through a very fine filter (historically a filter with a pore size of 0.45 μm, but later{{When|date=November 2022}} usually 0.2 μm).<ref name=key>{{cite journal|last=Pawlowicz|first=R.|title=Key Physical Variables in the Ocean: Temperature, Salinity, and Density|journal=Nature Education Knowledge|year=2013|volume=4|issue=4|pages=13|url=http://www.nature.com/scitable/knowledge/library/key-physical-variables-in-the-ocean-temperature-102805293}}</ref> Salinity can be expressed in the form of a [[Mass fraction (chemistry)|mass fraction]], i.e. the mass of the dissolved material in a unit mass of solution.

Seawater typically has a mass salinity of around 35 g/kg, although lower values are typical near coasts where rivers enter the ocean. Rivers and lakes can have a wide range of salinities, from less than 0.01 g/kg<ref name=dilute>{{cite journal|last1=Eilers|first1=J. M.|last2=Sullivan|first2=T. J.|last3=Hurley|first3=K. C.|title=The most dilute lake in the world?|journal=Hydrobiologia|year=1990|volume=199|issue=1 |pages=1–6|doi=10.1007/BF00007827|bibcode=1990HyBio.199....1E |s2cid=30279782}}</ref> to a few g/kg, although there are many places where higher salinities are found. The [[Dead Sea]] has a salinity of more than 200 g/kg.<ref name=anati>{{cite journal|last=Anati|first=D. A.|title=The salinity of hypersaline brines: concepts and misconceptions|journal=Int. J. Salt Lake. Res.| year=1999|volume=8|issue=1 |pages=55–70|doi=10.1007/bf02442137|bibcode=1999IJSLR...8...55A }}</ref> Precipitation typically has a [[Total dissolved solids|TDS]] of 20 mg/kg or less.<ref>{{cite web | url=http://www.salinitymanagement.org/Salinity%20Management%20Guide/ls/ls_3d.html | title=Learn about salinity and water quality | access-date=21 July 2018}}</ref>

Whatever pore size is used in the definition, the resulting salinity value of a given sample of natural water will not vary by more than a few [[percent]] (%). Physical oceanographers working in the [[Abyssal zone|abyssal ocean]], however, are often concerned with precision and intercomparability of measurements by different researchers, at different times, to almost five [[Significant figures|significant digits]].<ref name="teos10"/> A bottled seawater product known as IAPSO Standard Seawater is used by oceanographers to standardize their measurements with enough precision to meet this requirement.

===Composition===
Measurement and definition difficulties arise because natural waters contain a complex mixture of many different elements from different sources (not all from dissolved salts) in different molecular forms. The chemical properties of some of these forms depend on temperature and pressure. Many of these forms are difficult to measure with high accuracy, and in any case complete chemical analysis is not practical when analyzing multiple samples. Different practical definitions of salinity result from different attempts to account for these problems, to different levels of precision, while still remaining reasonably easy to use.

For practical reasons salinity is usually related to the sum of masses of a subset of these dissolved chemical constituents (so-called ''solution salinity''), rather than to the unknown mass of salts that gave rise to this composition (an exception is when [[artificial seawater]] is created).  For many purposes this sum can be limited to a set of eight major ions in natural waters,<ref name=wetzel>{{cite book|last=Wetzel|first=R. G.|title=Limnology: Lake and River Ecosystems, 3rd ed.|year=2001|publisher=Academic Press|isbn=978-0-12-744760-5}}</ref><ref name=limteos>{{cite journal|last1=Pawlowicz|first1=R.|first2=R.|last2=Feistel|title=Limnological applications of the Thermodynamic Equation of Seawater 2010 (TEOS-10)|journal=Limnology and Oceanography: Methods|year=2012|volume=10|issue=11|pages=853–867| doi=10.4319/lom.2012.10.853|bibcode=2012LimOM..10..853P |s2cid=93210746 |doi-access=free}}</ref> although for seawater at highest precision an additional seven minor ions are also included.<ref name=teos10>{{cite book|last=IOC, SCOR, and IAPSO|title=The international thermodynamic equation of seawater – 2010: Calculation and use of thermodynamic properties |year=2010 |publisher=Intergovernmental Oceanographic Commission, UNESCO (English) |pages=196pp |url=http://www.TEOS-10.org}}</ref>  The major ions dominate the inorganic composition of most (but by no means all) natural waters. Exceptions include some [[Cenote|pit lake]]s and waters from some [[hydrothermal spring]]s.

The concentrations of dissolved gases like [[oxygen]] and [[nitrogen]] are not usually included in descriptions of salinity.<ref name=key /> However, [[carbon dioxide]] gas, which when dissolved is partially converted into [[carbonates]] and [[bicarbonates]], is often included. [[Silicon]] in the form of [[silicic acid]], which usually appears as a neutral molecule in the [[pH]] range of most natural waters, may also be included for some purposes (e.g., when salinity/density relationships are being investigated).

===Seawater===
{{See also|Ocean#Salinity}}
[[File:Aquarius flat 2048x1024.ogv|thumb|300px|
[https://www.youtube.com/watch?v=5xQP_B18vMw <sup>Full 3 minute NASA video Feb 27,2013</sup>]
The NASA Aquarius instrument aboard Argentina's SAC-D satellite is designed to measure global sea surface salinity. This movie shows salinity patterns as measured by Aquarius from December 2011 through December 2012. Red colors represent areas of high salinity, while blue shades represent areas of low salinity.]]

The term ''salinity'' is, for oceanographers, usually associated with one of a set of specific measurement techniques. As the dominant techniques evolve, so do different descriptions of salinity. Salinities were largely measured using [[titration]]-based techniques before the 1980s. Titration with [[silver nitrate]] could be used to determine the concentration of [[halide]] ions (mainly [[chlorine]] and [[bromine]]) to give a [[chlorinity]]. The chlorinity was then multiplied by a factor to account for all other constituents. The resulting 'Knudsen salinities' are expressed in units of [[parts per thousand]] (ppt or [[Per mille|‰]]).

The use of [[electrical conductivity]] measurements to estimate the ionic content of seawater led to the development of the scale called the ''practical salinity scale 1978'' (PSS-78).<ref name="pss78a">Unesco (1981). The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980. ''Tech. Pap. Mar. Sci.'', 36</ref><ref name="pss78b">Unesco (1981). [http://unesdoc.unesco.org/images/0004/000479/047932eb.pdf Background papers and supporting data on the Practical Salinity Scale 1978]. ''Tech. Pap. Mar. Sci.'', 37</ref> Salinities measured using PSS-78 do not have units. {{anchor|PSU}}The suffix '''psu''' or '''PSU''' (denoting ''practical salinity unit'') is sometimes added to PSS-78 measurement values.<ref name=rant>{{cite journal|last=Millero|first=F. J.|journal=Oceanography| year=1993|volume=6| issue=3| pages=67|title=What is PSU?}}</ref> The addition of PSU as a unit after the value is "formally incorrect and strongly discouraged".<ref name=key />

In 2010 a new standard for the properties of seawater called the ''thermodynamic equation of seawater 2010'' ([[TEOS-10]]) was introduced, advocating absolute salinity as a replacement for practical salinity, and [[conservative temperature]] as a replacement for [[potential temperature]].<ref name="teos10"/> This standard includes a new scale called the ''reference composition salinity scale''. Absolute salinities on this scale are expressed as a mass fraction, in grams per kilogram of solution. Salinities on this scale are determined by combining electrical conductivity measurements with other information that can account for regional changes in the composition of seawater. They can also be determined by making direct density measurements.

A sample of seawater from most locations with a chlorinity of 19.37&nbsp;ppt will have a Knudsen salinity of 35.00&nbsp;ppt, a PSS-78 practical salinity of about 35.0, and a TEOS-10 absolute salinity of about 35.2&nbsp;g/kg. The electrical conductivity of this water at a temperature of 15&nbsp;°C is 42.9 mS/cm.<ref name="teos10"/><ref name=culk>{{cite journal|last1=Culkin|first1=F.|last2=Smith|first2=N. D.|title=Determination of the Concentration of Potassium Chloride Solution Having the Same Electrical Conductivity, at 15C and Infinite Frequency, as Standard Seawater of Salinity 35.0000‰ (Chlorinity 19.37394‰)|journal=IEEE J. Oceanic Eng.|year=1980|volume=OE-5|issue=1|pages=22–23|doi=10.1109/JOE.1980.1145443|bibcode=1980IJOE....5...22C}}</ref>

On the global scale, it is extremely likely that human-caused climate change has contributed to observed surface and subsurface salinity changes since the 1950s, and projections of surface salinity changes throughout the 21st century indicate that fresh ocean regions will continue to get fresher and salty regions will continue to get saltier.<ref name=":0">{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H.T. |author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S.S. |last6=Edwards |first6=T.L. |last7=Golledge |first7=N.R. |last8=Hemer |first8=M. |last9=Kopp |first9=R.E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Ocean, Cryosphere and Sea Level Change |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |publisher=Cambridge University Press |publication-place=Cambridge, UK and New York, New York, US |volume=2021 |pages=1211–1362 |bibcode=<!-- not 2021AGUFM.U13B..09F --> |doi=10.1017/9781009157896.011 |isbn=9781009157896 |url=https://www.vliz.be/imisdocs/publications/84/371584.pdf }}</ref>

Salinity is serving as a tracer of different masses. Surface water is pulled in to replace the sinking water, which in turn eventually becomes cold and salty enough to sink. Salinity distribution contributes to shape the oceanic circulation.

===Lakes and rivers===

[[Limnologist]]s and chemists often define salinity in terms of mass of salt per unit volume, expressed in units of mg/L or g/L.<ref name=wetzel/> It is implied, although often not stated, that this value applies accurately only at some reference temperature because solution volume varies with temperature. Values presented in this way are typically accurate to the order of 1%. Limnologists also use [[electrical conductivity]], or "reference conductivity", as a proxy for salinity. This measurement may be corrected for temperature effects, and is usually expressed in units of [[Siemens (unit)|μS/cm]].

A river or lake water with a salinity of around 70&nbsp;mg/L will typically have a specific conductivity at 25 °C of between 80 and 130 μS/cm. The actual ratio depends on the ions present.<ref>{{cite journal|last1=van Niekerk|first1=Harold|last2=Silberbauer|first2=Michael|last3=Maluleke|first3=Mmaphefo|title=Geographical differences in the relationship between total dissolved solids and electrical conductivity in South African rivers|journal=Water SA|date=2014|volume=40|issue=1|pages=133|doi=10.4314/wsa.v40i1.16|doi-access=free|bibcode=2014WatSA..40..133V }}</ref> The actual conductivity usually changes by about 2% per degree Celsius, so the measured conductivity at 5 °C might only be in the range of 50–80 μS/cm.

Direct density measurements are also used to estimate salinities, particularly in highly [[saline lake]]s.<ref name="anati"/> Sometimes density at a specific temperature is used as a proxy for salinity. At other times an empirical salinity/density relationship developed for a particular body of water is used to estimate the salinity of samples from a measured density.
{| id="SalinityTable" style="margin-left:auto; margin-right:auto;"
|-
! style="background:#B8E0F6" colspan="4"|Water   salinity
|-style="background:#87CEFA"
![[Fresh water]]
![[Brackish water]]
![[Saline water]]
![[Brine]]
|-style="background:#00BFFF"
! < 0.05%
! 0.05 – 3%
! 3 – 5%
! > 5%
|-style="background:#00BFFF"
! < 0.5 ‰
! 0.5 – 30 ‰
! 30 – 50 ‰
! > 50 ‰
|}