Salinity is the saltiness or amount of salt dissolved in a body of
water (see also soil salinity). This is usually measured in
displaystyle frac g textrm salt kg textrm sea textrm
(note that this is technically dimensionless).
Salinity is an
important factor in determining many aspects of the chemistry of
natural waters and of biological processes within it, and is a
thermodynamic state variable that, along with temperature and
pressure, governs physical characteristics like the density and heat
capacity of the water.
A contour line of constant salinity is called an isohaline, or
1.3 Lakes and rivers
2 Systems of classification of water bodies based upon salinity
2.1 Environmental considerations
3 See also
5 Further reading
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 nowadays usually 0.2 μm).
Salinity can be
expressed in the form of a 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 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.
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 ocean,
however, are often concerned with precision and intercomparability of
measurements by different researchers, at different times, to almost
five significant digits. A bottled seawater product known as IAPSO
Seawater is used by oceanographers to standardize their
measurements with enough precision to meet this requirement.
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, although for seawater at highest
precision an additional seven minor ions are also included. The
major ions dominate the inorganic composition of most (but by no means
all) natural waters. Exceptions include some pit lakes and waters from
some hydrothermal springs.
The concentrations of dissolved gases like oxygen and nitrogen are not
usually included in descriptions of salinity. 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).
Full 3 minute NASA video Feb 27,2013 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
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. The
distinctions between these different descriptions are important to
physical oceanographers but are obscure and confusing to
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 ‰).
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). Salinities measured
using PSS-78 do not have units. The suffix psu or PSU (denoting
practical salinity unit) is sometimes added to PSS-78 measurement
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. 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 ppt will have a Knudsen salinity of 35.00 ppt, a
PSS-78 practical salinity of about 35.0, and a TEOS-10 absolute
salinity of about 35.2 g/kg. The electrical conductivity of this
water at a temperature of 15 °C is 42.9 mS/cm.
Lakes and rivers
Limnologists and chemists often define salinity in terms of mass of
salt per unit volume, expressed in units of mg per litre or g per
litre. It is implied, although often not stated, that this value
applies accurately only at some reference 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
A river or lake water with a salinity of around 70 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.
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 lakes. 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
0.05 – 3%
3 – 5%
< 0.5 ‰
0.5 – 30 ‰
30 – 50 ‰
> 50 ‰
Systems of classification of water bodies based upon salinity
Marine waters are those of the ocean, another term for which is
euhaline seas. The salinity of euhaline seas is 30 to 35. Brackish
seas or waters have salinity in the range of 0.5 to 29 and metahaline
seas from 36 to 40. These waters are all regarded as thalassic because
their salinity is derived from the ocean and defined as homoiohaline
if salinity does not vary much over time (essentially constant). The
table on the right, modified from Por (1972), follows the "Venice
In contrast to homoiohaline environments are certain poikilohaline
environments (which may also be thalassic) in which the salinity
variation is biologically significant. Poikilohaline water
salinities may range anywhere from 0.5 to greater than 300. The
important characteristic is that these waters tend to vary in salinity
over some biologically meaningful range seasonally or on some other
roughly comparable time scale. Put simply, these are bodies of water
with quite variable salinity.
Highly saline water, from which salts crystallize (or are about to),
is referred to as brine.
Salinity is an ecological factor of considerable importance,
influencing the types of organisms that live in a body of water. As
well, salinity influences the kinds of plants that will grow either in
a water body, or on land fed by a water (or by a groundwater). A
plant adapted to saline conditions is called a halophyte. A halophyte
which is tolerant to residual sodium carbonate salinity are called
glasswort or saltwort or barilla plants. Organisms (mostly bacteria)
that can live in very salty conditions are classified as
extremophiles, or halophiles specifically. An organism that can
withstand a wide range of salinities is euryhaline.
Salt is expensive to remove from water, and salt content is an
important factor in water use (such as potability). Increases in
salinity have been observed in lakes and rivers in the United States,
due to common road salt and other salt de-icers in runoff.
The degree of salinity in oceans is a driver of the world's ocean
circulation, where density changes due to both salinity changes and
temperature changes at the surface of the ocean produce changes in
buoyancy, which cause the sinking and rising of water masses. Changes
in the salinity of the oceans are thought to contribute to global
changes in carbon dioxide as more saline waters are less soluble to
carbon dioxide. In addition, during glacial periods, the hydrography
is such that a possible cause of reduced circulation is the production
of stratified oceans. Hence it is difficult in this case to subduct
water through the thermohaline circulation.
Soil salinity control
Sodium adsorption ratio
CORA dataset salinity of global oceans
World Ocean Atlas 2009. nodc.noaa.gov
^ a b Pawlowicz, R. (2013). "Key Physical Variables in the Ocean:
Temperature, Salinity, and Density". Nature Education Knowledge. 4
^ Eilers, J. M.; Sullivan, T. J.; Hurley, K. C. (1990). "The most
dilute lake in the world?". Hydrobiologica. 199: 1–6.
^ a b Anati, D. A. (1999). "The salinity of hypersaline brines:
concepts and misconceptions". Int. J. Salt Lake. Res. 8: 55–70.
^ a b c d IOC, SCOR, and IAPSO (2010). The international thermodynamic
equation of seawater – 2010: Calculation and use of thermodynamic
properties. Intergovernmental Oceanographic Commission, UNESCO
(English). pp. 196pp. CS1 maint: Multiple names: authors
^ a b Wetzel, R. G. (2001). Limnology: Lake and River Ecosystems, 3rd
ed. Academic Press. ISBN 978-0-12-744760-5.
^ Pawlowicz, R.; Feistel, R. (2012). "Limnological applications of the
Thermodynamic Equation of
Seawater 2010 (TEOS-10)". Limnology and
Oceanography: Methods. 10 (11): 853–867.
^ Unesco (1981). The Practical
Salinity Scale 1978 and the
International Equation of State of
Seawater 1980. Tech. Pap. Mar.
^ Unesco (1981). Background papers and supporting data on the
Salinity Scale 1978. Tech. Pap. Mar. Sci., 37
^ Millero, F. J. (1993). "What is PSU?". Oceanography. 6 (3):
^ Culkin, F.; Smith, N. D. (1980). "Determination of the Concentration
of Potassium Chloride Solution Having the Same Electrical
Conductivity, at 15C and Infinite Frequency, as Standard
Salinity 35.0000‰ (
Chlorinity 19.37394‰)". IEEE J. Oceanic Eng.
OE–5 (1): 22–23. doi:10.1109/JOE.1980.1145443.
^ van Niekerk, Harold; Silberbauer, Michael; Maluleke, Mmaphefo
(2014). "Geographical differences in the relationship between total
dissolved solids and electrical conductivity in South African rivers".
Water SA. 40 (1): 133. doi:10.4314/wsa.v40i1.16.
^ Por, F. D. (1972). "Hydrobiological notes on the high-salinity
waters of the Sinai Peninsula". Marine Biology. 14 (2): 111.
^ Venice system (1959). The final resolution of the symposium on the
classification of brackish waters. Archo Oceanogr. Limnol., 11
^ Dahl, E. (1956). "Ecological salinity boundaries in poikilohaline
waters". Oikos. Oikos. 7 (1): 1–21. doi:10.2307/3564981.
^ Kalcic, Maria, Turowski, Mark; Hall, Callie. "Stennis Space Center
Salinity Drifter Project. A Collaborative Project with Hancock High
School, Kiln, MS". Stennis Space Center
Salinity Drifter Project.
NTRS. Retrieved 2011-06-16.
^ Hopes To Hold The Salt, And Instead Break Out Beet Juice And Beer To
Keep Roads Clear
Mantyla, A.W. 1987. Standard
Seawater Comparisons updated. J. Phys.
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MIT page of seawater properties, with Matlab, EES and Excel VBA
Equations and algorithms to calculate fundamental properties of sea
History of the salinity determination
Salinity Scale 1978.
Lewis, E. L. 1982. The practical salinity scale of 1978 and its
antecedents. Marine Geodesy. 5(4):350–357.
Equations and algorithms to calculate salinity of inland waters
Acid mine drainage
Adsorbable organic halides
Biochemical oxygen demand
Chemical oxygen demand
Total dissolved solids
Total suspended solids
Agricultural wastewater treatment
API oil-water separator
Decentralized wastewater system
Fecal sludge management
Industrial wastewater treatment
Rotating biological contactor
Sewage sludge treatment
Ultraviolet germicidal irradiation
Wastewater treatment plant
Septic drain field