Salinity is the saltiness or dissolved salt content of a body of water. 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
a isohalíne – or sometimes isohale.
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. Operationally, dissolved matter is defined as that which can pass through
a very fine filter. 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 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 %. 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 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 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, rather
than to the unknown mass of salts that gave rise to this composition. 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 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. Seawater
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 nonspecialists. 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 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. The use of electrical conductivity measurements
to estimate the ionic content of seawater led to the development of the Practical Salinity
Scale 1978. Salinities measured using PSS-78 do not have units. The 'unit' of PSU denoting
Practical Salinity Units) is sometimes added to PSS-78 measurements, however this is officially
discouraged. In 2010 a new standard for the properties
of seawater was introduced, the Thermodynamic Equation of Seawater 2010. This standard includes
a new scale, 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 of the
order of 1%. Limnologists also use electrical conductivity, or "reference conductivity",
as a proxy for salinity called. This measurement may be corrected for temperature effects,
and is usually expressed in units of μS/cm. 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 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 measured density.
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. The table on the right, modified from Por, follows the "Venice system".
In contrast to homoiohaline environments are certain poikilohaline environments 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, is referred to as brine.
Environmental considerations 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. 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 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.
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.
See also
References
Rezazadeh, A., Ghasemnezhad, A., Barani, M., & Telmadarrehei, T.. Effect of Salinity on
Phenolic Composition and Antioxidant Activity of Artichoke Leaves. Research Journal of Medicinal
Plant, 6(3). Further reading
Mantyla, A.W. 1987. Standard Seawater Comparisons updated. J. Phys. Ocean., 17: 543–548.
Equations and algorithms to calculate fundamental properties of sea water.
History of the salinity determination Practical Salinity Scale 1978.
Salinity calculator 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