Salinity
Salinity is the
saltiness or dissolved salt content of a body of
water.
Salinity in
Australian English may refer to salt in
soil (see
salinity in Australia).
The salt content of most natural lakes, rivers, and streams is so small that these waters are termed
fresh or even
sweet water. The actual amount of salt in fresh water is, by definition, less than 0.05%. Otherwise, the water is regarded as
brackish, or defined as
saline if it contains 3 to 5% salt by volume. At well over 5% it is considered
brine. The ocean is naturally saline at approximately 3.5% salt (see
sea water). Some inland salt lakes or
seas are even saltier. The
Dead Sea, for example, has a surface water salt content of around 15%.
The technical term for saltiness in the ocean is
halinity, from the fact that
halides—
chloride specifically—are the most abundant
anion in the mix of dissolved elements. In
oceanography, it has been traditional to express halinity not as percent, but as parts per thousand (ppt or ‰), which is approximately grams of salt per liter of solution. Prior to 1978, salinity or halinity was expressed as ‰ usually based on the
electrical conductivity ratio of the sample to "Copenhagen water", an artificial sea water manufactured to serve as a world "standard". In 1978, oceanographers redefined salinity in
Practical Salinity Units (
psu): the conductivity ratio of a sea water sample to a standard KCl solution. Ratios have no units, so it is not the case that 35 psu exactly equals 35 grams of salt per litre of solution.
These seemingly esoteric approaches to measuring and reporting concentrations may appear to obscure their practical use; but it must be remembered that salinity is the sum weight of many different elements within a given volume of water. It has always been the case that to get a precise salinity as a concentration and convert this to an amount of substance (sodium chloride, for instance) required knowing much more about the sample and the measurement than just the weight of the solids upon evaporation (one method of determining "salinity"). For example, volume is influenced by water temperature; and the composition of the salts is not a constant (although generally very much the same throughout the world ocean). Saline waters from inland seas can have a composition that differs from that of the ocean. For the latter reason, these waters are termed
saline as differentiated from ocean waters, where the term
haline applies (although is not universally used).
| THALASSIC SERIES |
| >300‰ | |
| hyperhaline |
| 60 - 80‰ | |
| metahaline |
| 40‰ | |
| mixoeuhaline |
| 30‰ | |
| polyhaline |
| 18‰ | |
| mesohaline |
| 5‰ | |
| oligohaline |
| 0.5‰ | |
Marine waters are those of the ocean, another term for which is
euhaline seas. The salinity range for 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 invariant). The table on the right, modified from Por (1972), follows the "Venice system" (1959).
In contrast to homoiohaline environments are certain
poikilohaline environments (which may also be
thallassic) in which the salinity variation is biologically significant (Dahl, 1956).
Poikilohaline waters 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. Organisms (mostly bacteria) that can live in very salty conditions are classified as
extremophiles,
halophiles specifically. An organism that can withstand a wide range of salinities is
euryhaline.
Salt is difficult to remove from water, and salt content is an important factor in water use (such as
potability).
*
Soil salination*
Desalination* Dahl, E. 1956. Ecological salinity boundaries in poikilohaline waters.
Oikos, 7(I): 1–21.
* Lewis, E.L. 1980. The Practical Salinity Scale 1978 and its antecedents.
IEEE J. Ocean. Eng., OE-5(1): 3-8.
* Mantyla, A.W. 1987. Standard Seawater Comparisons updated.
J. Phys. Ocean., 17: 543-548.
* Por, F. D. 1972. Hydrobiological notes on the high-salinity waters of the Sinai Peninsula.
Mar. Biol., 14(2): 111–119.
* Unesco. 1981a. The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980.
Tech. Pap. Mar. Sci., 36: 25 pp.
* Unesco. 1981b. Background papers and supporting data on the Practical Salinity Scale 1978.
Tech. Pap. Mar. Sci., 37: 144 pp.
* Unesco. 1985. The International System of Units (SI) in Oceanography.
Tech. Pap. Mar. Sci., 45: 124 pp.
* Venice system. 1959. Final resolution of the symposium on the classification of brackish waters.
Archo Oceanogr. Limnol., 11 (suppl): 243–248.
