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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).
Definitions
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).
Systems of classification of water bodies based upon salinity
| THALASSIC SERIES | |
| >300‰ | |
| hyperhaline | |
| 60 - 80‰ | |
| metahaline | |
| 40‰ | |
| mixoeuhaline | |
| 30‰ | |
| polyhaline | |
| 18‰ | |
| mesohaline | |
| 5‰ | |
| oligohaline | |
| 0.5‰ |
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.
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 (or by a groundwater). A plant adapted to saline conditions is called a halophyte (for salt loving). 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).
See also
References
- 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.
Chemical oceanography | Ecology | Oceanography
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"Salinity"