Salinity
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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).
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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. 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 ocean saltiness 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 salinity not as percent, but as concentration in parts per thousand (ppt or ‰), which is grams of salt per liter of water. After 1978, oceanographers defined salinity as the electrical conductivity ratio of a sea water to a standard KCl solution. Ratios have no units, so a salinity of 35 is essentially the same as a halinity of 35 ‰. This seemingly esoteric approach appears to obscure the practical use of the term; 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 and then convert this to an amount of a 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.
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‰ | -------------------- |
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 are or are about to crystallize out of, is referred to as brine.
Environmental Considerations
Salinity is an ecological factor of considerable import, 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.
Salt is difficult to remove from water, and salt content is an important factor in water use (such as potability).
- See also: biosalinity.
References
- Dahl, E. 1956. Ecological salinity boundaries in poikilohaline waters. Oikos, 7(I): 1–21.
- Por, F. D. 1972. Hydrobiological notes on the high-salinity waters of the Sinai Peninsula. Mar. Biol., 14(2): 111–119.
- Venice system. 1959. Final resolution of the symposium on the classification of brackish waters. Archo Oceanogr. Limnol., 11 (suppl): 243–248.