|Fresh water||Brackish water||Saline water||Brine|
|< 0.05%||0.05 – 3%||3 – 5%||> 5%|
|< 0.5 ‰||0.5 – 30 ‰||30 – 50 ‰||> 50 ‰|
Salinity in the ocean refers to the water's "saltiness". In oceanography, it has been traditional to express salinity not as percent, but as permille (parts per thousand) (‰), which is approximately grams of salt per kilogram of solution. Other disciplines use chemical analyses of solutions, and thus salinity is frequently reported in mg/L or ppm (parts per million). Prior to 1978, salinity or halinity was expressed as Cl ‰, usually based on comparison with IAPSO Standard Seawater ("Copenhagen water"), a natural sea water distributed to serve as a world standard. In 1978, oceanographers redefined salinity in the Practical Salinity Scale (PSS) as the conductivity ratio of a sea water sample to a standard KCl solution. Although PSS is a dimensionless quantity, its "unit" is usually called PSU. It is not the case that a salinity of 35 exactly equals 35 grams of salt per liter of solution.
These seemingly esoteric approaches to measuring and reporting salt 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 also 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).
Contour lines of constant salinity are called isohales.
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 system" (1959).
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.
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. 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).
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 
- World Ocean Atlas 2009
- Lewis, E. L. (1982). "The practical salinity scale of 1978 and its antecedents". Marine Geodesy 5 (4): 350. doi:10.1080/15210608209379432.
- Unesco (1981). The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980. Tech. Pap. Mar. Sci., 36
- Unesco (1981). Background papers and supporting data on the Practical Salinity Scale 1978. Tech. Pap. Mar. Sci., 37
- Unesco (1985). The International System of Units (SI) in Oceanography. Tech. Pap. Mar. Sci., 45
- Por, F. D. (1972). "Hydrobiological notes on the high-salinity waters of the Sinai Peninsula". Marine Biology 14 (2): 111. doi:10.1007/BF00373210.
- Venice system (1959). The final resolution of the symposium on the classification of brackish waters. Archo Oceanogr. Limnol., 11 (suppl): 243–248.
- Dahl, E. (1956). "Ecological salinity boundaries in poikilohaline waters". Oikos (Oikos) 7 (1): 1–21. doi:10.2307/3564981. JSTOR 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.
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 salinity.
- Practical Salinity Scale 1978.
- Practical 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