Salinity

Salinity is the saltiness or dissolved salt content of a body of water. Salinity in Australian English and North American English may also refer to the salt in soil (see soil salination).

Definitions
The technical term for saltiness in the ocean is salinity, from the fact that halides - chloride specifically - are the most abundant anions in the mix of dissolved elements. In oceanography, it has been traditional to express salinity not as percent, but as parts per thousand (ppt or ‰), which is approximately grams of salt per liter 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 ‰ 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 the Practical Salinity Scale (PSS) as the conductivity ratio of a sea water sample to a standard KCl solution. Ratios have no units, so it is not the case that a salinity of 35 exactly equals 35 grams of salt per litre 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 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 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. 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. 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).