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Salinity refers to the concept of saltiness of a body of water. Water is defined as saline if it contains 3 to 5% salt by volume. The ocean is naturally saline at ~3.5% salt (see sea water). Some inland salt lakes (seas) are even saltier. 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 solids. In oceanography, it has been traditional to express salinity as concentration in parts per thousand (ppt or o/oo), which is grams salt per liter of water.

The salt content of most lakes, rivers, and streams is so small that these waters are termed fresh or even sweet water. Salt is difficult to remove from water, and salt content is a factor in water potability. 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 a saline conditions is called a halophyte (for salt loving). See also biosalinity. Animals and bacteria that can live in very salty conditions are classified as extremophiles.

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 range for euhaline seas is 30 to 35 o/oo. brackish seas or waters have salinity in the range of 0.5 to 29 o/oo; and metahaline seas from 36 to 40 o/oo. These waters are all grouped as homoiohaline because their salinity is derived from the ocean (thalassic) and essentially invariant, in contrast to poikilohaline environments in which the salinity variation is biologically significant (Dahl, 1956).

Poikilohaline waters may range anywhere from 0.5 o/oo to greater than 300 o/oo. 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 variable salinity. The following table, modified from Por (1972) follows the "Venice system" (1959):

>300 o/oo --------------------
60 - 80 o/oo --------------------
40 o/oo --------------------
30 o/oo --------------------
18 o/oo --------------------
5 o/oo --------------------
0.5 o/oo --------------------

Highly saline water is referred to as brine.