Salinity () is the saltiness or amount of

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

dissolved in a body of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

, called

saline water Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish water, ...

(see also

soil salinity Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization. Salts occur naturally within soils and water. Salination can be caused by natural processes such as mineral weathering or by the ...

). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal to ‰). Salinity is an important factor in determining many aspects of the

chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

of natural waters and of

biological Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...

processes within it, and is a thermodynamic state variable that, along with

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

, governs physical characteristics like the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

and

heat capacity Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). Heat capacity i ...

of the water. A

contour line A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensional grap ...

of constant salinity is called an ''isohaline'', or sometimes ''isohale''.

Definitions

Salinity in rivers, lakes, and the ocean is conceptually simple, but technically challenging to define and measure precisely. Conceptually the salinity is the quantity of dissolved salt content of the water. Salts are compounds like

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

magnesium sulfate Magnesium sulfate or magnesium sulphate (in English-speaking countries other than the US) is a chemical compound, a salt with the formula , consisting of magnesium cations (20.19% by mass) and sulfate anions . It is a white crystalline solid, ...

potassium nitrate Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and nitrat ...

, and

sodium bicarbonate Sodium bicarbonate (IUPAC name: sodium hydrogencarbonate), commonly known as baking soda or bicarbonate of soda, is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cation ( Na+) and a bicarbonate anion ( HCO3−) ...

which dissolve into ions. The concentration of dissolved chloride ions is sometimes referred to as chlorinity. Operationally, dissolved matter is defined as that which can pass through a very fine filter (historically a filter with a pore size of 0.45 μm, but nowadays usually 0.2 μm). Salinity can be expressed in the form of a mass fraction, i.e. the mass of the dissolved material in a unit mass of solution. Seawater typically has a mass salinity of around 35 g/kg, although lower values are typical near coasts where rivers enter the ocean. Rivers and lakes can have a wide range of salinities, from less than 0.01 g/kg to a few g/kg, although there are many places where higher salinities are found. The

Dead Sea The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Bank ...

has a salinity of more than 200 g/kg. Precipitation typically has a TDS of 20 mg/kg or less. Whatever pore size is used in the definition, the resulting salinity value of a given sample of natural water will not vary by more than a few

percent In mathematics, a percentage (from la, per centum, "by a hundred") is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign, "%", although the abbreviations "pct.", "pct" and sometimes "pc" are also use ...

(%). Physical oceanographers working in the abyssal ocean, however, are often concerned with precision and intercomparability of measurements by different researchers, at different times, to almost five

significant digits Significant figures (also known as the significant digits, ''precision'' or ''resolution'') of a number in positional notation are digits in the number that are reliable and necessary to indicate the quantity of something. If a number expre ...

. A bottled seawater product known as IAPSO Standard Seawater is used by oceanographers to standardize their measurements with enough precision to meet this requirement.

Composition

Measurement and definition difficulties arise because natural waters contain a complex mixture of many different elements from different sources (not all from dissolved salts) in different molecular forms. The chemical properties of some of these forms depend on temperature and pressure. Many of these forms are difficult to measure with high accuracy, and in any case complete chemical analysis is not practical when analyzing multiple samples. Different practical definitions of salinity result from different attempts to account for these problems, to different levels of precision, while still remaining reasonably easy to use. For practical reasons salinity is usually related to the sum of masses of a subset of these dissolved chemical constituents (so-called ''solution salinity''), rather than to the unknown mass of salts that gave rise to this composition (an exception is when

artificial seawater Artificial seawater (abbreviated ASW) is a mixture of dissolved mineral salts (and sometimes vitamins) that simulates seawater. Artificial seawater is primarily used in marine biology and in marine and reef aquaria, and allows the easy preparation ...

is created). For many purposes this sum can be limited to a set of eight major ions in natural waters, although for seawater at highest precision an additional seven minor ions are also included. The major ions dominate the inorganic composition of most (but by no means all) natural waters. Exceptions include some pit lakes and waters from some

hydrothermal spring A hot spring, hydrothermal spring, or geothermal spring is a Spring (hydrology), spring produced by the emergence of Geothermal (geology), geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow ...

s. The concentrations of dissolved gases like

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

and

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

are not usually included in descriptions of salinity. However,

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

gas, which when dissolved is partially converted into

carbonates A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...

and

bicarbonates In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemica ...

, is often included.

Silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

in the form of

silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

, which usually appears as a neutral molecule in the pH range of most natural waters, may also be included for some purposes (e.g., when salinity/density relationships are being investigated).

Seawater

The term 'salinity' is, for oceanographers, usually associated with one of a set of specific measurement techniques. As the dominant techniques evolve, so do different descriptions of salinity. Salinities were largely measured using

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

-based techniques before the 1980s. Titration with

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar caustic' ...

could be used to determine the concentration of

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...

ions (mainly

chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...

and

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simila ...

) to give a

chlorinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

. The chlorinity was then multiplied by a factor to account for all other constituents. The resulting 'Knudsen salinities' are expressed in units of

parts per thousand In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...

(ppt or

‰ Per mille (from Latin , "in each thousand") is an expression that means parts per thousand. Other recognised spellings include per mil, per mill, permil, permill, or permille. The associated sign is written , which looks like a percent sig ...

). The use of

electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...

measurements to estimate the ionic content of seawater led to the development of the scale called the ''practical salinity scale 1978'' (PSS-78).Unesco (1981). The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980. ''Tech. Pap. Mar. Sci.'', 36Unesco (1981)
Background papers and supporting data on the Practical Salinity Scale 1978
''Tech. Pap. Mar. Sci.'', 37 Salinities measured using PSS-78 do not have units. The suffix psu or PSU (denoting ''practical salinity unit'') is sometimes added to PSS-78 measurement values. The addition of PSU as a unit after the value is "formally incorrect and strongly discouraged". In 2010 a new standard for the properties of seawater called the ''thermodynamic equation of seawater 2010'' (

TEOS-10 TEOS-10 (Thermodynamic Equation of Seawater - 2010) is the international standard for the use and calculation of the thermodynamic properties of seawater, humid air and ice. It supersedes the former standard EOS-80 (Equation of State of Seawater 1 ...

) was introduced, advocating absolute salinity as a replacement for practical salinity, and

conservative temperature Conservative temperature (\Theta) is a thermodynamic property of seawater. It is derived from the potential enthalpy and is recommended under the TEOS-10 standard (Thermodynamic Equation of Seawater - 2010) as a replacement for potential temper ...

as a replacement for

potential temperature The potential temperature of a parcel of fluid at pressure P is the temperature that the parcel would attain if adiabatically brought to a standard reference pressure P_, usually . The potential temperature is denoted \theta and, for a gas well-a ...

. This standard includes a new scale called the ''reference composition salinity scale''. Absolute salinities on this scale are expressed as a mass fraction, in grams per kilogram of solution. Salinities on this scale are determined by combining electrical conductivity measurements with other information that can account for regional changes in the composition of seawater. They can also be determined by making direct density measurements. A sample of seawater from most locations with a chlorinity of 19.37 ppt will have a Knudsen salinity of 35.00 ppt, a PSS-78 practical salinity of about 35.0, and a TEOS-10 absolute salinity of about 35.2 g/kg. The electrical conductivity of this water at a temperature of 15 °C is 42.9 mS/cm. On the global scale, it is extremely likely that human-caused climate change has contributed to observed surface and subsurface salinity changes since the 1950s, and projections of surface salinity changes throughout the 21st century indicate that fresh ocean regions will continue to get fresher and salty regions will continue to get saltier.

Lakes and rivers

Limnologist Limnology ( ; from Greek λίμνη, ''limne'', "lake" and λόγος, ''logos'', "knowledge") is the study of inland aquatic ecosystems. The study of limnology includes aspects of the biological, chemical, physical, and geological characteristi ...

s and chemists often define salinity in terms of mass of salt per unit volume, expressed in units of mg/L or g/L. It is implied, although often not stated, that this value applies accurately only at some reference temperature because solution volume varies with temperature. Values presented in this way are typically accurate to the order of 1%. Limnologists also use

, or "reference conductivity", as a proxy for salinity. This measurement may be corrected for temperature effects, and is usually expressed in units of μS/cm. A river or lake water with a salinity of around 70 mg/L will typically have a specific conductivity at 25 °C of between 80 and 130 μS/cm. The actual ratio depends on the ions present. The actual conductivity usually changes by about 2% per degree Celsius, so the measured conductivity at 5 °C might only be in the range of 50–80 μS/cm. Direct density measurements are also used to estimate salinities, particularly in highly

saline lake A salt lake or saline lake is a landlocked body of water that has a concentration of salts (typically sodium chloride) and other dissolved minerals significantly higher than most lakes (often defined as at least three grams of salt per litre). ...

s. Sometimes density at a specific temperature is used as a proxy for salinity. At other times an empirical salinity/density relationship developed for a particular body of water is used to estimate the salinity of samples from a measured density.

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 Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for br ...

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

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

s that will grow either in a water body, or on land fed by a water (or by a

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...

). A plant adapted to saline conditions is called a

halophyte A halophyte is a salt-tolerant plant that grows in soil or waters of high salinity, coming into contact with saline water through its roots or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs and seashores. T ...

. A halophyte which is tolerant to residual sodium carbonate salinity are called

glasswort The glassworts are various succulent, annual halophytic plants, that is, plants that thrive in saline environments, such as seacoasts and salt marshes. The original English glasswort plants belong to the genus ''Salicornia'', but today the glass ...

saltwort Saltwort is a common name for various genera of flowering plants that thrive in salty environments, typically in coastal salt marshes and seashores, including: :*''Salsola'' and related genera within subfamily ''Salsoloideae'' :*''Salicornia'' :*'' ...

barilla ''Barilla'' refers to several species of salt-tolerant (halophyte) plants that, until the 19th century, were the primary source of soda ash and hence of sodium carbonate. The word "barilla" was also used directly to refer to the soda ash obtained ...

plants. Organisms (mostly bacteria) that can live in very salty conditions are classified as

extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...

s, or

halophile The halophiles, named after the Greek word for "salt-loving", are extremophiles that thrive in high salt concentrations. While most halophiles are classified into the domain Archaea, there are also bacterial halophiles and some eukaryotic species, ...

s specifically. An organism that can withstand a wide range of salinities is

euryhaline Euryhaline organisms are able to adapt to a wide range of salinities. An example of a euryhaline fish is the molly (''Poecilia sphenops'') which can live in fresh water, brackish water, or salt water. The green crab (''Carcinus maenas'') is an e ...

. Salts are expensive to remove from water, and salt content is an important factor in water use, factoring into

potability Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, ag ...

and suitability for

irrigation Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow Crop, crops, Landscape plant, landscape plants, and Lawn, lawns. Irrigation has been a key aspect of agriculture for over 5,00 ...

. Increases in salinity have been observed in lakes and rivers in the United States, due to common

road salt Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

and other salt de-icers in runoff. 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 Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary p ...

is such that a possible cause of reduced circulation is the production of stratified oceans. In such cases, it is more difficult to subduct water through the thermohaline circulation. Not only is salinity a driver of ocean circulation, but changes in ocean circulation also affect salinity, particularly in the subpolar North Atlantic where from 1990 to 2010 increased contributions of Greenland meltwater were counteracted by increased northward transport of salty Atlantic waters. However, North Atlantic waters have become fresher since the mid-2010s due to increased Greenland meltwater flux.

Definitions

Composition

Seawater

Lakes and rivers

Classification of water bodies based upon salinity

Environmental considerations

See also

References

Further reading