Salinity

Salinity in rivers, lakes, and the ocean is conceptually simple, but technically challenging to define and measure precisely.

[5] 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 (%).

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.

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.

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 is created).

Silicon in the form of silicic acid, 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).

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.

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.

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.

Salinity is an ecological factor of considerable importance, influencing the types of organisms that live in a body of water.

A halophyte which is tolerant to residual sodium carbonate salinity are called glasswort or saltwort or barilla plants.

In addition, during glacial periods, the hydrography is such that a possible cause of reduced circulation is the production of stratified oceans.

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.

[13][21][22][23] However, North Atlantic waters have become fresher since the mid-2010s due to increased Greenland meltwater flux.