While common 'table salt' is sodium chloride (NaCl), salts can more broadly be defined as the product formed from neutralizing an acid, where a metal atom (or positively charged radical) replaces one or more of the acid's hydrogen atoms. Salts are thus neutral ionic compounds composed of positively charged ions ('cations'; e.g., calcium, magnesium, potassium, sodium) and negatively charged ions ('anions'; e.g., bicarbonate, carbonate, chloride, nitrate, sulfate).
Salinity is a measure of the 'saltiness' or concentration of salt in water or soil. Salinity is synonymous with halinity, which derives from the word halides meaning chloride, and means the total concentration of soluble salts. An oft-ignored convention is for oceanographers to use the term 'halinity' when referring to oceanic conditions, due to the dominance of NaCl in seawater, and 'salinity' being used for soil or freshwater systems. Salinity historically has been measured as the ratio of the mass of dissolved salts to the solution in which it is dissolved (e.g., parts per thousand or ppt). More recently, salinity has been measured in terms of practical salinity units (psu) - the ratio of the conductivity of the sample water to a potassium chloride standard (32.435 6gKCl/kg water); psu is a ratio, and thus is a dimensionless measure of salinity.
Salinity is often measured using conductivity meters, which measure the conductance of electricity through solution. Since salt ions conduct electricity, conductivity is proportional to the concentration of salts in a solution. While measures of conductivity can be converted to salinity, and vice versa, the algorithms can be complex as they also depend upon temperature and pressure. Measurements of salinity can also be inferred through refractance, water density, and sound speed. Refractometers measure refractance, or the bending of light waves as they pass through a solution. Both refract-ometers and salinity meters require a fluid solution, while conductivity meters can measure conductance of moist sediment slurry.
There are a number of systems for classifying aquatic environments based on salinity. For marine waters, the 'Venice system' (or some variation thereof) is commonly used, with freshwater (<0.5), oligohaline (0.5-5), meso-haline (5-18), polyhaline (18-30), euhaline (30-40), and hyperhaline (>40). Other commonly used categories include brackish (0.5-30) and brine (>50). Seawater salinity usually ranges from 32 to 38 and is an average of 35.
Organisms can also be classified with regard to their salt tolerance. Those that can tolerate a wide range of salinities are called euryhaline (e.g., many intertidal fishes, mussels), while those with narrow salinity tolerances are 'stenohaline' (e.g., most ascidians, freshwater fishes). Some organisms complete different stages of their life cycle in different salinity regimes. For example, fish that live most of their lives in the sea but that breed in freshwater are called anadromous (e.g., salmon). Conversely, catadromous organisms are those that live in freshwater but breed in saltwater (e.g., eels, Chinese mitten crabs).
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