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Biodiversity & Conservation


Salinity is a measure of the concentration of dissolved salts in seawater. Salinity is defined as the ratio of the mass of dissolved material in sea water to the mass of sea water (UNESCO, 1985). But this 'absolute' definition is not practical. Salinity was measured by a chlorinity titration but with the development of the salinometer, which utilizes conductivity, a new definition was developed. The 'practical salinity' (S) of a sea water sample is defined as the ratio of the electrical conductivity of the sample (at 15 C, and one standard atmospheric pressure) to that of a standard solution of potassium Chloride (KCl). A ratio of 1 is equivalent to a 'practical salinity' of 35 (UNESCO, 1985).

Until recently, salinity was expressed as parts per thousand (ppt or ). Subsequently, adoption of the 'practical salinity' gave rise to the 'practical salinity unit' (psu). However, 'salinity', defined as the ratio of two quantities of the same unit, is a 'dimensionless quality', i.e. takes no units. Therefore, it is correct to speak of a salinity of 35 (UNESCO, 1985). Baretta-Bekker et al. (1992) suggested that, in most cases, where a high degree of accuracy is not required, old and new figures for salinity can be used interchangeably.

Nevertheless, for the sake of accuracy when referring to salinity in our on-line reviews, the units used by the original authors are quoted in the text.

Freshwater is regarded as < 0.5 (limnetic), seawater as > 30 (euhaline), and brackish water as intermediate, including oligohaline, mesohaline and polyhaline waters (based on McLusky, 1993).

Salinity levels (adapted from Hiscock, 1996)
Full salinity 30-40
Variable salinity 18-40
Reduced salinity 18-30
Low salinity < 18
Unknown salinity ???