Salinity is the concentration of dissolved salts in a body of water, a measure that influences everything from aquatic ecosystems to ocean currents. The salts are typically compounds like sodium chloride and magnesium sulfate, which dissociate into ions when dissolved. The most commonly used historical unit for measuring this concentration is Parts Per Thousand (ppt).
In modern oceanography, the widely accepted standard has transitioned to the Practical Salinity Unit (PSU). This shift reflects the need for greater precision and standardization across global measurements.
Parts Per Thousand and the Practical Salinity Unit
The traditional unit, Parts Per Thousand (ppt), is a measure of mass, representing the total mass of dissolved salts in grams per 1,000 grams of water (g/kg). A salinity reading of 35 ppt means there are 35 grams of dissolved salt components in every kilogram of water. This gravimetric approach was the standard for over a century, relying on chemical analysis or physical evaporation.
The scientific community began moving away from this unit in the late 1970s with the introduction of the Practical Salinity Scale (PSS-78), which yielded the Practical Salinity Unit (PSU). The primary motivation was the challenge of accurately measuring the mass of salt, which required complex and time-consuming chemical analysis. The new scale is based on measuring the water’s electrical conductivity instead of its mass.
PSU is a unitless ratio derived from comparing the electrical conductivity of a water sample to a standardized potassium chloride solution. Saltier water conducts electricity more efficiently due to the higher concentration of dissolved ions, offering a precise and rapid measurement. For typical seawater, the numerical value of PSU closely approximates the historical ppt value, allowing for continuity with older data while providing increased accuracy.
Methods for Measuring Salt Content
The most accurate and widespread modern technique for determining salinity is through the measurement of electrical conductivity. Instruments like salinometers or Conductivity-Temperature-Depth (CTD) sensors pass an electrical current between two immersed electrodes. Higher concentrations of dissolved salt ions result in higher electrical conductivity, which is then mathematically converted into a PSU value using the PSS-78 equation.
Chemical Titration
Before conductivity sensors, chemical titration was the established method, particularly the Mohr-Knudsen method, which measured chlorinity. This process involved reacting a water sample with silver nitrate to precipitate the halide ions, primarily chloride. Total salinity was then calculated from the chlorinity value, assuming the relative proportions of salts in seawater were constant.
Refractometry
A third, less precise method used for quick field checks or in aquaculture is refractometry. A refractometer measures how much light bends as it passes through a water sample. Dissolved salts increase the refractive index, and this property is correlated to salinity, typically displayed in ppt or specific gravity. Refractometers offer a simple, portable, and rapid way to estimate salt content.
Salinity Ranges in Natural Water Bodies
Salinity values vary widely across the planet. Freshwater, found in rivers and most lakes, has a very low salt concentration, typically less than 0.5 PSU. This low ionic content is characteristic of water bodies where evaporation is low and constant replenishment from precipitation occurs.
Estuaries and coastal areas, where fresh river water mixes with the ocean, are classified as brackish and have highly variable salinity. These regions can range from oligohaline (0.5 to 5.0 PSU) near the river mouth to polyhaline (18.0 to 30.0 PSU) closer to the open sea. This dynamic range dictates which organisms can survive in these transitional habitats.
The global average for open ocean seawater is consistent, sitting around 35 PSU, though surface values range from 30 to 40 PSU depending on location. Areas with high evaporation and low precipitation, such as the Red Sea, can see surface salinity rise to about 41 PSU. Conversely, regions with high levels of melting ice or river discharge, like the Baltic Sea, may have significantly lower salinities.
Water bodies that experience extreme evaporation with limited outflow are known as hypersaline, exhibiting values far exceeding that of the ocean. The Great Salt Lake, for instance, can reach salinities over 250 PSU, and specific deep brine pools in the Red Sea have been measured at up to 256 PSU. These extreme environments demonstrate the maximum concentrations possible before salts begin to precipitate.