Water covers most of Earth’s surface, forming oceans, lakes, and rivers. The composition of this water varies significantly across different environments. A key characteristic influencing water’s nature and behavior is its “saltiness,” which is important for comprehending a wide range of natural and human systems.
Understanding Salinity
Salinity refers to the total concentration of dissolved salts in water. These dissolved substances exist as ions, with sodium (Na+) and chloride (Cl-) being the most abundant in seawater, accounting for over 85% of its dissolved solids. Other ions include magnesium (Mg2+), sulfate (SO42-), calcium (Ca2+), and potassium (K+).
Salinity can be expressed in various units. Parts per thousand (ppt), or permil (‰), represents grams of salt per kilogram of water. For instance, typical ocean water averages around 35 ppt, meaning 35 grams of dissolved salts per 1,000 grams of seawater. Another common expression is the Practical Salinity Unit (PSU), a dimensionless unit often considered numerically equivalent to ppt in many contexts.
The Practical Salinity Scale of 1978 (PSS-78) is a modern standard used to define salinity based on electrical conductivity measurements. This scale provides a consistent and reproducible method for reporting salinity data, particularly in oceanography.
How Salinity is Measured
Modern salinity measurement relies on a water sample’s electrical conductivity. Dissolved salts increase a solution’s ability to conduct an electrical current, so higher conductivity indicates greater salinity. Instruments like conductivity meters or Conductivity, Temperature, Depth (CTD) sensors measure this property by passing an electrical current between electrodes.
CTD sensors are widely used in oceanography to collect simultaneous measurements of conductivity, temperature, and pressure at various depths. The measured conductivity is then converted into salinity using established empirical relationships, such as the PSS-78 scale, which account for temperature and pressure effects. Historically, methods like refractometry or chemical titration were used to estimate salinity. Electrical conductivity measurements have become the most practical and accurate approach for routine salinity determination.
The Importance of Salinity
In oceanography, salinity, alongside temperature, influences seawater density, which drives global ocean currents. Denser, saltier water tends to sink, creating a circulation pattern that distributes heat and affects global climate. Understanding salinity changes helps scientists trace ocean water movement and its interaction with the atmosphere and ice.
For marine life, salinity is a significant environmental factor shaping species distribution and ecosystem structure. Organisms have specific salinity tolerances; freshwater and saltwater species are adapted to different salt concentrations, and estuarine environments support life forms that tolerate fluctuating salinity. Changes in salinity can impact an aquatic species’ growth, respiration, reproduction, and survival.
In human applications, salinity affects agriculture, drinking water quality, and industrial processes. High salinity irrigation water can reduce crop yields by making it difficult for plants to absorb water. Desalination technologies, such as reverse osmosis and thermal distillation, remove salts from water for consumption or industrial use, especially in regions with limited freshwater. The energy required for desalination is directly related to the water’s salinity.