Salinity is the measure of the total concentration of dissolved salts in a body of water. These dissolved solids are primarily ionic compounds, such as sodium chloride (common table salt), along with ions like magnesium, sulfate, calcium, and potassium. These components dissociate into ions when submerged, contributing to the water’s overall “saltiness.” Salinity is a fundamental property of aquatic environments.
Quantifying Salinity
Salinity was historically determined by a gravimetric method, which involved measuring the mass of salt remaining after evaporating a known mass of water. Today, salinity is measured indirectly by assessing the water’s electrical conductivity. Since dissolved ions conduct an electrical current, a higher conductivity reading indicates a greater concentration of dissolved salts.
The modern standard for reporting this measurement is the Practical Salinity Unit (PSU), a dimensionless unit closely related to parts per thousand (ppt). One PSU is approximately equivalent to one gram of salt per kilogram of water. The average salinity of the world’s oceans is around 35 PSU.
How Salinity Varies Across Water Bodies
Salinity levels vary significantly across the globe, defining water bodies as freshwater, brackish, or marine. Freshwater, such as in rivers and most lakes, has very low salinity, often less than 0.5 PSU. Marine water in the open ocean averages about 35 PSU. Brackish water, found in estuaries where rivers meet the sea, represents an intermediate zone.
These variations are controlled by geophysical processes that add or remove fresh water. Evaporation, especially in hot, arid regions, removes pure water and leaves the salt behind, which increases salinity, as seen in the Red Sea. Conversely, processes that add fresh water, such as precipitation, river runoff, and melting ice, dilute the concentration of salt. Large rivers like the Amazon create plumes of low-salinity water extending far into the ocean.
Salinity’s Role in Ocean Systems and Biology
The concentration of salt affects the physical properties of water, most notably its density. Saltier water is denser than less salty water, causing it to sink. This density difference, combined with temperature variations, is the primary driver of the global ocean circulation system known as thermohaline circulation.
This deep-ocean current system distributes heat and nutrients around the planet, making salinity a factor in global climate regulation.
Osmoregulation in Marine Life
For marine life, salinity presents a challenge known as osmoregulation, the process by which organisms maintain a stable internal salt and water balance. Most aquatic organisms are adapted to a narrow salinity range and cannot easily transition between high and low salt concentrations. Some species, like salmon, are anadromous and have specialized physiological mechanisms that allow them to adjust their gills and kidneys to move between marine and freshwater environments.