Salinity is a measure of all of the salts dissolved in seawater, although this is frequently represented as the content of the major constituent NaCl. Many regions of the ocean are freshening (decreasing in salinity) due to increases in freshwater addition from runoff, ice melt and precipitation. However, salinity can also increase due to increased evaporation, with patterns of salinity change in the ocean depending on region. Salinity is often measured and reported in Practical Salinity Units (PSU, which are formally unitless), parts per thousand (ppt) or, more recently, as Absolute Salinity with units of g/kg.
Changes in salinity affect osmoregulatory processes (such as ion pumps). However, the salt tolerance of organisms varies widely, from stenohaline (narrow tolerance) to euryhaline (broad tolerance). Salinity has mainly been studied in the context of physiological processes.
Physical and chemical interactions with other drivers: Salinity affects water density, which will affect water column mixing . Salinity also affects gas solubility and carbonate chemistry (alkalinity).
References and resources
Pawlowicz, R. (2013) Key Physical Variables in the Ocean: Temperature, Salinity, and Density. Nature Education Knowledge 4(4):13