salinities

Salinity is a measure of the concentration of dissolved salts in water. In oceanography, salinity is commonly expressed in practical salinity units (PSU), a dimensionless scale derived from conductivity ratios. Although PSU and parts per thousand (ppt) are often used interchangeably, PSU is defined from seawater conductivity and temperature and reflects the salt content of the water rather than a direct mass fraction.

Ranges: Freshwater 0–0.5 PSU; brackish waters 0.5–30 PSU; seawater around 34–36 PSU; hypersaline environments exceed about 40 PSU, with some salt lakes and brines reaching well over 100 PSU.

Effects: Salinity influences water density and buoyancy, freezing point, and osmotic pressure for aquatic organisms. Salinity tolerance shapes species distributions and community structure; many organisms are stenohaline (narrow tolerance) or euryhaline (wide tolerance).

Variability: Salinity is governed by evaporation and precipitation, river discharge, ice formation and melt, and ocean circulation. In the oceans, salinity varies with latitude, season, and proximity to freshwater sources; estuaries show strong salinity gradients. Globally, the mean oceanic salinity is relatively stable over short timescales but can shift with climate-driven changes in the water cycle.

Measurement: Salinity is measured with conductivity sensors on CTD instruments, refractometers, and salinometers. Historically, salinity was inferred from chlorinity titration in seawater samples. Modern sensors provide high-resolution, in situ measurements used in weather forecasting, climate monitoring, and marine research.

Relevance: Salinity balances are central to desalination and water resource management, coastal salinity intrusion, and the interpretation of paleoceanographic records. Understanding salinities helps explain marine chemistry, ecosystem dynamics, and the global hydrologic cycle.