Water rarely exists in its pure form in nature, typically containing various dissolved substances. Salinity quantifies the amount of these dissolved salts. Understanding salinity is important because it influences many natural processes, from vast oceans to local freshwater systems.
Understanding Salinity
Salinity refers to the concentration of dissolved salts within a body of water. These dissolved salts are not solely common table salt (sodium chloride); they are a mixture of various ionic compounds. Primary ions contributing to salinity include sodium, chloride, magnesium, sulfate, calcium, and potassium. These ions dissolve as water interacts with rocks and sediments.
For instance, the average salinity of ocean water is approximately 35 parts per thousand (ppt) or 35 Practical Salinity Units (PSU). This means that for every 1,000 grams of seawater, there are about 35 grams of dissolved salts. Freshwater, by contrast, has a much lower salt concentration, typically less than 1 ppt.
How Salinity is Measured
Measuring salinity primarily involves assessing electrical conductivity. Dissolved salts increase water’s ability to conduct electricity because ions carry an electrical charge. A salinity meter, often called a conductivity meter, passes an electric current between two electrodes in a water sample. The strength of this current directly correlates with the concentration of dissolved ions, indicating salinity. Most conductivity meters also include temperature compensation, as temperature influences conductivity readings.
While electrical conductivity is the most common method, other techniques exist. Refractometers measure how light bends through a water sample, with the degree of bending related to salt concentration. Titration, a chemical method, can also be used to determine the amount of specific ions, such as chloride, from which salinity can be inferred. Salinity is typically expressed in Practical Salinity Units (PSU) or parts per thousand (ppt), which are often considered approximately equivalent. Other units, like grams per kilogram (g/kg), are also used, with 1 ppt roughly equivalent to 1 g/kg.
The Significance of Salinity
Salinity plays an important role in various environmental systems. In oceans, it determines seawater density; saltier water is denser. This density difference, combined with temperature variations, drives global ocean currents, including the thermohaline circulation that distributes heat and influences climate patterns. Salinity also impacts density stratification, affecting how water masses layer in the ocean and influencing marine life distribution. Estuaries, where freshwater rivers meet the ocean, exhibit a wide range of salinity, creating unique transitional habitats.
Salinity levels affect living organisms, as different species adapt to specific salinity ranges, influencing their geographic distribution. Marine organisms, for example, adapt to high salt concentrations, while freshwater organisms are sensitive to them. Organisms employ osmoregulation to maintain internal salt and water balance when external salinity fluctuates. Estuarine species are euryhaline, tolerating a wide range of salinity changes, whereas many open ocean species are stenohaline, adapted to a narrow range. Changes in salinity can affect an organism’s survival, growth, reproduction, and overall fitness.
Human activities and natural processes both influence salinity levels. Natural factors include evaporation, which increases salinity by removing fresh water, and precipitation and river runoff, which decrease it by adding fresh water. The formation of sea ice also increases the salinity of the surrounding water as salts are excluded from the ice, while melting ice decreases it. Human activities, such as agricultural irrigation, can lead to increased soil salinity, hindering crop growth and reducing yields. Industrial wastewater discharge, road salt application, saltwater intrusion due to over-pumping, and improperly disposed desalination brine can elevate local salinity, impacting drinking water and infrastructure.