How Salinity is Measured

A body of water's salinity affects its density

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The simplest definition of salinity is that it is a measure of dissolved salts in a concentration of water. "Salts" in seawater aren't just sodium chloride (what makes up our table salt), but other elements including calcium, magnesium and potassium.

These substances get into the ocean through complex processes including volcanic eruptions, hydrothermal vents and less-complex ways such as the wind and rocks on land which dissolve into sand and then salt.

How Salinity is Measured

Salinity in seawater may be measured in parts per thousand (ppt), or practical salinity units (psu). These measurement units, according to the National Snow and Ice Data Center, are relatively equivalent.

The average salinity of ocean water is 35 parts per thousand, and can vary from about 30 to 37 parts per thousand. Deeper ocean water may be more saline, as is ocean water in regions where there is a warm climate, little rainfall and lots of evaporation. In areas close to shore where there is more flow from rivers and streams, or in polar regions where there is melting ice, the water may be less saline.

Which Bodies of Water Have the Most Salinity?

The Mediterranean Sea has a high level of salinity because it's mostly closed off from the rest of the ocean. It also has warm temperatures that results in frequent humidity and evaporation. Once the water evaporates, the salt remains (and the cycle begins all over again).

If the salinity in a body of water changes, it can affect the water's density. The higher the saline levels, the denser the water. Cold water with high salinity, such as that found in the northern Atlantic Ocean, is denser than warm, fresh water.

Why Does Salinity Matter? 

Salinity can affect the density of ocean water - more saline water is denser and heavier and will sink underneath less saline, warmer water. This can affect the movement of ocean currents. It can also affect marine life, who may need to regulate their intake of salt water.

Sea birds can drink salt water, and they release the extra salt via the "salt glands" in their nasal cavities. Whales can't drink much salt water - instead, the water they need comes from whatever is stored in their prey. They do have kidneys that can process extra salt, however. Sea otters can drink salt water, because their kidneys are adapted to process the salt.