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 include not just sodium chloride (table salt) but other elements such as calcium, magnesium, and potassium.

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

Measuring Salinity

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

The average salinity of ocean water is 35 parts per thousand. This means that in every liter of water, there are 35 grams of salt, or about 3.5 percent of the weight of seawater comes from salts. The salinity of the ocean remains fairly constant over time. It does differ slightly in different areas, though, and can vary from about 30 to 37 ppt.

Deeper ocean water may be more saline, as is ocean water in regions with a warm climate, little rainfall, and plenty 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. Even so, according to the U.S. Geological Survey, there is enough salt in the world's oceans that if you removed it and spread it evenly over the Earth's surface, it would create a layer about 500 feet thick.

Bodies of Water With High Salinity

The Mediterranean Sea has a high level of salinity because it is mostly closed off from the rest of the ocean. It also has warm temperatures that result in frequent humidity and evaporation. Once the water evaporates, the salt remains, and the cycle begins all over again. In 2011, the salinity of the Dead Sea, which is situated between Israel and Jordan, was 34.2 percent, though its average salinity is 31.5 percent.

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. For example, visitors are often astonished that they can simply float on their backs, without any effort, on the surface of the Dead Sea, due to its high salinity, which creates high water density. Even cold water with high salinity, such as that found in the northern Atlantic Ocean, is denser than warm, fresh water.

Salinity and Marine Life

Salinity can affect the density of ocean water — water that has higher salinity 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, which may need to regulate its intake of saltwater.

Seabirds can drink saltwater, and they release the extra salt via the salt glands in their nasal cavities. Whales can't drink much saltwater; 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 saltwater because their kidneys are adapted to process the salt.