Salinity: Definition and Importance to Marine Life

The simplest salinity definition 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.

What Salinity Is

Salinity in seawater is measured in parts per thousand (ppt) or practical salinity units (psu). Normal seawater has an average of 35 parts of dissolved salt per thousand parts of water, or 35 ppt. That equates to 35 grams of dissolved salt per kilogram of seawater, or 35,000 parts per million (35,000 ppm), or 3.5% salinity, but it can range from 30,000 ppm to 50,000 ppm.

By comparison, fresh water has just 100 parts of salt per million parts of water, or 100 ppm. The water supply in the United States is restricted to a salinity level of 500 ppm, and the official salt concentration limit in U.S. drinking water is 1,000 ppm, while water for irrigation in the United States is limited to 2,000 ppm, according to The Engineering Toolbox.

History

Throughout Earth's history, geological processes, such as the weathering of rocks, have helped make the oceans salty, says NASA. Evaporation and the formation of sea ice caused the salinity of the world's oceans to rise. These "salinity rising" factors were counterbalanced by the inflow of water from rivers as well as rain and snow, NASA adds.

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How Salty Is the Ocean?

By Jennifer Kennedy

Studying the salinity of the oceans has been difficult throughout human history due to limited sampling of ocean waters by ships, buoys, and moorings, NASA explains.

Still, as far back as the years 300 to 600 "awareness of changes in salinity, temperature, and smell helped Polynesians explore the southern Pacific Ocean," says NASA.

Much later, in the 1870s, scientists on a ship named the H.M.S. Challenger measured salinity, temperature, and water density in the world's oceans. Since then, techniques and methods for measuring salinity have changed drastically.

Why Salinity Is Important

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 salt water, 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 salt water because their kidneys are adapted to process the salt.

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.

In 2011, NASA launched Aquarius, the agency's first satellite instrument designed to study the salinity of the world's oceans and predict future climate conditions. NASA says the instrument, launched aboard Argentine spacecraft Aquarius/Satélite de Aplicaciones Científicas, measures the salinity in the surface—about the top inch—of the world's oceans.

Saltiest Bodies of Water

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 again.

In 2011, the salinity of the Dead Sea, which is situated between Israel and Jordan, was measured at 34.2%, though its average salinity is 31.5%.

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.

References

• Barker, Paul, and Anoosh Sarraf. (TEOS-10)Thermodynamic Equation of SeaWater 2010.
• "Salinity and Brine." National Snow and Ice Data Center.
• Stout, P.K. "Salt: in the Oceans and in Humans." Rhode Island Sea Grant Fact Sheet. 
• U.S. Geological Survey: Why Is the Ocean Salty?