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Salinity
Definition and Importance to Marine Life
A body
of water's salinity affects its density
by Jennifer Kennedy
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.
Key Takeaways: Defining
Salinity
· Seawater has an average of 35 parts of dissolved salt per
thousand parts of water, or 35 ppt. By comparison, tap water has a salinity
level of 100 parts per million (ppm).
· Salinity levels can affect the movement of ocean currents. They
can also affect marine life, which may need to regulate its intake of
saltwater.
· The Dead Sea,
located between Israel and Jordan, is the saltiest body of water in the world
with a salinity level or 330,000 ppm, or 330 ppt, making it nearly 10 times
saltier than the world's oceans.
Salinity Definition
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 percent 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 U.S. 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 U.S. 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.
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 Is Salinity 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 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 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 Sea and Bodies of
Water in the World
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 measured at 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.
Jennifer
Kennedy
Scientist,
educator, and naturalist with more than two decades of experience studying and
educating the public about the oceans
The
executive director of the Blue Ocean Society for Marine Conservation
Whale
watch naturalist who conducts educational programs about the marine environment
Experience
Jennifer
Kennedy is a former writer for ThoughtCo, where she contributed 260 stories
about marine life during her more than seven-year tenure. Jennifer worked
in marine research and education for over 20 years. She is the executive
director of the Blue Ocean Society for Marine Conservation, based in
Portsmouth, New Hampshire. With this organization, Jennifer works as a whale
watch naturalist, directs educational programs about the marine environment,
coordinates local aquatic conservation activities, and manages the
organization's operations.
Jennifer
is a scientist, educator, and naturalist with more than two decades of
experience studying and educating the public about the oceans. She was a
naturalist and human resources manager with the Isles of Shoals Steamship
Company before joining Blue Ocean. Her master's thesis involved studying
cooperative research between fishers and scientists. Jennifer is a member of
the Gulf of Maine Marine Education Association and the New Hampshire Sea Grant
Policy Advisory Committee.
Education
Jennifer
Kennedy earned a Master Science (M.S.) in Resource Administration and
Management from the University of New Hampshire in 2003. She also holds a
Bachelor Science (B.S.) in Natural Resources from Cornell University.
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and Dotdash
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