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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 sea water aren't just sodium chloride (what makes up our table salt), but other elements including calcium, magnesium and potassium.

Salinity in sea water may be measured in parts per thousand (ppt), or more recently, 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.

Why Does Salinity Matter? 

For one, 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 cavity. Whales can't drink much salt water - instead, the water they need comes from that 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.

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