Water Education - Water Quality

Sea Water vs. Fresh Water

If there is one thing that just about everyone knows about the ocean is that it is salty. The two most common elements in seawater, after oxygen and hydrogen, are sodiumsodium and chloride. Sodium and chloride combine to form what we know as table salt. Seawater salinity is expressed as a ratio of salt (in grams) to a liter of water. In seawater, there are typically close to 35 grams of dissolved salts in each liter. It is written as 35‰. The normal range of ocean salinity ranges between 33-37 grams per liter (33‰ - 37‰).

But as in weather, where there are areas of high and low pressure, there are areas of high and low salinity. Of the five ocean basins, the Atlantic Ocean is the saltiest. On average, there is a distinct decrease in salinity near the equator and at both poles, although for different reasons.

Near the equator, the tropics receive the most rain on a consistent basis. As a result, the freshwater falling into the ocean helps decrease the salinity of the surface water in that region. As one moves toward the poles, the region of rain decreases and with less rain and more sunshine, evaporation increases.

In comparison freshwater, in the form of water vapor, moves from the ocean to the atmosphere through evaporation causing the higher salinity. Toward the poles, freshwater from melting ice decreases the surface salinity once again. The saltiest locations in the ocean are the regions where evaporation is highest or in large bodies of water where there is no outlet into the ocean. The saltiest ocean water is in the Red Sea and in the Persian Gulf region (around 40‰) due to very high evaporation and little freshwater inflow.

So just how do the two compare? If you take one cubic foot of seawater and evaporate it you would have about 2.2 pounds of salt left behind. If you do the same with a cubic foot of water from one of the Great Lakes of North America only about 0.01 pounds of salt or about one-sixth of an ounce of salt would be left behind. Therefore, the sea is about 220 times saltier than our primary freshwater resources.

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