All of the waters of the ocean, exclusive of the sea bed or other landforms, is referred to as the water column. (For a more complete discussion of the water column, see the chapter titled "Ocean.")

Every element known on Earth can be found in ocean water. It is 3.5 percent dissolved salts by weight. The percentage of these salts determines the ocean's salinity (level of salts). These salts also make seawater heavier than fresh water. The ocean water closest to the surface is usually less salty because of rainfall and fresh water flowing in from rivers.

The temperature of the oceans varies. In general, however, temperature changes are greatest near the surface where the heat of the Sun can be absorbed. In the warmest regions of the world, this heat absorption occurs to depths of 500 to 1,000 feet (150 to 305 meters).

Zones in the Ocean

Different...