Put simply, an objects density is related to the amount of 'stuff' in a given space. In better terms, density is the ratio of an objects mass to its volume. Adding more salt to the water made it more dense, making it sink below the freshwater above it. On average, sea water has around 35 grams of salt for every litre of water.
Density gradients drive currents to and from the polar regions of the world. When dense water masses are first formed they move towards the ocean floor. This moving mass of dense water is seeking to find a stable position along the ocean floor. As the cold salty water falls to the ocean floor, water must fill the space above it, setting up a situation for mass movement of water. As equatorial seawater warms up it expands, and is driven by wind and the differing density gradient toward the polar regions.
This movement is known as Thermohaline circulation.
The movement of cold polar water towards equatorial regions provides an important source of nutrients for equatorial waters. Australian southern fisheries thrive on the upwelling of these deep ocean currents, as fish populations expand when their natural food sources have more nutrients to grow.
Reference:
Brady, J. E. & Holum, J. R. (1993). Chemistry. The Study of Matter and Its Changes.
John Wiley & Sons, New York
Williams, W.D. (1980). Australian Freshwater Life. Macmillan Education Australia, Melbourne.
