Aquascope|Facts|Over-fertilization 6

Salts are heavy. The higher the salt content of the water, the higher its density, which means that a given volume of saltwater will weigh more than the same volume of freshwater. In addition, cold seawater is heavier than warmer seawater. clines

A halocline, a layering of water depending on its salt content, is a boundary between heavy saltwater lying under a layer of less saltier and lighter water. A thermocline, a layering of the water depending on its temperature, is a boundary between the underlying, heavier and colder water and the lighter and warmer water above. At low temperatures and salt content special circumstances exist, for example, water just a few degrees above zero can lay above warmer water.

The sea moves

Because of varying factors, water seldom stands still. This can happen when the sun unevenly heats the earth and creates winds and currents. Furthermore, tidal currents occur when the sun and moons gravitational pull effect the seas.
The earth and the moon rotate around a common axis that is slightly displaced from the earths centre. Gravitational forces work against each other, while centrifugal force press them apart. At the centre of the earth, equalibrium exists between these forces, but on the side of the earth facing the moon, an attractive (pull) force dominates, while on the opposite side a centrifugal force dominantes. Therefore, there are "proturberances" on both sides of the earth, as portrayed in the digram to the right. The earths rotation round its axis results in that we experience as two high and two low tides daily. The gravitational pull is about half of the moons. When there is a full or new moon, the sun and the moon are in line with each other.

At this time both planets work together, resulting in a spring tide. When there is a half moon, the difference between high and low tide is the smallest because the gravitational pull from the sun and moon counteract each other. This is known as a nip tide.

Tides