The oceans covering the Earth are rich in animal and plant life, but neither of these communities could survive without the chemical or gaseous constituents of seawater.

To phytoplankton, for instance, the silicates, nitrates and phosphates found in this water are essential. Oxygen, also necessary for marine life, is found in abundance in the sea - an average of 6ml is found per liter of water.

The dependence on the salinity (salt level) of water is also reflected in the movement of marine communities around its increase and reduction.

Light also affects them in this way. The blueness of the sea depends on the depth of penetration of blue light which, in turn, depends on the smoothness of the waters. In clear waters such as the Caribbean, light may reach depths of 160m, whereas on rough coasts it may only go down to 15m.