What lives underwater is determined by a number of factors, namely water currents, light intensity, water depth, salinity, concentration of nutrients, etc.

With increasing depth, light intensity decreases rapidly – in fact, no organisms using light are found below 50m. In addition, the salinity (concentration of dissolved salt) is highest in warm seas and is lowest in cold seas, due to the evaporation of water from the surface of the oceans under hot conditions.

Dissolved nutrients mainly reach our seas via rivers, which discharge into the sea. Nutrients are most abundant in coastal areas close to the land, whilst they are found in very low concentrations in the open seas far from land. The Mediterranean Sea is described as being ‘an oligotrophic sea’, in view of the low nutrient concentrations found in its waters – this means is a sea which is poor in food content and in fish numbers but which is very transparent in return (that’s why it’s called the Blue Mediterranean and it’s so popular with divers).

With depth, pressure increase greatly. For example, at about 10 m below the surface, the water exerts twice the pressure found at the surface. Water pressure at the deepest point in the ocean is more than 8 tons per square inch, the equivalent of one person trying to hold 50 jumbo jets.

Try this for yourself! Obviously not by diving to such depths but by lowering a sealed aluminium can tied to a string into water – choose a spot where water is deep (from a cliff top or from a boat for example) and the string must be at least 30m long.