Why is the Mediterranean sea so blue?

Answer 1

Excellent answer by Lila Koumandou here:

http://www.bluesci.org/index.php?option=com_content&task=view&id=127&Itemid=504

To summarize, it's because there is very little phosphate in the water to support plankton.

The nearly land locked waters are heated by the sun. This drives evaporation. The water grows more saline, and denser, until it sinks. This sinking occurs at the east end of the Mediterranean. Surface waters flow east to replace the sinking water. The dense salty water flows west on the bottom, and out the straight of Gibralter, carrying most of the nutrients with it. Cold surface water from the Atlantic flows in to replace it.

Interesting related points:

A similar process drives the Atlantic current, which is responsible for northwest Europe's relatively mild climate. The major difference is that the Atlantic current is mainly north-south. Heating takes place at tropical latitudes. The surface water flows north, getting saltier due to evaporation but also getting colder as it goes. It is losing heat to the winds that blow towards England and France. Massive sinking of cold, salty water in the extreme north Atlantic is what drives the rapid flow of the "gulf stream," which is just the most famous part of this system.

Flows of this sort occur in all the oceans and redistribute heat from the tropics toward to poles. Interruptions in these patterns may be what trigger ice ages. We are in an ice age now (no really, look it up) probably because the current positions of the continents tend to isolate the poles from ocean currents.

Answer 2

The name derives from Latin 'Mediteranneous' meaning 'island' or in the 'middle of the land' (as the sea is surrounded by land)

Answer 3

The sea is blue because of the reflection from the sky.