1.2 The temperature variable
The temperature is a fundamental physical characteristic of marine waters, in fact, its variations, both during the changing seasons and at different depths, greatly affect the organic life and the movement of water masses.
Temperature creates different layers in bodies of water. An important phenomenon generated by the differences in temperature of superimposed layers of water masses is the thermocline (thermal = temperature, cline = layer, Fig.1_sez 1.2 ®Wikimedia commons). You have known water for yours whole life.. drinking it, bathing in it.. even our body is 70% water. In the same container, the sea, there are layers that separate colder water below from warmer water above. How does this happen?
The surface layer of water, sometimes called the “ocean skin” or “sunlight zone”, is heated by solar irradiation. The interaction with the wind and the waves distributes the warmth and the heat is gradually transmitted to the underlying layers until it arrives at a transition point.
Here the temperature rapidly decreases (10 degrees in only a few meters) when approaching the deepest areas of the water, where the water is cold and no more relevant and abrupt changes occur.
If you SCUBA diving or free diving in apnea, you could come across several local thermoclines. Suddenly while diving you get as chill as if someone drops a bucket of ice water over your head! (Fig.2_SES1.2)
The clarity of water visibility differs inside areas of the thermocline too. The colder more dense water has more particles floating. This cause a clear demarcation in the water that looks like crinkled glass that is flowing.
In the Mediterranean, as in other temperate seas, during the summer months a seasonal thermocline is formed between 25 and 40 m; this temperature gradient along the water column results in an increase in density (pycnocline) that produces a barrier for the free spread of nutrients and plankton (i.e, jellyfishes) between the bottom and the surface and vice versa: this influences the whole food chain.
Global climate change, which inter alia implies an increase in average water temperatures, may contribute to the expansion or variation of the distribution range of a species, generally towards higher latitudes, and create new favorable conditions for the settlement of species from regions with warmer climates. This is the case of the “meridionalization” or “tropicalization” of the Mediterranean Sea that brings thermophilic species, native to this basin, to go northwards such as the coral Astroides calycularis and the fish Thalassoma pavo (Fig.3_SES1.2).