Dive Atlas Of The World: An Illustrated Reference To The Best Sites - Jack Jackson (2016)
by Jack Jackson
ALMOST LANDLOCKED, THE MEDITERRANEAN SEA LIES between Africa and Eurasia, exhibiting evidence of a complex geological history. Elongated from west to east, in the west the Mediterranean extends to the Strait of Gibraltar between Spain and Morocco, only 13km (8 miles) wide at its narrowest point and its only connection with the Atlantic Ocean. To the east it is bordered by the shores of Turkey, Syria, Lebanon and Israel. Nearly 4000km (2500 miles) wide, including the Sea of Marmara, it occupies an area of approximately 2,510,000 sq km (970,000 sq miles).
To the northeast the Mediterranean Sea connects with the Black Sea through the Dardanelles, the Sea of Marmara, and the Bosphorus Strait. To the southeast the Suez Canal, an artificial waterway across the Isthmus of Suez, connects with the Red Sea.
Diving in the Mediterranean has considerable variety. Seismic action, uneven deposition of river sediments, some of which were remelted, and uneven erosion together with the movements of the sea and the emergence and submergence of the land have resulted in a variety of coastlines. The northern shores of the eastern Mediterranean are complex and have variable fold mountains. The north coast of Africa bordering the eastern Mediterranean is low-lying and uniform except for the Cyrenaica highlands east of the Gulf of Sidra in Libya. Diving is popular everywhere with the local inhabitants, but there are some areas where only local nationals are allowed to dive and others that do not attract visiting divers. At Gibraltar and the marine reserves and Balearic Islands of Spain, the marine life is still good, but diving resorts on the Mediterranean coast of Morocco have not proved popular. The Mediterranean coast of France has good wrecks. Corsica and Sardinia have everything from wrecks to caverns. Algeria would have good diving but has never developed it; Tunisia has and its diving is quite good. The Tyrrhenian and Adriatic coasts and Sicily are popular and have plenty of wrecks. Malta, Gozo and Comino are particularly popular with UK clients and the government has scuttled six ships for divers. While not a problem for divers, Great White Sharks have been seen in Sardinia, Tunisia, Malta, Sicily and the mouth of the Adriatic. Greece and Turkey limit the areas where foreigners are allowed to dive to protect ancient shipwrecks, although Turkey has just opened some of these areas. Libya, Syria and Lebanon have never developed their diving. Cyprus has good diving, Israel limits access to its Mediterranean underwater archaeological sites, but Egypt has opened up the recent finds near Alexandria.
Much of the Mediterranean coastline has rugged hills rising sharply from the water, but Egypt and Libya have plains lying next to the sea. The largest island is Sicily while in decreasing size other large islands include Sardinia, Cyprus, Corsica and Crete. There are myriads of smaller islands and those of the Aegean are so numerous that the name Archipelago was formerly applied to the Aegean Sea. These islands have frequent earthquakes.
The large volume of warm water gives the surrounding land a subtropical climate known as a Mediterranean climate, even when it occurs elsewhere. Most Mediterranean countries have hot, dry summers and mild, wet winters. In summer, the temperature may reach 27° (80°F) while in winter it rarely drops below 4°C (39°F). Egypt and Libya have tropical climates. Off the coast of Libya, the Gulf of Sidra can reach 31°C (88°F) in August.
WINDS AND RAINFALL
Airflow into the Mediterranean Sea is mainly through breaks in the mountain ranges. The cold, dry, northwesterly mistral passes through the Alps-Pyrenees gap. The strong northeasterly bora passes through the Trieste gap. The cold easterly levanter and the westerly vendaval pass through the Strait of Gibraltar. Hot, dry southeasterly winds, known locally as the sirocco, ghibli or khamsin, frequently blow into the Mediterranean from the Arabian Peninsula and the Sahara desert when low-pressure zones cross the sea in late winter and early spring. These winds reduce the heat of the surface water by evaporative cooling. This is enough to increase its density so that it sinks and increases the salinity of incoming Atlantic surface water.
The distribution and quantity of rainfall is variable and unpredictable. Maximum rainfall is found in mountainous coastal areas. The Dalmatian coast of Croatia can get 2540mm (100 in) per year, while the north African coast from Gabès in Tunisia to Egypt gets less than 250mm (10in). Overall, the loss of water by evaporation is three times the combination of rainfall and river drainage into the sea.
Most regions of the Mediterranean have semidiurnal tides. The average tidal range is only about 300mm (12 in), but the change in water level caused by strong winds can be four times greater. However, the Gulf of Gabès off Tunisia has a range of nearly 2m (6½ft). The Atlantic affects tides in the Strait of Gibraltar, but its influence declines further east. A strong current flows into the Mediterranean from the Black Sea.
The tides are significant in the Aegean Sea where the Euripus Phenomenon (violent currents, variable in speed and direction) is named after the tide of the strait lying between continental Greece and the island of Euboea (Évvoia). These currents are mainly influenced by the winds.
Density currents are currents that move by the force of gravity acting on small density differences caused by variations in salinity or temperature. The surface layer, which is disturbed by the waves, is there because it is less dense than the deeper waters due to being warmer or less saline. The oceans are composed of layers of water that have distinctive chemical and physical properties, which move more-or-less independently of each other and which do not lose their individuality by mixing even after they have flowed for hundreds of kilometres (miles) from their point of origin.
The water from the Mediterranean Sea flowing through the Strait of Gibraltar into the Atlantic Ocean is an example of this type of density current or stratified flow. The Mediterranean is enclosed in a basin that is relatively small compared with the ocean basins and, because it is located in a relatively dry climate, evaporation exceeds the supply of fresh water from river drainage. The result is that the Mediterranean Sea contains water that is both warmer and more saline than normal deep-sea water. Overall the Mediterranean water is denser than the water in the upper parts of the North Atlantic. This difference in density causes the lighter Atlantic water to flow into the Mediterranean in the top 200m (660ft) of the Strait of Gibraltar and the denser Mediterranean water to flow out into the Atlantic between 200m (660ft) and the top of the sill separating the Mediterranean from the Atlantic at a depth of 320m (1050ft). Because the strait is relatively narrow, both inflow and outflow achieve quite high speeds. Near the surface the inflow may reach speeds of 2m (6½ft) per second, and the outflow 1m (40in) per second. A result of the high current speeds in the strait is a considerable amount of mixing, which reduces the salinity of the outflowing Mediterranean water, which sinks until it encounters colder, denser Atlantic water, where it then spreads out.
A submarine ridge with a sill depth of about 365m (1200ft) between the African coast and the island of Sicily divides the Mediterranean Sea into western and eastern sections. The sea has an average depth of 1501m (4926ft). The eastern basin is the deeper than the western one, reaching a maximum depth of 5093m (16,302ft) in the Hellenic Trough between Greece and Italy.
The western section is characterized by broad, smooth, abyssal plains and is further subdivided into three main basins. The Alborán Basin is east of Gibraltar, between the coasts of Spain and Morocco. The Balearic Basin, sometimes called the Algerian or Algero-Provençal Basin, is east of the Alborán Basin and west of Sardinia and Corsica, extending from the coast of Algeria to the coast of France. The Tyrrhenian Basin, covered by the Tyrrhenian Sea, lies between Italy and the islands of Sardinia and Corsica.
In contrast, the Mediterranean’s eastern section is dominated by the Mediterranean Ridge system and is further subdivided into two main basins. The Ionian Basin, the area known as the Ionian Sea, lies to the south of Italy and Greece. A submarine ridge between Libya’s Cyrenaica and the western end of Crete separates the Ionian Basin from the Levantine Basin to the south of Anatolia (Turkey). The island of Crete separates the Levantine Basin from the Aegean Sea. The Aegean is that part of the Mediterranean Sea lying north of Crete, bounded on the west and north by the coast of Greece; and on the east by the coast of Turkey. The Aegean Sea contains the many islands of the Grecian Archipelago. To the northwest of the main body of the Mediterranean lies the Adriatic Sea, which is bounded by Italy to the west and north; and by Croatia, Bosnia-Herzegovina, Serbia and Albania to the east.
Modern studies suggest that the structure and present form of this tectonically active basin and its bordering mountain system have been determined by the convergence and recession of the continental plates of Eurasia and Africa squeezing and stretching the Earth’s crust. As they drifted apart, Eurasia turned clockwise and Sardinia, Corsica and Africa turned anticlockwise, opening a waterway to the ocean at the western end of the sea. There were many alternating phases of flooding and evaporation. Geological data suggests that there are currently at least six main areas of collision between Africa and Eurasia, resulting in volcanism, mountain building, and land submergence. Earthquakes occur frequently throughout the region, especially in Greece and western Turkey. Volcanic action formed many of the islands in the Mediterranean Sea and volcanoes, including Mount Etna, Stromboli, and Vesuvius, still erupt in the region.
SURFACE CIRCULATION AND TIDES
Mediterranean surface circulation basically consists of separate anticlockwise movements of the water in the western and eastern basins, but many small eddies and other local currents occur because of the complexity of the northern coastline and the many islands. Although only significant in the Gulf of Gabès and the northern Adriatic, tides add complications to the currents in narrow channels such as the Strait of Messina.
Historically, large seasonal variations were caused by the flooding of the Nile, which reduced the salinity of coastal waters of the southeastern Mediterranean and increased the stratification and productivity of these waters. This influence ended with the completion of the Aswan High Dam. The amount of Red Sea water passing through the Suez Canal into the Mediterranean is negligible.
The opening of the Suez Canal turned the Mediterranean into one of the world’s busiest shipping lanes and it became heavily polluted with oil, agricultural run-off, industrial pollutants and sewage, even spreading typhoid and infectious hepatitis. Eventually the United Nations Environment Program (UNEP) sponsored the Barcelona Convention for the Protection of the Mediterranean Sea Against Pollution, calling on all Mediterranean countries to clean things up. Recently there have been spills of toxic chemicals, leaks from offshore oil installations and a virulent alien alga discharged from an aquarium, but generally things are much improved.
Blackfaced Blenny (Tripterigion delaisi). Territorial males have black heads where the coloration does not extend to the pectoral fins, and yellow bodies.
Found in much of the Atlantic Ocean, including the Mediterranean, Salema or Saupe (Sarpa salpa) usually congregate on rocky or sandy seabeds with lots of vegetation.
Wide-eyed Flounders (Bothus podas) lie camouflaged on sandy bottoms where they feed on small fish and invertebrates. Both eyes are on the left side, with the lower eye forward.
L’Estartit, is the staging port for all sub aquatic activities to the Medas Islands. There are also several diving schools.
The Medas Islands became a marine preserve in 1983 and are renowned for the large number of friendly groupers, sea bream and colourful Gorgonian Sea Fans.