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.
SEAFLOOR TOPOGRAPHY
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.
SPAIN AND FRANCE
by Lawson Wood
THE MEDITERRANEAN, WHERE THE FIRST sport diving took place, has received a bad press over the years. Population and industrial growth has resulted in vastly increased pollution levels. Much of the original coastline has been changed irrevocably by marinas, harbours and housing developments. Monaco has now lost some 75 per cent of its original coastline. The Mediterranean Sea is undoubtedly one of the world’s most threatened seas, due to the increased demand on its natural resources and pollution from homes, industry and intensive agriculture. Fortunately, the influx of fresh seawater from the Atlantic has managed to slow the deterioration.
While small areas such as the waters around Venice and the northern Adriatic, sections of the Greek coast and parts of Tunisia are under increased threat due to the pressures of tourism, this is usually on a seasonal, temporary basis and the marine life does regenerate itself. However, there is now a new threat: the introduction of alien species of algae to this enclosed sea as well as commercial fishing.
There are protected areas around the shores of the Mediterranean, but enforcement is difficult, particularly when it involves huge factory ships. Most countries now accept that a successful tourist industry relies on strict conservation policies. For this industry to succeed and prosper, tourists also have to be made aware of the impact they have on small areas. Membership of conservation agencies is an important
