Mediterranean Sea

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Oceans and seas (main)

November 9, 2009, 12:00 am
May 13, 2013, 9:44 pm
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Cliffs of Santorini. Source: C.Michael Hogan

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The Mediterranean Sea is a large saline water body between Europe, North Africa and Asia, connected to the Atlantic Ocean in the west through the Strait of Gibraltar; the Black Sea in the northeast through the Dardanelles, Sea of Marmera and Bosporus; and to the Red Sea in the southeast through the man-made Suez Canal. In ancient times the Mediterranean Basin was home to some of the earliest human civilisations developing sophisticated art and advanced engineering technology.

Marine species in the Mediterranean have been under intense pressure for the past millennium, due to overfishing and intense development leading to water pollution and coastal habitat reduction. Water quality degradation is ongoing with chief sources being surface runoff (Surface runoff) of nitrates and phosphates, and discharge of inadequately treated sewage from coastal communities fringing on the Mediterranean.


Hvarcroatia04-116.jpg Harbour at Hvar, an island on the Dalmatian coast of Croatia
in the Adriatic. @ C.Michael Hogan

The Mediterranean Sea covers an area of 2,700,000 km2 (1,042,000 miles2) or about one percent of the Earth surface. It extends approximately 3860 km (2400 miles) west-east from the Strait of Gibraltar to the coast of the Middle-east. The north-south dimension of the Mediterranean Sea at its widest is 1600 km (990 miles), while the average width is half of that distance.

The Mediterranean contains two major basins, eastern and western, separated by the Sicily Channel, which have horizontal communication to a depth of 430 m (1400 ft), known as the sill depth, and the 80 m (260 ft) sill depth Strait of Messina. The major sub-basins include a number of regional seas.

  • The Eastern Basin, 4000 to 5000 m (13,000 - 16,400 ft) in depth, includes the Aegean Sea (which itself includes the Myrtoan Sea, Sea of Crete and Thracian Sea), Ionian Sea and Leventine Sea.
  • The Western Basin, 2500 to 3500 m (8200 - 11,500 ft) in depth, includes the Alboran Sea, Balearic Sea, Ligurian Sea and the Tyrrhenian Sea.

In addition, the Adriatic Sea is separated from the Eastern Basin by the Strait of Otranto (800 m/2600 ft sill depth). Generally shallower in the northern areas, the Mediterranean has an overall average depth of about 1500 m (4921 ft). The deepest point in the Mediterranean is 5267 m (17,280 ft) at a point known as the Calypso Deep in the Ionian Sea.

Mediterraneannasa.jpg Satellite image of the Mediterranean Basin. Source: NASA The Mediterranean receives the inflow from relatively few rivers because of a number of mountain ranges that lay close to the sea, the Sierra Nevada mountains of southern Spain, the Pyrenees, Alps, Dinaric Alps, Taurus Mountains, Lebanon Mountains, the Aurès, Tell Atlas Mountains and Rif Mountains. These mountains, combined with the fact that North Africa is dominated by the Sahara Desert results in only a few major rivers flowing into the Mediterranean Sea include: the Ebro (Spain), Rhone (France), Po River, (Italy), and Nile River (Egypt).

Due to the small number of major rivers feeding the Mediterranean, evaporation within the Mediterranean basin water from the sea is greater than the inflow of water from rivers and rain producing a water deficit in the sea (approximately 2500 cubic km per annum) that requires additional inflows to maintain the sea level. This water deficit also makes the water of the Mediterranean more saline than that of the connected Atlantic Ocean: 39 practical salinity units (psu) compared to 36 psu in the Atlantic . The Mediterranean's high salinity and water deficit leads to thermohaline circulation with the Atlantic. A swift current brings water from the Atlantic into the regional Alboran Sea through the Strait of Gibraltar at and near the surface. As the water flows east it becomes saltier and mixes with Mediterranean waters. However, the current remains distinct as is sinks in the eastern part of the Mediterranean Sea before turning westward in a kind of "conveyor belt" flow which returns west and exits through the lower part of the Strait of Gibraltar. The water within this flow (known as the Mediterranean Intermediate Water), that completes the entire conveyor belt loop resides within the Mediterranean basin for approximately 80 to 100 years.

This thermohaline circulation also occurs at the Mediterranean Sea's eastern end with the Black Sea through the Dardanelles, Sea of Marmera and Bosporus.

Water Quality

The Mediterranean Sea has a number of significant water quality issues including discharges of untreated or inadequately treated sewage, non-point runoff from agricultural uses and sedimentation from terrestrial land development in the basin. At the basin scale, sewage effluents released along the southern coasts typically are entrained either parallel to the shoreline or seaward, before eventually coming back. Effluents from the eastern and northern coasts are characterisically entrained parallel to the coasts.

Algal blooms and jellyfish population explosions in the Mediterranean are due to and high nutrient influxes, chiefly from intensive terrestrial agriculture. Algal blooms are enhanced by nitrate and phosphate influxes from both farming and inadequately treated human wastes. The Mediterranean eddy currents are such that floatables and non-dissolved pollutant materials tend to concentrate on the North African coastline. Many beaches in southern Europe as well as North Africa have high levels of bacteria, chiefly due to the wastewater load of undertreated sewage and circulation anomalies of the Mediterranean.


Countries and regions that border the Mediterranean Sea include (from the northwest moving clockwise): Gibraltar (British territory), Spain, France, Monaco, Italy, Malta, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, Albania, Greece, Turkey, Cyprus, Syria, Lebanon, Israel, Gaza Strip, Egypt, Libya, Tunisia, Algeria and Morocco. This article does not address certain contentious issues related to the borders and status of some areas within the Mediterranean.

Marine Flora and Fauna

Posidonia 2 alberto romeo.jpg Dense stand of Posidonia oceanica. Source: Alberto Romeo

The marine ecosystem represents a significant area of species occurrence and endemism; however, considerable degradation is present and ongoing, due to the influence of the expanding human population over the last several millennia. Chief threats to the marine ecosystem are overfishing, coastline land development and water pollution.


There are more than 1000 species of macro-flora present in the Mediterranean Sea and its arms; approximately twenty percent of these are endemic to the Mediterranean, representing a high level of aquatic plant endemism. The most striking plant community is Neptune Grass, Posidonia oceanica, meadows system, which has a high biomass density, primary productivity and level of endemism; these communities are being steadily degraded particularly due to water pollution in the coastal seanear unban areas such as Barcelona, Athens, Nice, Naples, Toulon and Marseilles. One large clonal colony has been discovered off the island of Ibiza, measuring eight kilometres in characteristic dimension, which some researchers feel may be the largest and oldest clonal community on Earth, at an estimated age of 100,000 years.

Other significant marine flora are Zostera marina, Z. noltii, Calerpa olivieri, Cystoseira amentacea, C. mediterranea, C. sedoides, C. Spinosa, C. zosteroides, Goniolithon byssoides, Laminaria rodriguezii, Lithophyllum lichenoides, Ptilophora mediterranea and Schimmelmannia shousboei.


There are significant occurrences of cetaceans, marine turtles and pinnipeds in the Mediterranean sea as well as numerous fish species, invertebrates and lower lifeforms. Threats to cetaceans include intentional taking, water pollution and bycatch. Nineteen cetacean species are found in the Mediterranean, including the Fin Whale, Balaenoptera physalus, Sperm whale, Physeter macrocephalus,Striped dolphin, Stenella coeruleoalba, Risso's dolphin, Grampus griseus, Long-finned Pilot whale, Globicephala melas, Bottlenose dolphin Tursiops truncatus, Common dolphin Delphinus delphis, Cuvier's beaked whale, Ziphius cavirostris.Four of the cetaceans are occasional visitors: Minke whale, Balaenoptera acutorostrata, Killer whale, Orcinus orca, False killer whale, Pseudorca crassidens, and Rough toothed dolphin, Steno bredanesis.

All of the five species of marine turtles occurring in the Mediterranean are listed as endangered. Specific locales of the marine turtle habitat are off the coasts of Croatia, Bosnia, Algeria, Albania, Egypt, Cyprus, Spain, Greece, Italy, France, Libya, Morocco and Malta. The Loggerhead turtle, Caretta caretta, is an important instance of the marine turtles in the Mediterranean. Particularly important sea turtle migration occurs in the Straits of Gibrltar and the Alboran Sea and connecting waters. The United Nations has expressed specific concern over bycatch mortality in fisheries governed by Libya, Malta, Morocco and Algeria; furthermore, the UN has noted particular failures in Syria and Egypt to identify properly the nesting areas and protect these areas.

The Mediterranean monk seal is restricted to a handful of small and scattered colonies in the Ionian and Aegean Seas and the southern coast of Turkey in the Mediterranean, as well as scattered populations on the coasts of the western Sahara and Mauritania, and the Portuguese Desertas Islands, Madeira. It is thought that just two of these populations are viable, in Greece and northwest Africa. Mediterranean monk seals are thought to have been extirpated along the Libyan coast, due to water pollution and lack of protection in that locale. Sea cave breeding areas for this seal have been identified in Sardina, Greece (Northern Sporades) and Tunisia (Galite). There are only about 400 Mediterranean monk seals remaining, so that strong protection measures are urgently needed by the European Union and North African countries.

There are 85 species of chondrichthyan fish in the Mediterranean Sea including several cartilagenous fishes that are presently protected including the Basking shark, Cetorhinus maximus; Great white shark, Carcharodon carcharias; and the Mediterranean devil ray, Mobula mubular. Other species that are endangered, but in need of further protection are: Sand sharks, Carcharias taurus and Odontaspis ferox, and the Gray skate, Dipturus batis. The Speckled skate, Raja polystigma, is a species in potential danger of extinction, but there is a defiency of data on which to base an action plan. The United Nations has expressed concern about alien species of chnodrichthyan fishes entering the eastern Mediterranean from the Red Sea via the Suez Canal.

There is a pressing need to develop programs to sustainably manage with respect to target as well as bycathch species the commercially important Porbeagle, Lamna nasus; Dogfish, Squalus acanthias; Blue shark, Prionace glauca; Catshark, Galeus melastomus; Houndshark, Galeorhinus galeus; Requem sharks, Carcharhinus falciformis, C. limbatus, C. obscurus and C. plumbeus. The Bluefin tuna populations have been decimated by overfishing.

There are several hundred bird species found in the Mediterranean Basin, although there are relatively few true pelagic birds present. There are several robust breeding colonies of the endangered Cory's Shearwater, Calonectris diomedea. The endangered Levantin Shearwater, Puffinus yelkouan, the Balearic Shearwater, P. mauretanicus and the endangered European Storm-petrel, Hydrobates pelagicus can be observed on certain coastal cliff topography at rocky islands and skerries.

Other avafauna species that are threatened or endangered in this region include: European Shag, Phalacrocorax aristotelis; Pygmy Cormorant, P. pygmeus; White Pelican, Pelacanus onocrotalus; Dalmatian Pelican, P.crispus; Greater Flamingo, Phoenicopterus ruber; Osprey, Pandion haliaetus; Eleonora's Falcon, Falco eleonorae; Slender-billed Curlew, Numenius tenuirostris; Audouin's Gull, Larus audouinii; Lesser Crested Tern, Sterna bengalensis; Sandwich Tern, S. sandvicensis; Little Tern, S. albifrons.

Geological and Climate History

The most notable geological evolution of the basin involves the Mediterranean Sea beginning a cycle of almost complete desiccation in the latter part of the Messinian Age of the Miocene epoch, enduring from approximately 5.96 to 5.33 million years before present. This basin isolation from the Atlantic terminated with the Zanclean flood, when the sea again invaded the Basin; that flooding ended the low-sunken hypersaline pools that occupied the Mediterranean Basin during the cutoff of the Atlantic Ocean connection.

Pliocene Mediterranean temperatures have been estimated to be one to two and a quarter degrees Celsius warmer than at Present (Dowsett et al., 2009) and atmospheric carbon dioxide concentrations were about 30% higher than pre-anthropogenic values (Tripati et al., 2009), consequently climate during this time period has been considered a good analogue for future climate conditions. On the basis of climate model simulations the climate of Western Europe and Mediterranean was warmer and wetter due to the enhanced atmospheric and oceanic transport of heat and moisture to the North Atlantic and Mediterranean from the Equatorial Atlantic, especially during winter (Haywood et al.,2000).

The increase in high-latitude summer insolation that followed the Last Glacial Maximum favoured the northern hemisphere ice sheet retreat and triggered a 120 to 130 metre increase in global sea level (starting about 19 thousand years before present; (Stanford et al., 2006) that led to a gradual deepening of shallower straits and to flooding of the northern Adriatic Sea (Asioli et al., 2001). The last deglaciation is marked by a succession of accelerated melting events superimposed upon a smooth continuous sea level rise, the meltwater occurring at 14 thousand years before present.. Millennial-scale climate variability, which includes two extreme cold episodes. The younger Dryas interrupted the deglacial-warming trend. Subsequently abrupt and massive icebergs discharged into the North Atlantic.

These events led to the interglacial period in which we presently live, the Holocene. This relatively warm period started at about 11.000 years before present. and has generally been considered to be an epoch of climate stability, compared to the rapid and intense variability that characterized the last glacial. ( Abrantes, 2012) Climatic oscillations of short duration are, to some extent, superimposed on this general Holocene evolution; this period is associated with a combination of natural internal climate system variability and external forcings. (Debret et al., 2009). Variability at this timescale was first identified in the North Atlantic and linked to external forcing of solar activity (Bond et al., 2001). The periods of less intense solar activity were associated with relatively cold conditions and larger number of icebergs; the last such period was the Little Ice Age, that followed warmer climatic conditions known as the Medieval Warm Period.

In summary, proxy methods for paleoclimate period ensuing about 19000 years before present reveal natural oscillations of Pre Industrial Revolution Mediterranean climate data indicating temperature swings and droughts that produced extreme events wider than the post Industrial Revolution period; thus the anthropogenic forcings of modern times can be viewed as a relatively moderate forcing event compared to the entire Holocene and pre-Holocene natural oscillations. It is also notable that the periods of comparable warming to the present (e.g. Roman Warming Period and Medieval Warm Period) produced climatic conditions most suitable for human health, crop production and overall flourishing of civilisation.

Ancient History

Phaistosurnsgreece05-063.jpg Ancient Minoan urns, Phaistos, Crete. @ C. Michael Hogan Several of the earliest advanced human civilizations began near the eastern Mediterranean sea. In Mesopotamia beginning with Sumer in the 4th millennium BC, there was evidence of symbol writing and advanced art. Slightly later, the Nile River valley of ancient Egypt was the center of highly advanced architecture, art and hieroglyphic writing also in the 4th millennium BC; other notable Mediterranean cultures arose on Crete and other Greek Islands, where significant advances in writing, art and architecture occurred in the 3rd through 1st centuries BC. These early technologies dispersed rapidly throughout the Levant, most notably involving Lebanon, and Israel, with certain advanced metal technologies proceeding as far as the Iberian Peninsula, but it was more difficult to spread agricultural technologies and crops, such as flax, lentil, peas, barley, and cotton throughout the Mediterranean Basin. In short, the emergence of advanced art and technology in the Mediterranean rivalled similar early awakenings of similar advancements in such distant places as China and in the Orkney Islands.

In ancient Macedonia, technology developed in concert with cavalry warfare. Alexander the Great led the Macedonians to defeat the Persian threat from the east and vanquished the Persian empire, resulting in the dispersion of Greek dominance and technology throughout the north and east of the Mediterranean Basin, including authority as far north and east as ports on the Black Sea such as Histria. Power centres moved to the west with the Phoenician colony of Carthage achieving primacy in North Africa with territories extending to Volubilis and Lixis in present day Morocco. The Roman Empire expanded to control the whole Mediterranean region. Rome vanquished Carthage in the Punic Wars. Rome extended its hegemony eastward conquering Greece and much of the Middle East. Judea was a portion of the Roman Empire, where Christianity was founded, ultimately becoming the official faith of the Empire. Rome collapsed after 476 AD, as a result of heavy taxation and extensive immigration from poorer countries. The Byzantine Empire emerged to govern the eastern portion of the prior Roman Empire. The western elements of Gaul, Iberia and the Maghreb were overrun by nomadic peoples from the Eurasian steppe, leading to piecemeal fiefdoms in the sest and among the Germanic peoples to the north.


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  • Asioli, A. et al., 2001. Sub-millennial scale climatic oscillations in the central Adriatic during theLateglacial: palaeoceanographic implications. Quaternary Science Reviews, 20(11), 1201-1221
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  • Debret, M. et al., 2009. Evidence from wavelet analysis for a mid-Holocene transition to globalclimate forcing. Quaternary Science Reviews, 28, 2675-2688
  • Dowsett, H.J., Chandler, M.A. and Robinson, M.M., 2009. Surface temperatures of the Mid-Pliocene North Atlantic Ocean: implications for future climate. Philosophical Transactions of the Roya lSociety A: Mathematical, Physical and Engineering Sciences, 367(1886), 69-84.
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  • Stanford, J.D. et al., 2006. Timing of meltwater pulse 1a and climate responses to meltwater injections. Paleoceanography, 21, PA4103, doi: 10.1029/2006PA001340
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  • United Nations Environment Programme. Action Plan for Conservation of Marine Vegetation in the Mediterranean. Simpact Publishers, Tunisia


C. Michael Hogan (2013) Mediterranean Sea. ed. Peter Saundry. Encyclopedia of Earth. National Council for Science and Environment. Washington DC Retrieved from