Oceans and seas

Aegean Sea

May 13, 2013, 10:26 pm
Content Cover Image

Aegean Sea, observed from satellite. Source: United States National Aeronautics and Space Administration

The Aegean Sea is a saltwater body in the northeast part of the Mediterranean Sea, centered at approximately 25 oE and 38 oN lying between Turkey and the mainland of Greece at the west and north margin.

It is bounded in the south by an arc extending along the Greek islands of Rhodes near the Turkish mainland, across to Crete and Kythera (near the Peloponnese peninsula).

Three parts of the Aegean are recognized as smaller regional seas:

  • The Sea of Crete in the southern part of the Aegean between the Cretan Arc and the Kiklades Plateau (defined as the 400 metre isobath)
  • The Thracian Sea in the north after the traditional name for the region now covers parts of Greece, Bulgaria and Turkey.
  • The Myrtoan Sea west of the Cyclades

Climate

The climate in the Aegean Sea area is characterized by the presence of two distinct periods, summer and winter, with spring and autumn relatively short and transitional. The topography and continual alternation of land and sea make the climate highly variable. Annual river runoff averages about 18,800 cubic meters, and evaporation exceeds precipitation and river runoff. The most prominent wind pattern is the Etesian winds, which are persistent, northerly, cold and dry winds that often reach gale force in July and August. When this wind approaches the southern Aegean is bifurcates, becoming northeasterly over the Kitherian Straits and northwesterly-westerly over the southeastern Aegean. The Etesians vanish in late autumn to be replaced by violent cyclonic storms and highly variable prevailing winds.

Hydrology and Bathymetry

caption Cliffs of Santorini, Cyclades island group. @ C.Michael Hogan The Aegean contains an extended plateau (Thermaikos, Samothraki, Limnos and Kyklades) as well as the deep basins the North Aegean Trough (1600 m maximum depth), the Chios Basin (1160 m) and the Cretan Sea (two depressions in the east 2561 m and 2295 m deep). It covers an area of approximately 214,000 km2, has a volume of around 3,000,000 km3, and a maximum depth of 3543 m. The sea extends approximately 610 km (380 miles) north-south and approximately 300 km (186 miles)  in an east-west direction, with a total area of about 214,000 km2 (83,000 miles2).

The Aegean Sea is connected to the Levantine Sea to the southeast via the Cassos or Kasos Strait (67 km wide, 1000 m deep) between Crete and Karpathos, the Karpathos Strait (43 km wide, 850 m deep) between Karpathos and Rhodes, and the Rodos or Rhodos Strait (17 km wide, 350 m deep) between Rhodes and Turkey. It joins the Ionian Sea and Cretan Sea to the southwest through the Antikithira Strait between Crete and Antikithira (32 km wide, 700 m deep), the Kithira Strait between Antikithira and Kithira (33 km wide, 160 m deep), and the Elafonissos Strait between Kithira and Peloponnese (11 km wide and 180 m deep). There is considerable and complicated interchange of water with the eastern Mediterranean through these passages. The Strait of Dardanelles provides a northern link to the Black Sea

The Aegean is connected to the Black Sea via a narrow straight, the Dardanelles (55 m deep, 0.45-7.4 km wide), the small Sea of Marmara and a second narrow strait, the Bosphorus. The Aegean receives around 190 km3 per year of water from the Black Sea.  Currents carry the less saline water of the Black Sea at and near the surface, while lower down more saline water of the Mediterranean flows in the opposite direction.

caption Aegean bay scene, Santorini. @ C.Michael Hogan The Aegean includes many islands that are collectively known as the Greek Archipelago including the Cyclades, the Dodecanese, Euboea, Lesbos, Samos, and many others. The Aegean has an average depth of about 350 m (1,150 feet) and a maximum depth of 3543 m (11,627 ft) east of Crete.

A narrow strait between the island of  Euboea and the Greek mainland, known as the Euripus is notable for powerful erratic currents. The Aegean sea is relatively warm with the average temperatures of surface waters varying from 16°C (60 °F) to 25 °C (77°F) over the course of the year.

The surface circulation is most affected by the summer Etesian winds and the low salinity inflow from the Black Sea. The winds cause upwelling along the western coasts of the islands in the eastern Aegean, and a accompanying cold surface zone with temperatures 2-3C lower than in the northern and western Aegean. During the summer, this colder water is present in the eastern Aegean from Rodos Island up to the Limnos Plateau. In winter, the warmer waters of Levantine origin are found in the same area, while the cold waters arriving from the Strait of Dardanelles spread over the Samothraki Plateau and follow the general cyclonic circulation of the north Aegean. In addition to the overall cyclonic circulation, there is also a Samothraki anticyclonic gyre located in the northeastern part of the North Aegean, a semi-permanent feature that can be detected through most of the year, and an anticyclone near Athos.

The surface flow in the south is into the Aegean between Kithira and Crete, Crete and Karpathos, Karpathos and Rhodes, and Rhodes and Turkey, and into the Mediterranean between Kithira and the Peloponnese coast. There is systematic wind-driven upwelling along the northern coasts of the Patraikos and Korinthiakos Gulfs.

The main water masses found in the Aegean are (from shallowest to deepest):

  • Black Sea Water (BSW);
  • Levantine Intermediate Water (LIW);
  • modified Atlantic Water (AW); and
  • Eastern Mediterranean Deep Water (EMDW).

The BSW enters from the Strait of Dardanelles, producing a pronounced halocline in the norther Aegean with a maximum depth from 20-80 m. It moves southward and westward, following the general cyclonic circulation, and can be detected by a surface salinity minimum as far south as the Kithira Straits. LIW is the saltiest water mass of the eastern Mediterranean. It is generated in the Levantine and southern Aegean Seas in February and March. It flows eastwards and westwards from the Aegean, and also flows into the Aegean via the eastern straits of the Cretan Arc. It predominates in the subsurface layers of the Cretan Sea as well as in the eastern parts of the Aegean as far north as the southern boundary of the Limnos Plateau, and is easily identified by its salinity maximum. The modified AW enters the Aegean through the straits of the Cretan Arc and is identifed in several regions as a subsurface (30-200 m) salinity maximum. The Aegean deep water mass extends from about 400 to 500 m to the bottom, with temperatures ranging from 12 to 14.5C and salinities from 38.68-38.9. 

Thus the greater depth basins of the Mediterranean are very uncommon deep-sea systems. With water temperatures at 4000 m in excess of 14°C (rather than 4°C or colder for the deep oceanic basins). All the entire benthic habitat is as hot as the water around a typical hydrothermal vent system, but lacks a vent's rich chemical energy supply.

One traditional origin of the sea's name is from a King Aegeus, who threw himself into the sea, when mistakenly believe that his son, Theseus, had died while facing the Minotaur on Crete. The sea was also called the "Archipelago" in ancient times and provided the word for groups of islands like that which exists in the Aegean Sea.

Island Groups

There are seven principal island groups within the Aegean: The North Aegean Islands, Euboea, Northern Sporades, Southern Sporades, Cyclades, Saronic Islands and Crete. The southern rim of the Aegean is actually defined by an island arc consisting of (east to west) Rhodes, Karpathos, Kasos, Crete, Antikythera and Kythera. These islands were very important to the ancient world, being the sites of key trading ports and early civilizations that had contact with Cairo to the south, Phoenicia to the east and mainland Greek cities to the north.

Biodiversity

caption The critically endangered monk seal. The Mediterranean Sea is generally considered a biodiversity hotspot for coastal marine species, but has been thought to be depauperate in deep-sea species richness. However, Danovaro et al have recently shown that in contrast to what was expected from the sharp decrease in organic carbon fluxes and reduced faunal abundance, the deep-sea biodiversity of the entire eastern basin of the Mediterranean Sea is similarly high. All of the biodiversity components, except bacteria and archaea, evinced a diminution of richness with increasing water depth below the surface. Moreover, deep sea bacterial diversity at the Cretan rim is thought to be among the highest in the world comparable with environments found in Antarctica. Nematode diversity exhibits a maximum at a depth of approximately 1000 metres, above and below which it is in monotonic decline.

The fishes and megafauna of the Aegean have been severely depleted during the last 10,000 years of exploitation and overfishing. These practises have greatly reduced stocks of such commercially significant fish as the Blue-finned tuna, but also driven to near extinction such megafauna as the Caribbean monk seal, whose last major populations are in the Aegean; however, this species may have already entered an extinction vortex, since its total numbers are fewer than 450 animals.

Comparing biodiversity among open slopes, deep basins, canyons, and deep-water corals demonstrates that the deep basins are least biodiverse. Danovaro estimates the total number of distinct taxa in the whole Mediterranean Sea to be almost 3000, with only one third having been presently described; this estimate excludes prokaryotes.

In the entire Eastern Mediterranean it has been conventionally held that there is low-abundance and low-diversity conditions of marine invertebrates. The Gibraltar sill constitutes a physical barrier for the colonization of Mediterranean habitats by larvae and deep-sea benthic organisms from the more rich Atlantic fauna, which could explain the low diversity observed form deep Mediterranean macrofauna. Other than strictly deep-dwelling taxa (e.g., the deep-water decapod crustacean family Polychelida), the Gibraltar sill is not a totally impenetrable barrier for all deeper-water macrobenthic species.

Species composition significantly within canyons compared to the surrounding slopes. Canyon assemblages generally display lower diversity for the meiofaunal components because of the high dominance of a few species and the lower evenness. On the other hand, certain canyons may contain a higher diversity of megafauna than the slopes and can be considered as hotspots of diversity as they may display high rates of endemism.

In the deeper basins fishes, decapod crustaceans, mysids, and gastropods are much less abundant in the deep Aegean and entire Mediterranean than in the northeastern Atlantic, by contrast.

Ancient History

caption Ancient urn at Phaistos, Crete. @ C.Michael Hogan Bronze Age Greek and Minoan civilizations around the Aegean Sea were two of the significant highly developed cultures in this basin: The Minoans of Crete and the Mycenean culture of the Peloponnese. Athens and Sparta emerged as mainland city-states, while Knossos. Phaestos and Cydonia were key city-states on Crete in the Bronze Age. Persians and Romans later invaded the mainland Greek perimeter, while mainland Greeks and subsequently Romans invaded Crete and some of the other significant island cultures such as Akritiri. Later contol of the Byzantine Empire, Venetians, the Seljuk Turks and the Ottomans occurred in many of the perimeter cities. The Aegean was the center of some of the earliest original democracies, and its sea lanes have always been prominent transects of commerce for maritime powers of the Eastern Mediterranean.

Further Reading

  • Peter Saundry. 2011. Seas of the world. Topic Ed. C.Michael Hogan. Ed.-in-chief Cutler J.Cleveland. Encyclopedia of Earth
  • Roberto Danovaro et al. 2010. Deep-Sea Biodiversity in the Mediterranean Sea: The Known, the Unknown, and the Unknowable. PLoS ONE 5(8)
  • H. M. Denham. 1983. The Aegean: A Sea Guide to Its Coasts and Islands, John Murray Publishers; 5th edition, ISBN: 0719539803
  • Y. Acer, 2007, The Aegean Sea in Its Contemporary Context - Part I, Turkish Weekly
  • T. Taymaz, Y. Yilmaz and Y. Dilek. 2008. The Geodynamics of the Aegean and Anatolia, Geological Society of London. ISBN: 1862392390
  • POEM Group. General circulation of the Eastern Mediterranean. Earth-Science Reviews, 32: 309, 1992.
  • K. I. Stergiou, E. D. Christou, D. Georgopoulos, A. Zenetos, and C. Souvermezoglou. 1997. The Hellenic Seas: Physics, chemistry, biology and fisheries. Oceanog. Marine Biol. Ann. Rev., 35: 538
  • E. Th. Balopoulos, A. Theocharis, H. Kontoyiannis, S. Varnavas, F. Voutsinou-Taliadouri, A. Iona, A. Souvermezoglou, L. Ignatiades, O. Gotsis-Skretas, and A. Pavlidou. 1999. Major advances in the oceanography of the southern Aegean Sea Cretan Straits system (eastern Mediterranean). Progr. Oceanogr. 44: 130
  • Tjeerd H.van Andel and Judith C.Shackleton. 1982. Late Paleolithic and Mesolithic Coastlines of Greece and the Aegean], Journal of Field Archaeology. Vol. 9, No. 4
Glossary

Citation

Hogan, C. (2013). Aegean Sea. Retrieved from http://www.eoearth.org/view/article/149849

0 Comments

To add a comment, please Log In.