Oceans and seas

Thracian Sea

May 13, 2013, 10:28 pm
Content Cover Image

Thracian Sea from space. Note Marmara Sea connecting at upper right. Source: NASA

The Thracian Sea is the northern part of the Aegean Sea, itself the northeastern part of the Mediterranean Sea.

The greatest marine biodiversity within the Mediterranean Basin is found in the Thracian and other eastern elements of the Mediterranean; however, extensive historical loss of fish numbers and the critically endangered Mediterranean monk seal are noted in the Thracian due to human actions in the basin beginning in ancient times, and accelerating with the human population explosion of the last century.

Relatively recent research suggests that biodiversity at the single-cell organism level is much greater in the deep ocean portions of the Thracian Sea, than formerly thought; however, species diversity at shallow and intermediate levels is low.

The Thracian Sea is one of the earliest seas noted in recorded history and literature, being a body traversed by the ancient Greeks in colonisation of Black Sea margins and Anatolia. The Black Sea is accessed via the Dardinelles and Sea of Marmara, which water channel supplies the bulk of freshwater input to the Thracian Sea.

Extent and Geography

The Thracian Sea extends from the three-fingered Chalkidiki peninsula south of the Greek city of Thessaloniki in the west to the Turkish Gallipoli peninsula that flanks the Dardenelles 130 miles to the east. The Greek and Turkish coast forms the northern sea boundary, with the southerm limit being the island of Lemnos. 

The Thracian Sea is rimmed by Macedonia and Thrace and Turkey. The total longitudinal extent of the sea from east to west varies from approximately 23.00 E to 25.80 E, and the latitudinal length from north to south varies from 40.25 N to about 41.00 N.

caption The island of Samothrace, Greece. Source: Wikipedia

The Thracian Sea is the northernmost element of the Aegean Sea. The longitudinal extent of the Thracian Sea from east to west varies from 23.00 degrees E to about 25.80 degrees E, and the latitudinal length from north to south varies from 40.25 N to about 41.00 N.  

The Greek islands Thasos and Samothrace and Turkish islands of Gökçeada (Imbros) and Bozcaada (Tenados) are the principal islands in the Sea. Chief gulfs rimming the Thracian Sea are the Gulf of Ierissos to the west, Strymonian Gulf (also Orfanos Gulf), the Kavala Gulf and the Saros Gulf in Turkey. The Nestos and the Evros River/Meriç are the rivers that discharge into the Thracian Sea. Loutra Eleftheron in Kavala are noted thermal springs in the basin.


The Thracian Basin contains an extended plateau (e.g. Samothraki, Lemnos) as well as a deep basin: the North Aegean Trough (1600 m maximum depth). 

The Thracian  Sea 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 Thracian 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 Thracian flows in the opposite direction.

The Thracian 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. Salinity is high at approximately 3.5 percent.

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 two to three degrees C 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 eastern part of the Thracian Sea, a semi-permanent feature that can be detected through most of the year, and an anticyclone near Athos.

The main water masses found in the Aegean, and Thracian by extension, 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 Thracian Sea with a maximum depth from 20 to 80 meters (m). This water mass 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, but has only slight interaction with the Thracian Sea. 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 (southern edge of the Thracian), 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 deep water mass extends from about 400 to 500 m to the bottom, with temperatures ranging from 12.0 to 14.5 degrees C and salinities from 38.68-38.90. 

Thus the greater depth basins of the Mediterranean are very uncommon deep-sea systems. With deep water temperatures 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.

Marine Ecology

caption Mediterranean monk seal, whose remnant
populations are centred in the Aegean.

The entire Mediterranean Sea is generally considered a biodiversity hotspot for coastal marine species, but is somewhat 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 Thracian 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 critically endangered Mediterranean 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 individuals.

Comparing biodiversity among open slopes, deep basins, canyons, and deep-water corals demonstrates that the deep basins are least biodiverse, except for microscopic organisms and possibly megafaunal fishes. 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 in 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 Thracian trenches, fishes, decapod crustaceans, mysids, and gastropods are much less abundant in the deep Aegean and entire Mediterranean than in the northeastern Atlantic, by contrast.

Terrestrial Margin Ecology

See main article: Aegean and Western Turkey sclerophyllous and mixed forests

caption Pinus brutia forest along the Thracian coast,
Limenas, Greece. Source: Sten Porse
Situated in parts of Turkey, Greece, and on some of the the Aegean Sea islands, the Aegean and Western Turkey sclerophyllous and mixed forests ecoregion enjoys a Mediterranean climate and encompasses islands, coastal areas and some inland plains around the Thracian Sea and islands therein. This ecoregion supports a few areas of pine forest, and hosts rare and endemic species such as oriental sweetgum and Datça palm. The endangered loggerhead turtle nests here, and fox, wolf and wild boar are included among its mammal populations. Many resident and migratory birds are found here, including threatened species such as the pygmy cormorant, Dalmatian pelican, white-headed duck, and the lesser kestrel. The dense human populations that have inhabited this ancient area of human civilization have severely degraded most of the original habitat, beginning in the earlier Holocene, and accelerating in more modern times with the recent human population explosion.

The Thracian Sea margin has one of the most important populations of Callabrian pine (Pinus brutia). Communities of maquis, dominated by evergreen shrubs and Pinus brutia, are the most common formations in this ecoregion. However, there is considerable variation in the vegetation composition, and the species of the maquis change from south to north.

Arbutus andrachne, A. unedo, Spartium junceum, and Laurus nobilis are the main species of maquis vegetation along the Thracian coasts. In the areas where Pinus brutia has been removed, Kermes Oak (Quercus coccifera), Calicotome villosa, Palirus spina-cristii, and Erica arborea are the main species. Common associations in the maquis formation are:

  1. Olea europea-Ceratonia siliqua – widespread from southern parts up to Izmir;
  2. Arbutus andrachne-Acer sempervirens-Quercus ilex – one of the rarest Mediterranean formations and well represented on Samsun Dag; and
  3. Quercus infectoria forest – one of the most important coastal formations; it forms a mixture with other oak species.

Quercus cerris, Q. frainetto, Q. pubescens and Q. ithaburensis once made up deciduous formations in the ecoregion, but these are now highly degraded.

In the Ancient World

caption Attic red-figure kylix, Achilles tending Patrochlus
ca. 500 BC. Source: Altes Museum, Berlin
The Thracian Sea is one of the earliest seas of the world mentioned by name in world literature. The ancient region of Thrace covered much of the region north and east of the sea, a region which includes parts of Greece, Bulgaria and Turkey. Homer noted the voyage of Achilles across the sea, using its present day name: "And o'er the Thracian Sea, that groaned and heaved, beneath their passage, home the Winds returned; and weary, from the pyre, a space withdrawn, Achilles lay, o'ercome by gentle sleep."

The ancient Greeks plied the Thracian Sea not only for the notable war with Troy, but in colonisation of Anatolia and the perimeter of the Black Sea, creating far reaching settlements at locations such as Histria in present day Romania.


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