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Oceanography of the Aegean

      Ωκεανογραφία του Αιγαίου (3/5/2006 v.1) Oceanography of the Aegean (4/5/2006 v.1)
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Συγγραφή : Valiakos Ilias (10/7/2006)
Μετάφραση : Valiakos Ilias (13/2/2007)

Για παραπομπή: Valiakos Ilias, "Oceanography of the Aegean", 2007,
Πολιτιστική Πύλη του Αρχιπελάγους του Αιγαίου

URL: <http://www.ehw.gr/l.aspx?id=10540>

 
 

1. Introduction

The Aegean sea is, perhaps, the first whose oceanography was studied. Aristotle (384-322 BC), who had important contribution to oceanography and marine biology, begins actually his second book with a treatise in oceanography. He was the first who made estimates for bathymetry in various seas and recognized that the seas and the continents are changing as time passes. He also described and named 24 species of crustaceans and worms, 40 species of clams and echinoderms, and 116 species of fish, all coming from the Aegean sea.

2. The Aegean Sea basin

The Aegean Sea is one of the four biggest basins of Eastern Mediterranean covering a surface of 240.000 km2. It has unique characteristics and plays an important role in the wider hydrography and dynamics of the Mediterranean.

2. 1. Geographic limits

The Aegean sea is geographically situated in the north-eastern Mediterranean. To the North and West it is surrounded by the Hellenic mainland country, to the east by the coast of Asia Minor and to the south by the islands of the Cretan arc. The Aegean sea connects with the sea of Marmara and the Black sea via the Dardanelle channel. To the south it is separated from the Mediterranean sea via a line of six channels (the channels of the Cretan arc), which include from East to West: the channel of Rhodes (width 17 km, depth 350 m.), the channel of Karpathos (width of 43 km, depth 850 m.), the channel of Kasos (width of 67 km, depth 1.000 m.), the channel of Antikythira (width of 32 km, depth 700 m.), the channel of Kythira (width of 33 km, depth 160 m.) and the channel of Elafonissos (width of 11 km, depth 180 m.).

2. 2. The parts of the Aegean Sea

Since antiquity, the various parts of the Aegean have already had special names: a) Myrtoo sea, between Sounio and Kythira b) Thrakiko sea, between Thassos, Samothrace and Thracian coasts c) Ikario sea, between Chios and Kos d) Cretan sea, north of Crete e) Carpathian sea, between Karpathos and the coasts of Minor Asia e) Euboian sea, at the island of Euboia f) Dodecanese sea, which surrounds the islands of Dodecanese.

2. 3. Coastline, shelf and sea bottom

The coastline of the Aegean sea is very irregular and its topographic structure very complex. There are more than 2.000 islands of various sizes that are scattered in the whole basin and as a consequence the morphology of the sea bottom presents abrupt precessions. There are three deep basins: 1. The basin of northern Aegean with direction West Southwest-East Southeast. This basin includes also the basins of Northern Sporades, Athos and Limnos, with greatest depth 1.500 metres. 2. The basin of Chios in central Aegean with biggest depth 1.100 metres and 3. the Cretan basin in southern Aegean, that has also the biggest depth (2.500 metres). The basin of Chios borders to the South with the extensive plateau of the Cyclades, where depths do not exceed the 350 metres. This plateau is referred in the bibliography as the limit between the northern and southern Aegean.

The shelf of the Aegean (the area where the depths do not exceed the 200 metres) is rather limited, as it covers approximately 15.000 km2.

The seabed consists mainly of limestone, but there are also metamorphic rocks covering a significant area in the North and central Aegean. Almost everywhere a sedimentary cover is present, with thickness that varies, consisting of land or organic sediments. The volcanic materials are more limited.

3. The circulation of waters

Despite the progress in direct observations and in the creation of models during the last decades, the circulation of waters of the Aegean sea still has not been determined completely and has not become comprehensible. The circulation of waters is rather complex and variable. This is due to many factors, like the distribution of various islands and channels, the irregular topography of the sea bottom, the seasonal variability of the atmospheric pressure, the presence of powerful meteorological phenomena that can alter the local systems of circulation of waters and the presence of many different masses of water.

3. 1. Surface circulation

The surface circulation of waters is influenced mainly by the aestival annual winds and the low surge of saltiness from the Black sea. The winds cause the rise of waters along the West coasts of the East Aegean islands, resulting in the creation of a cold area in the surface with temperatures 2-3oC lower comparing to that of the North and West Aegean. During summertime, this colder water appears in east Aegean from the island of Rhodes up to the Limnos plateau. In the winter, the hotter waters that emanate from the south can be found in the same region, while the cold waters that arrive from the Dardanelle channel are expanded in the Samothrace plateau and follow the general cyclonic circulation of the north Aegean. Apart from the general cyclonic circulation, there is also an anticyclonic motion in Samothrace, which is observed in the north-east part of North Aegean and an anticyclone near the peninsula of Athos.

The rate of evaporation above the Aegean sea is approximately 1,3-1,5 m./year. The annual medium flow of heat is calculated in 26 W/m2, which means that the Aegean loses heat via its surface. This loss is being equalized by the shift of hot waters that emanates from the South via the Cretan channel as the exchange of heat with the Black sea is considered negligible.

Cold waters from the Black sea flow to the Aegean via Bosporus, the sea of Marmara and the Dardanelle channel. Under this layer, there is a flow of the saltier waters of the Aegean to the north. The quantity of water of the Black sea that pours into the Aegean is 1.257 km3/year, while the water of the Aegean that outpours is 957 km3/yaer. Moreover, the Aegean Sea accepts quantities of sweet water that emanates from the rivers that debouch along the Greek and Turkish coasts. The annual medium rate of evaporation exceeds the sum of fall and the flow of rivers (0,5 m./year and 0,11 m./year). Nevertheless, if the influence of waters of the Black sea is taken into consideration, the balance of water of the Aegean Sea is negative. The excessive volume of water per region of units has been calculated between 0,7 and 1,4 m./year. The balance of water of the region is restored by the surge of water via the Cretan arc channels.

Trying to summarise the known characteristics of circulation (from the various studies that cover departments of the region, recent placement of special bodies and various models) it appears that a general circular circulation exists in the Aegean Sea. Nevertheless, the most active dynamic characteristics are the cyclonic and anticyclonic swirls of intermediate scale. The territorial and time variability of these characteristics is not really known. Some of these characteristics appear to be permanent (like the cyclonic swirl in the basin of southern Chios), while others are periodical. In the surface layers, the circulation of the water is generally cyclonic (counter clockwise). The intensity of currents for depths less than 100 m is bigger than that of deeper layers. In the configuration of surface circulation the entry and the exit of masses of water from Dardanelle and the Cretan arc channels play an essential role. Masses of cold and brackish water from the Black sea enter from Dardanelle into the North-East Aegean and are directed westwards mixed with the much saltier and hot surface waters that emanate from the South-East. Those enter into the Aegean from South-East and reach the area North-West of Limnos. The Black sea waters are turned then to South following the coastline of east continental Greece and can be detected at Kythira and Antikythira channel. In these channels, the exit of water towards the Ionian Sea is observed.

3. 2. Tides

The Aegean is characterized by weak tides. The tides barely exceed 12cm, but there have been observed tides up to 50cm. The changes in the sea level happen with a period of 12 hours, without however the absence of fluctuations of longer periods that are owed mainly to perseverance for many days of the very strong north or south winds. The tide currents have a very small or no contribution in the configuration of the general circulation of the Aegean. Exception is the tide current of the strait of Euripos between north and south Euboian gulf, that owes his genesis to the morphologic particularities of the region.

3. 3. Variability factors of the circulation of waters

Between the natural factors that play important role in the biochemical activities that take place in the Aegean, are the variability in the circulation of waters, the formations of deep waters, the topography of the bottom, the surge of waters from the Black sea and the flow of rivers. The atmospheric conditions as well as their territorial and time variability play also important role, while their territorial and time variability is also important in the determination of coastal regions where a remarkable rise of the sea level is observed, which is usually connected with the powerful annual winds.

4. Biodiversity

The irregular coastline with the big number of gulfs, the islands and channels, as well as the complex topography of the sea bottom with the complex hydrology and circulation of waters in the Aegean Sea led to the big biodiversity. The Aegean sea can be characterized as an oligotrophic basin that presents however powerful gradation between north and south in production of biomass plankton and productivity. North Aegean is characterized as a special environment because of the existence of important rivers and hydrologic fronts. This is portrayed also in the higher trophic levels (fishes and benthos) of the north Aegean, where a more efficient energy transport via the sea web of foods is realised.


 

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