Greece covers an area of about 131,000 square kilometres, of which 20% is made up of roughly 3,053 islands. Its coastline is about 17,000 km long, about half the total Mediterranean coastline.
The coastal zone (up to 50 km from the sea) is home to 85% of the population, 80% of industry, 90% of tourism, a large percentage of farming activity and almost the whole of the fishing and fish-farming industries.
The sea temperature below a depth of 400 m. is constant at about 13° C. Their blue, very clear water is a distinctive feature of the Greek Seas. This glorious blue colour is due partly to the reflection of the blue skies and partly to the fact that the sea does not contain large quantities of solid matter such as plankton, mud and dust floating in the water.
The Aegean Sea, with a volume of 7.4×10.4 cubic kilometres, is the third-largest sea of the Eastern Mediterranean after the Ionian Sea and the Levantine Seas. The changing contours of the seabed include an extensive continental shelf under the Thermaic Gulf, Samothrace, Lemnos and the Cyclades, and deep basins such as the North Aegean Trough (maximum depth 1,600 metres), the Chios Basin (maximum depth 1,160 metres) and the Sea of Crete with two deep troughs to the east (2,561 and 2,295 metres). The Greek part of the Ionian Sea includes the Hellenic Trench, which is deepest off the coast of the SW Peloponnese (5,121 metres), the deepest point in the Mediterranean. The Hellenic Trench continues along the Cretan Arc, at depths usually over 4,000 metres. The are also deep points in the Rhodes Basin (maximum depth 4,433 metres).
All seven main groups of marine and coastal Mediterranean ecotope are found in Greece. 31 marine and 132 coastal (land and wetland) plant species have been recorded and described. The Greek Seas contain about 450 species of fish( see the list of Fish and Sea Animals of Greece), some of which are recent Lessepsian migrants from the Indian Ocean and the Red Sea via the Suez Canal. There are also 12 types of cetacean (whales and dolphins), from sperm whales to Cuvier’s beaked whales.
About 110 Mediterranean Monk Seals (Monachusmonachus) still live in the Greek seas, sheltering in remote caves. The loggerhead sea turtle (Caretta caretta) lays its eggs on a number of Greek beaches, while two other types of sea turtle, the green sea turtle (Chelonia mydas) and the leatherback turtle (Dermochelys coriacea), are also present.
The first and only marine park in Greece, the National Marine Park of Alonissos – Northern Sporades, includes a complex of islands with rocky coasts, large and small bays and sea caves. The continental shelf around the islands is very narrow and most of the Park’s marine environment is a huge volume of very deep water. To the west it includes part of the Sporades Basin, reaching depths of over 1,400 metres, while to the east the sea is no more than 600 metres deep. The primary reason for the Park’s establishment was to protect the extremely endangered Mediterranean Monk Seal (Monachusmonachus). Information on the biodiversity of its habitat has so far come only from occasional research, establishing that the area contains a wide variety of hard and soft substrate biotopes of great ecological value.
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