Report | December 15, 2018
Deep-Sea Lebanon Results of the 2016 Expedition: Exploring Submarine Canyons
The Mediterranean Sea is a semi-enclosed basin, surrounded by 21 African, Asian, and European states. It extends over an area of 2.5 million km and is connected to the Atlantic Ocean though the Strait of Gibraltar, to the Black Sea through the Dardanelles and Bosforus Straits, and to the Red Sea through the artificially opened Suez Canal. When compared to the average depth of the world’s oceans (3733 m), the Mediterranean Sea is relatively shallow, with a mean depth of ca. 1650 m, yet it is the deepest of the enclosed seas. The entire Mediterranean Sea is considered a biodiversity hotspot, hosting up to 18% of the global total of macroscopic species (in less than 1% of the global ocean surface), with 25% to 30% of them considered endemic.
The Eastern basin of the Mediterranean is defined as lying to the east of the Strait of Sicily. It encompasses the Adriatic, Aegean, Ionian, and Levantine Seas, and includes the deepest waters of the Mediterranean (5121 m), in the Hellenic Trench. 4 The nutrient concentration of the Mediterranean is relatively low, and it is more oligotrophic or ultra-oligotrophic in the Eastern basin, where the exhausted surface layer is thicker and deeper due to stronger stratification. 5 In the Levantine Sea, the easternmost region of the Mediterranean, temperatures are also more extreme than in the rest of the Mediterranean, such that it is also referred to as the ‘warm-water corner of the Mediterranean’. 6 This characteristic appears to have become more pronounced during recent decades, possibly due to climate change.7,8,9 The relatively warmer waters of the Levantine may be responsible for the fact that, even though community composition throughout the Mediterranean Sea is relatively homogenous, some important habitat ‑forming species are absent from Levantine waters (e.g., Posidonia oceanica seagrass and sea fans such as Paramuricea clavata and Eunicella spp.).
Within the Levantine Sea, the waters of Lebanon host a wide variety of ecosystems, in an area representing 0.005% of the global marine surface. These ecosystems range from shallower features such as coralligenous habitat, seagrass meadows and vermetid reefs, to deep-sea ones such as underwater canyons.13 Lebanese offshore waters are characterised by a narrow continental shelf, which is perpendicularly crossed by various canyon systems that connect coastal zones to deep ‑sea habitats. These geographic features range approximately from 50 to 1600 m deep in Lebanese waters, and are important ‘keystone structures’ due to their role in supporting deep-sea communities acting as nursery and shelter habitats, benefiting fisheries, enhancing carbon sequestration, and providing other ecosystem goods and services to human society.14
