Marine Ecology

Alboran Sea

Content Cover Image

Resort on the Alboran Sea near Cabo del Gato, Spain. Source: C. Michael Hogan

The Alboran Sea is the western-most part of the Mediterranean Sea between the coast of Spain and the coast of Morocco.

caption Alboran Sea. Source: Norman Einstein

It extends from the The Strait of Gibraltar in the west to the Alboran Island in the northeast where it meets the Balearic Sea. The Alboran Sea extends from between about 35 and 38 degrees N and 6 degrees N and the Equator.

The sardine and anchovy fisheries have historically been the most important pelagic target species; however, overfishing and water pollution threatens the sustainability of these activities; furthermore the use of driftnets is causing ongoing heavy mortality of small cetaceans in the Alboran Sea. Heavy tourism along the Iberian Alboran coast contributes to water pollution that adversely affects many marine organisms in the Alboran Sea.

The Alboran Sea was traversed in the first millennium BC by Phoenician navigators, and subsequently by Carthaginians and Romans who were in conflict over the rich Mauritanian colonies first established by the Phoenicians.

Circulation and Hydrography

The classical circulation pattern in the upper layer of the Alboran Sea is a swift Atlantic current surrounding and feeding two anticyclonic gyres, which together are termed the Alboran Gyre: The Western Alboran Gyre and the Eastern Alboran Gyre (WAG, EAG).

The circulation exhibits considerable variability, characterized by the stability of the two–gyre system in the summer months, and by a coastal jet sometimes called the Algerian Current flowing close to the African shore in the winter. The Alboran's average depth is just 445 metres (1461 feet) with a maximum depth of 1500 m (4920 feet).

caption Anticyclonic gyres of the Alboran Sea. Source: Hauschildt et. al.

The currents of the Alboran Sea reflect the interesting water balance between the Mediterranean Sea and the Atlantic Ocean. Within the Mediterranean basin, the evaporation of water from the sea is greater than the inflow of water from rivers and rain. This makes the water of the Mediterranean more saline that that of the Atlantic and produced a water deficit in the sea that requires additional inflow to maintain the sea level.

The Mediterranean's high salinity and water deficit leads to thermohaline circulation with the Atlantic which has two parts:

  • A swift current brings water from the Atlantic into the Alboran Sea through the Strait of Gibraltar at and near the surface. As a result, the surface water of the Alboran sea is lower saline Atlantic water mixing as it progresses eastward with the higher saline Mediterranean water. The Atlantic current surrounds and feeds two anticyclonic gyres: The Western Alboran Gyre (WAG) and the Eastern Alboran Gyre (EAG).
     
  • The higher saline water of the Mediterranean Sea is denser and sinks to the lower depths of the Alboran Sea and flows out into the Atlantic Ocean through the Strait of Gibraltar

Marine Ecology

The mixture of Atlantic and Mediterranean water also results in an amalgamation of Atlantic and  Mediterranean influenced marine organisms in the Alboran Sea. While the Mediterranean Sea is generally considered an oligotrophic (low primary productivity) marine body, the Alboran Sea is considered one of the highest centres of productivity within the Mediterrranean Basin.

A special element of biodiversity found in the Alboran Sea are limited colonies of cold water coral reefs, dominated by the reef-building species Lophelia pertusa (Deepwater White Coral). These reef systems are biodiversity hotspots in only a few other locales within the Mediterranean Basin, such as in the Gulf of Lion.

Driven by water pollution and overfishing the deeper reaches of the Alboran are somewhat impoverished of marine fauna; for example, below 600 metres in depth there are virtually no copepods. However, historically the Alboran has been one of the most productive parts of the Mediterranean Basin for the pelagic fisheries of sardine and anchovy. By 2001 both of these species were considered overfished, although efforts have been made in the most recent decade to restore these fisheries to sustainable levels.

The Spanish shores of the Alboran have been affected by water pollution driven by the intensity of tourism in that region. According to the OECD residents of the Spanish Alboran coast have a strong liking for consuming sea larvae (called whitebait). Even though the sale of such larvae has been outlawed, the practise continues throughout the region and places great stress on other members of the marine food chain, due to the depletion of the larval resource.

An even more significant impact arising from the Spanish fishing fleet is cetacean driftnet bycatch, which kills a documented number of dolphins exceeding 400 per annum; another 5300 small cetaceans are entrained in the driftnets annually, which are claimed to be released. The chief species taken in bycatch are the Common dolphin and the Striped dolphin, although other cetaceans claimed in the killing are Risso's dolphin, Long-finned pilot whale and Sperm whale. Frequency of cetacean injuries is exacerbated by the dolphin and whale attraction to captured fishes, but also enhanced by the artificial lighting used on many handlines, which lights are attractive to cetaceans in their pursuit of squid prey. The resultant cetacean mortality is likely understated of the bycatch, evidenced by  strandings of juvenile Sperm whales along the Iberian and Spanish Balearic shores being inexplicably high.

On the North African coast of the Alboran Sea, fishing is virtually the only means of livelihood for most of the people who live along the coastal zone; anchovies and sardines are also the chief species taken in these Moroccan and Algerian ports.

Terrestrial Margin

See main articles: Mediterranean woodlands and forests  and Southwest Iberian Mediterranean sclerophyllous and mixed forests

The southern margin of the Alboran Sea consists largely of the Mediterranean woodlands and forests ecoregion stretches from the coastal plains to the hills of northern Morocco, Algeria and Tunisia, and eventually surrounds the Atlas Mountains. To the north is the Alboran Sea, the westernmost element of the Mediterranean Sea. The variety of substrates and climates leads to a diverse mix of vegetation including holm oak forests, cork oak forests, wild olive and carob woodlands, as well as extensive Berber thuya forest. This old, endemic North African conifer species is representative of the great diversity and endemism of both flora and fauna in this ecoregion. Reptile diversity is high and the region harbors charismatic large mammals, including the rare and endangered Barbary leopard. Unfortunately, this region contains high human populations and widespread deforestation.

The northern margin of the Alboran Sea consists substantially of the Southwest Iberian Mediterranean sclerophyllous and mixed forests ecoregion, located on the southeast coast of the Iberian Peninsula, is characterized by a human-made semi-natural landscape, formed by extensive semi-natural sylvopastoral woodlands known as montados in Portugal and as dehesas in Spain. Historically, these forests, often dominated by oaks, have represented intense multipurpose management systems, adapted to adverse environmental conditions imposed by low-quality soils and sometimes harsh climate. This ecoregion serves as habitat to some of the most endangered species in Europe such as the Iberian lynx (Lynx pardinus), the imperial eagle (Aquila heliaca), and the great bustard (Otis tarda). Change in land use has led to soil degradation; tourism, urbanization, and road building, which factors comprise the most serious threats to this ecoregion.

Ancient History

The Alboran Sea was an early exploration route for Phoenician navigators probing the Mediterranean coast along the Moroccan and Algerian coasts, as well as the southern Iberian shores. These explorations led to establishment of the westernmost settlements of the Phoenicians and their successors, the Carthaginians. The most extreme colonies of the Phoenicians involved not only the crossing of the Alboran Sea, but sailing through the Strait of Gibraltar to reach the Atlantic Moroccan coast and establish substantial permanent coastal colonies at Lixus, Chellah and Mogador as well as the inland city of Volubilis.

Further Reading

  • Peter Saundry. 2011. Seas of the world.. topic ed. C.Michael Hogan, Ed in chief Cutler J.Cleveland. Encyclopedia of Earth
  • Donde Va. Donde Va? An oceanographic experiment in the Alboran Sea: The oceanographic report. EOS, Trans. Amer. Geophys. Union, 65:682–683, 1984.
  • Rhodes W. Fairbridge, editor. The Encyclopedia of Oceanography. Van Nostrand Reinhold Co., 1966.
  • J. C. Gascard and C. Richez. Water masses and circulation in the western Alboran Sea, and in the Straits of Gibraltar. Progress in Oceanography, 15:157–216, 1985.
  • C.Michael Hogan. 2007. Chellah, The Megalithic Portal, ed. A. Burnham,
  • IUCN Centre for Mediterranean Cooperation. 2004. The Mediterranean deep-sea ecosystems: an overview of their diversity, structure, functioning and anthropogenic impacts, with a proposal for their conservation. IUCN. 64 pages
  • Organisation for Economic Co-Operation and Development. 2003. Review of Fisheries in OECD Countries: Policies and Summary Statistics (Google eBook) OECD Publishing. 400 pages
  • Sergi Tudela. 2004. Ecosystem effects of fishing in the Mediterranean: an analysis of the major threats of fishing gear and practices to biodiversity and marine habitats. General Fisheries Council for the Mediterranean (Food and Agriculture Organization of the United Nations). Food & Agriculture Org. 44 pages
  • Paul A. Tyler. 2003. Ecosystems of the deep oceans. (Google eBook) Elsevier. 569 pages
  • Jorge Vazquez-Cuervo, Jordi Font, and Juan J. Martinez-Benjamin. Observations on the circulation of the Alboran Sea using ERS-1 altimetry and sea surface temperature data. J. Phys. Oceanogr., 26:1439, 1996
  • A. Viudez, J.-M. Pinot, and R. L. Haney. On the upper layer circulation in the Alboran Sea. JGR, 103: 21,653–21,666, 1998.
  • M. Vargas-Yanez, F. Plaza, J. Garcia-Lafuente, T. Sarhan, J. M. Vargas, and P. Velez-Belchi. About the seasonal variability of the Alboran Sea circulation. Journal of Marine Systems, 35:229–248, 2002.
  • Pew Charitable Trusts. 2010. Alborán Sea Conservation
Glossary

Citation

Hogan, C. (2013). Alboran Sea. Retrieved from http://www.eoearth.org/view/article/51cbece37896bb431f68e437

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