The Mediterranean Basin is one of the most highly valued seas in the world. The region comprises a vast set of coastal and marine ecosystems that deliver valuable benefits to all its coastal inhabitants, including brackish water lagoons, estuaries, or transitional areas; coastal plains; wetlands; rocky shores and nearshore coastal areas; sea grass meadows; coralligenous communities; frontal systems and upwellings; seamounts; and pelagic systems. The Mediterranean is not only complex in ecology, but also socio-politically – twenty-one countries border this heavily used sea (UNEP/MAP, 2012).
The region enclosing the Mediterranean Sea encompasses portions of three continents: Europe and its southern peninsulas to the north, southwestern Asia to the east, and the Maghreb region of northern Africa to the south. Overall, it is a densely populated region with an intricate political history involving many different ethnic groups. This has led to a complex and patchy political map. Today 21 countries, with surface areas from 2 km2 to 2.4 million km2, have coastlines on the Mediterranean Sea. They are Albania, Algeria, Bosnia and Herzegovina, Croatia, Cyprus, Egypt, France, Greece, Israel, Italy, Lebanon, Libya, Malta, Monaco, Montenegro, Morocco, Slovenia, Spain, Syria, Tunisia, and Turkey.
The Mediterranean region has historically been the scene of intense human activity. The Mediterranean Sea and its coasts are the source of many of the resources harvested in the region, but also the conveyor belt for trade, and often the sink for the cumulative impacts of these activities. The Mediterranean is a relatively small, enclosed sea with limited exchange with the oceanic basins, intense internal mesoscale circulation, and high diversity of sensitive ecosystems. These characteristics, combined with the political complexity of the region, mean the management and protection of the coastal and marine environment will require multilateral environmental agreements and regulations, abided by at a supranational level. This approach is essential to sustainable development in all nations bordering on bodies of water that extend beyond their boundaries.
In order to be able to analyse the different environmental problems and issues that affect the Mediterranean marine and coastal ecosystems it is important to be aware of the natural characteristics of the Mediterranean Basin and have an overview of the major drivers in the Mediterranean region, including all economic sectors within the Mediterranean basin and specially those devoted to the exploitation of the coastal and marine natural resources. This allows increased understanding of the overall interrelation between Mediterranean ecosystems and the human drivers.
Geography, physiography and landscapes.
A general overview of the Mediterranean region’s physical geography reveals an irregular, deeply indented coastline, especially in the north, where the Iberian, Italian, and Balkan peninsulas jut southward from the main body of Europe. Numerous islands correspond to isolated tectonic blocks, the summits of submarine ridges, or the tips of undersea volcanoes. The largest islands are Sicily, Sardinia, Corsica, Cyprus, and Crete, and the major island groups include the Balearics off the coast of Spain and the Ionian, Cyclades, and Dodecanese islands off Greece. Apart from the coastal plains and the deltaic zones of large rivers (Ebro, Rhone, Po and Nile), the coastlines are mostly rimmed by mountain ranges. Only the coastal plains from eastern Tunisia to the Sinai Peninsula, bordered mainly by low-lying desert, are free of mountains. In fact, the highest reaches of the main mountain ranges generally mark the limit of the hydrographic basin that drains towards the Mediterranean Sea. These mountain ranges include the Atlas, the Rif, the Baetic Cordillera, the Iberian Cordillera, the Pyrenees, the Alps, the Dinaric Alps, the Hellenides, the Balkan, and the Taurus.
The Mediterranean Sea stretches from the Atlantic Ocean on the west to the Asian continent on the east, and separates Europe from Africa. The basin expands up to 2.6 million square kilometres with an average depth of 1,460 meters, and a maximum depth of 5,267 meters, making it the largest enclosed sea on Earth (Coll et al, 2010). The Mediterranean has narrow continental shelves and a large area of open sea. Therefore, a large part of the Mediterranean basin can be classified as deep sea and includes some unusual features such as variation of temperatures from 12.8°C–13.5°C in the western basin to 13.5°C–15.5°C in the eastern and high salinity of 37.5–39.5 psu.
The coasts of the western Mediterranean, just as those of the eastern basin, have been subjected in recent geologic times to the uneven action of deposition and erosion. This action, together with the movements of the sea and the emergence and submergence of the land, resulted in a rich variety of types of coasts. The Italian Adriatic coast, revealing the Apennines, is typical of an emerged coast. The granite coast of north eastern Sardinia and the Dalmatian coast where the eroded land surface has sunk, producing elongated islands parallel to the coast, are typical submerged coasts. The deltas of the Rhône, Po, Ebro, and Nile rivers are good examples of coasts resulting from silt deposition.
Circulation and water masses.
The Mediterranean Sea is a semi-enclosed sea characterized by high salinities, temperatures and densities. The net evaporation exceeds the precipitation, driving an anti-estuarine circulation through the Strait of Gibraltar, contributing to very low nutrient concentrations. The Mediterranean Sea has an active overturning circulation, one shallow cell that communicates directly with the Atlantic Ocean, and two deep overturning cells, one in each of the two main basins (Tanhua et al. 2013). It acts like an ocean system in which several temporal and spatial scales (basin, sub-basin and mesoscale) interact to form a highly complex and variable circulation. It is one of the few locations in the world where deep convection and water mass formation take place. The Mediterranean is also an important marginal basin to the North Atlantic producing very saline waters, the outflow of which through the Strait of Gibraltar may play an indirect role in the deep circulation of the North Atlantic. The inflowing waters are altered by an excess of evaporation over precipitation and slight cooling within the Mediterranean basin during their 100-year-long journey before returning back to the Atlantic (El-Geziry & Bryden 2010).
The Mediterranean hydrodynamics are driven by three layers of water masses: a surface layer, an intermediate layer, and a deep layer that sinks to the bottom. The Mediterranean Sea receives from the rivers that flow into it only about one-third of the amount of water that it loses by evaporation. In consequence, there is a continuous inflow of surface water from the Atlantic Ocean. After passing through the Strait of Gibraltar, the main body of the incoming surface water flows eastward along the north coast of Africa. This current is the most constant component of the circulation of the Mediterranean. It is most powerful in summer, when evaporation in the Mediterranean is at a maximum. This inflow of Atlantic water loses its strength as it proceeds eastward, but it is still recognizable as a surface movement in the Sicilian channel and even off the Levant coast. A small amount of water also enters the Mediterranean from the Black Sea as a surface current through the Bosporus, the Sea of Marmara, and the Dardanelles (Coll et al., 2010).
Hydrological and climatic setting.
The Mediterranean region is characterized by winter dominated rainfall and hot dry summers. Even though large spatial climate variability and diversity exist within the Mediterranean basins, many areas can be classified as arid or semiarid. The Mediterranean is an area of transition between a temperate Europe with relatively abundant and consistent water resources, and the arid African and Arabian deserts that are very short of water. The Mediterranean region is experiencing a large stress on its water resources due to a combination of effects ranging from climate change to anthropogenic pressures due to an increasing water demand for domestic and industrial use, expansion of irrigated areas, and tourism activities. More than half of the water-poor population of the world is concentrated in the Mediterranean basin, which holds only 3% of the world's fresh water resources. These resources are unevenly distributed over space. Half are located in Italy and Greece and 25% in catchments in France and Turkey. Catchments on the southern and eastern rims provide, respectively, only 4% and 2% of Mediterranean water resources (Milano et al., 2013).
Water resource availability in the Mediterranean has already been affected by environmental change, and is seriously jeopardized in future environmental, economic, and demographic scenarios (Garcia-Ruiz et al., 2011). Most global hydrological models are based on expected trends in precipitation and temperature. However, a number of studies have demonstrated the influence of land cover on river discharge and water resources. Climate and land cover change (artificial and natural reforestation, deforestation, expansion of farming areas) are likely to amplify water stress in the Mediterranean region, caused by a combination of decreased water resource availability (lower precipitation and increased evapotranspiration) and increased water use pressure resulting from economic growth and urban expansion. Special attention to mountain areas is required, as they are the most important sites for water resource generation worldwide, and particularly in temperate and semi-arid areas including the Mediterranean basin. However, mountain areas are facing increasing hydrological stress caused by a combination of i) increasing temperature and decreasing precipitation, exceeding that in the lowlands; ii) land use change, including natural and deliberate reforestation of abandoned farmland, thus increasing evapotranspiration and water consumption; and, (iii) increasing pressures on surface and groundwater resources, thus reducing river discharge and lowering the depth of the water table in groundwater-dependent areas.
The amount and distribution of rainfall in Mediterranean localities is variable and unpredictable. Along the North African coast from Gabès in Tunisia to Egypt, more than 10 inches (250 mm) of rainfall per year is rare, whereas on the Dalmatian coast of Croatia there are places that receive 100 inches (2,500 mm). Maximum precipitation is found in mountainous coastal areas (Figure 1.3). The climate in the region is characterized by hot, dry summers and cool, humid winters. The annual mean sea surface temperature shows a high seasonality and important gradients from west to east and north to south.
Coastal aquifers provide another source of freshwater discharge to the Mediterranean. The submarine groundwater discharge from the coastal aquifers, estimated at 2.200 m3/s, accounts for almost one-fifth of the total freshwater inflow into the Mediterranean, with more than one-third of this discharge entering from the sea’s European shores. Seepage inflows are prevalent on the eastern coast of the Adriatic, dominated by karstic aquifer systems, as well as on the eastern and southern Mediterranean coast with semi-arid and arid conditions, limited precipitation and runoff, and limited surface watercourses and discharge points. Coastal seepage and submarine discharges are critical to the water balance and seawater quality in the marine sub-basins. They also support wetlands and brackish water habitats, important to biodiversity, and fishery nursery areas. The coastal aquifers are threatened by over-exploitation and consequent seawater intrusion and water and land salinisation, which will add to the deficit in recharge of the Mediterranean. Submarine groundwater discharge is also a significant source of nutrient input in some regions and could provide pathways for pollutants to disperse into the sea (UNEP/MAP, 2012, and UNEP/MAP, UNESCO, 2015).
Water and nutrient characteristics.
With a typical tidal range of less than 50 cm, the Mediterranean Sea is microtidal. This reduces the potential for dilution and dispersion of dissolved and particulate wastes. It is also one of the most oligotrophic (i.e. poor in nutrients) oceanic systems, and is characterised by an eastwards longitudinal gradient in this oligotrophy. The main source of nutrients in the Mediterranean lies in the inflowing Atlantic surface waters at the level of the Gibraltar Strait. These inflowing waters flow eastward along the African coasts in the western Mediterranean, then cross the Sicily Strait and continue their flow again along the northern African coasts. As the waters move eastwards from the Gibraltar Strait, they become depleted in nutrients. By the time they reach the Egyptian coasts, their nutrient signature has almost disappeared. Additionally, the Nile River nutrient signature has disappeared due to the 1960s Nile Dam construction. All this contributes towards making the Levantine Basin (at the eastern part of the Mediterranean Sea) one of the most oligotrophic areas in the world ocean (EEA-UNEP/MAP 2014).
Additional sources of nutrients exist in the Mediterranean, but these have localised and rather small impacts. One is the outflow of Black Sea surface waters into the Aegean, which have an influence limited to the north Aegean; a second source is the Po River, emptying into the Adriatic on its western coast. The most eutrophic waters in the western basin are located on the north shore, at the mouth of the large rivers Rhone and Ebro. Riverine nutrient inputs are relatively low, as most river systems discharging in the Mediterranean Sea are small. High nutrient inputs to small rivers may be important in most North African oueds, as they collect rich effluents in large quantities. In these rivers/oueds, metals, nitrates and organic carbon reach concentrations that could affect biological populations after heavy rains following dry periods (EEA-UNEP/MAP 2014).
Rivers also are a contributor of nutrients to the sea accounting about 50% for Nitrogen and 75% for Phosphorus which together with Silica are crucial elements for maintaining biological productivity in the sea (Figure 1.4). River basins accumulate the products of various natural and anthropogenic activities (agriculture, urbanisation, wastewaters, industry, etc.) emitted into surface waters which are transported downstream to the river mouths and eventually to the sea (PERSEUS-UNEP/MAP, 2015).
The Mediterranean is one of the world’s 25 hot spots for biodiversity. Its highly diverse marine ecosystem hosts around 4 to 18% of the world’s marine biodiversity (Coll et al. 2010, Gabrié C., et al. 2012). The Mediterranean provides vital areas for the reproduction of pelagic species: the Atlantic bluefin tuna’s main spawning areas, the great white shark’s unique breeding areas and sea turtles, such as the green and loggerhead turtles, nesting areas along its eastern coast. These high oceanic productivity areas host a particularly rich marine mammal fauna and the eastern part of the basin is one of the last shelters for the threatened Mediterranean monk seal. The shallow coastal waters are home to key species and sensitive ecosystems such as seagrass beds and coralligenous assemblages, whilst the deep waters host a unique and fragile fauna. Many of these species are rare and / or threatened and are globally or regionally classified by IUCN as threatened or endangered.
This natural heritage has profoundly influenced the development of populations, transforming this basin into a rich and heterogeneous mosaic of cultures. It is defined as “under siege” due to historical and current impacts of multiple stressors. Among them, fishing practices, habitat loss and degradation, eutrophication, and more recently, the introduction of alien species and climate change effects. Since the intensity of these stressors is increasing throughout most of the Mediterranean basin, temporal analyses are increasingly needed to inform effective current and future marine policies and management actions.
Almost 86 000 km² of the Mediterranean is classified Marine Protected Areas (MPAs) or Natura 2000 site (Figure 15). In 2016, only 3 % of the Mediterranean Sea is protected. The target of 10% protection of the CBD convention is far from being achieved. New Marine Protected Areas must be created in high and deep sea, which are not represented in the current network.