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Oceanography


Being nearly landlocked affects conditions in the Mediterranean Sea: for instance, tides are very limited as a result of the narrow connection with the Atlantic Ocean. The Mediterranean is characterised and immediately recognised by its deep blue colour.

Evaporation greatly exceeds precipitation and river runoff in the Mediterranean, a fact that is central to the water circulation within the basin. Evaporation is especially high in its eastern half, causing the water level to decrease and salinity to increase eastward. The average salinity in the basin is 38 PSU at 5 m depth. The temperature of the water in the deepest part of the Mediterranean Sea is 13.2 °C.

General ciruclation
Water circulation in the Mediterranean can be described from the surface waters entering from the Atlantic through the Strait of Gibraltar. These cool and relatively low-salinity waters circulate westwards along the North African coasts. A part of these surface waters does not pass the Sicily Strait, deviates towards Corsica before exiting the Mediterranean. The surface waters entering the eastern Mediterranean basin circulate along the Lybian and Israelian coasts. Upon reaching the Levantine Sea, the surface waters have experienced warming and saltening from their initial Atlantic state, they have now a more important density and dive to form the Levantine Intermediate Waters (LIW). Most of the water found anywhere between 50 and 600 m deep in the Mediterranean originates from the LIW. LIW are formed along the coasts of Turkey and circulate eastwards along the Greek and South Italian coasts. LIW are the only waters passing the Sicily Strait eastwards. After the Sicily Strait, the intermediate waters circulate along the Italian, French and Spanish coasts before exiting the Mediterranean through the depths of the Strait of Gibraltar. Deep water in the Mediterranean is formed in three main areas: the Adriatic Sea, from which originates most of the deep water in the eastern Mediterranean, the Aegean Sea, and the Gulf of Lion. Deep water formation in the Mediterranean is triggered by strong winter convection fueled by intense cold winds like the Bora. When new deep water is formed, the older waters mix with the overlaying intermediate waters and eventually exit the Mediterranean. The residence time of water in the Mediterranean is approximately 100 years, making the Mediterranean especially sensitive to climate change.

Other events affecting water circulation
Being a semi-enclosed basin, the Mediterranean experiences transitory events that can affect the water circulation on short time scales. In the Mid 1990s, the Aegean Sea became the main area for deep water formation in the eastern Mediterranean after particularly cold winter conditions. This transitory switch in the origin of deep waters in the eastern Mediterranean was termed Eastern Mediterranean Transient (EMT) and had major consequences on water circulation of the Mediterranean. Another example of transient event affecting the Mediterranean circulation is the periodic inversion of the North Ionian Gyre, which is a anticyclonic ocean gyre observed in the northen part of the Ionian Sea, off the Greek coast. The transition from anticylonic to cyclonic rotation of this gyre changes the origin of the waters fueling it. When the circulation is anticyclonic (most common), the waters of the gyre originate from the Adriatic Sea. When the circulation is cyclonic, the waters originate from the Levantine Sea. These waters having different physical and chemical characteristics, the periodic inversion of the North Ionian Gyre (called Bimodal Oscillating System or BiOS) has important consequences on the Mediterranean circulation and biogeochemistry.

Climate change
Because of the short residence time of waters, the Mediterranean Sea is considered a hot-spot for climate change effects. Bottom water temperatures have increased by 0.12°C between 1959 and 1989. According to climate projections, the Mediterranean Sea could become warmer. The decrease in precipitations over the region could lead to more important evaporation ultimately increasing the Mediterranean Sea salinity. Because of the changes in temperature and salinity, the Mediterranean Sea may become more stratified by the end of the 21st century, with important consequences on water circulation and biogeochemistry.

Biogeochemistry
In spite of its great biodiversity, concentrations of chlorophyll and nutrients in the Mediterranean Sea are very low, making it one of the most oligotrophic ocean regions in the world. The Mediterranean Sea is commonly referred to as an LNLC (Low-Nutrient, Low-Chlorophyll) area. The Mediterranean Sea fits the definition of a desert as it experiences little precipitations and its nutrient contents are low, making it difficult for plants and animals to develop.

There are intense gradients in nutrient concentrations, chlorophyll concentrations and primary productivity in the Mediterranean. Nutrient concentrations in the western part of the basin are approximately two times higher than the concentrations in the eastern basin. The Alboran Sea, close to the Strait of Gibraltar, has a daily primary productivity of about 0.25 gC m-2 day-1 whereas the eastern basin has an average daily productivity of 0.16 gC m-2 day-1. For this reason, the eastern part of the Mediterranean Sea is termed "ultraoligotrophic". The productive areas of the Mediterranean Sea are few and have a small spatial extent. High (i.e. more than 0.5 grams of chlorophyll a per cubic meter) productivity occurs in coastal areas, close to the river mouths which are important suppliers of dissolved nutrients. The Gulf of Lion has a relatively high productivity because it is an area of high vertical mixing, bringing nutrients to the surface waters that can be used by phytoplankton to produce chlorophyll a.

Primary productivity in the Mediterranean is also marked by an intense seasonal variability. In Winter, the strong winds and precipitation over the basin generate important vertical mixing, bringing nutrients from the deep waters to the surface, where phytoplankton can convert it into biomass. However, in Winter, light may be the limiting factor for primary productivity. Between March and April, Spring offers the ideal trade-off between light intensity and nutrient concentrations in surface for a Spring bloom to occur. In Summer, high atmospheric temperatures lead to the warming of the surface Mediterranean waters. The resulting density difference virtually isolates the surface Mediterranean waters from the rest of the water column and nutrient exchanges are limited. As a consequence, primary productivity is very low between June and October.

Oceanographic expeditions uncovered a characteristic feature of the Mediterranean Sea biogeochemistry: most of the chlorophyll production does not occur in surface, but in sub-surface between 80 and 200 meters deep. Another key characteristic of the Mediterranean is its high nitrogen-to-phosphorus ratio (N:P). Redfield demonstrated that most of the world's oceans have an average N:P ratio around 16. However, the Mediterranean Sea has an average N:P between 24 and 29, which translates a widespread phosphorus limitation.

Because of its low productivity, plankton assemblages in the Mediterranean Sea are dominated by small organisms such as picophytoplankton and bacteria.