discharge, particularly to sub-basins such as the South Levan- tine, Alboran, Southwestern Basin, Aegean, Central, and North Levantine. Given the importance of water resources to local economies in this part of the world, a 20 % reduction in only 40 years is significant. Climate monitoring and modelling studies re- veal a general trend toward drier and warmer conditions, which started in the last century and is expected to worsen in the fu- ture. The trend could have serious repercussions for riverine wa- ter discharges (Ludwig et al. 2009).
Due to damming, several major Mediterranean rivers, such as the Rhone and Ebro, have seen a reduction in freshwater discharge. River flow regulation for irrigation purposes is es- timated to have caused a further reduction of up to 40 % in freshwater discharge to the Mediterranean Sea (Poulos 2011). In the case of the Ebro River in Spain, trend analysis shows a reduction of the river’s discharge of about 50 % since the 1950s. This change is attributed to increased human water use (urban demand, agriculture, industry, and tourism) and regulation, but also to an increase in shrub and afforestation-related vegeta- tion cover where grazing and traditional agriculture have dis- appeared (Lopez-Moreno et al. 2011). Similarly, Zahar and Al- bergel (1999) reported that the closure of the Sidi Salem Dam in Tunisia led to a reduction of the mean annual discharge of the Medjerdah River by 65 % due to diversion for irrigation and evaporative losses.
Coastal aquifers provide another source of freshwater discharge to the Mediterranean. The submarine groundwater discharge from the coastal aquifers, estimated at 2.200 m3
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 ma- rine 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 conse- quent seawater intrusion and water and land salinisation, which will add to the deficit in recharge of the Mediterranean (UNEP/ MAP/MED POL 2005). 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 (Lobko- vsky et al. 2003).
Climatically, the Mediterranean is characterised by warm tem- peratures, winter-dominated rainfall, dry summers, and a profu- sion of microclimates (UNEP/MAP/MED POL 2003). Mean annual temperature follows a marked north-to-south gradient, with lo- cal variations superimposed by geography.
Mediterranean circulation and water masses /s, accounts for
almost one-fifth of the total freshwater inflow into the Mediter- ranean, with more than one-third of this discharge entering from the sea’s European shores (Zektser et al. 2006). Seepage inflows are prevalent on the eastern coast of the Adriatic, dominated by
22 STATE OF THE MEDITERRANEAN MARINE AND COASTAL ENVIRONMENT
A large thermohaline cell (affected by both temperature and sa- linity) characterises the general circulation in the Mediterranean Sea. Circulation is driven by the water balance deficit and by the heat fluxes between the sea and the atmosphere. The water defi- cit, caused by greater evaporation than precipitation and river run-off, is mainly compensated for by the inflow of Atlantic water through the Straits of Gibraltar and by the water contribution from the Black Sea through the Straits of the Dardanelles. The exchange of heat with the atmosphere, leading to the cooling and subsequent sinking of surface waters, also contributes to the thermohaline circulation.