Levantine deep water

Figure 4.2 shows a typical vertical temperature and salinity profile from the eastern Levantine Basin in May. Beneath a shallow mixed layer and sharp seasonal thermocline, there is a modified Atlantic water, which has a characteristic low salinity. As the surface water flows to the east the modified Atlantic water becomes progressively more saline. Eventually it is converted into LIW and Levantine Deep Water (LDW). The transformation occurs in rather limited areas within the eastern basin where the oceanographic and climatic conditions combine to cause downward flow of water in winter. As occurs in all parts of the ocean, it results in the downward flow of biogeochemically active chemical substances such as oxygen, carbon dioxide and nutrients from the surface into intermediate and deep water. 

The concentration of dissolved nutrients in the deep waters of the Eastern Mediterranean is much lower than those in other oceans of the world, and when these are mixed into the surface waters they support very low primary productivity. The basic reason for this ultra-oligotrophic status is that the Mediterranean has an anti-estuarine (reverse thermohaline) circulation in which nutrient-depleted surface waters flow into the western basin at the Straits of Gibraltar and then on into the eastern basin at the Straits of Sicily. The deeper counter current consists of Levantine Intermediate Water (LIW) which contains a significant amount of dissolved nutrients. 

Sea and the Levantine Basin. The deep waters of these basins used to be very homogenous in temperature and salinity displaying only small horizontal gradients (Fig. 4.2 Schlitzer etal., 1991). Roether and Schlitzer (1991) computed an annual mean formation rate of 0.3 Sv for LDW formed in the Adriatic. Using freon and tritium measurements, Roether etal. (1996) calculated a mean residence time of LDW in the eastern basin which was 50-80 years. 

Observations on this new deep water have shown that it contains CFC s characteristic of water formed recently (Roether etal., 1996). The water has higher dissolved oxygen and lower silicate than the adjacent LDW formed in the Adriatic. Once this deep water flowed into the Levantine Basin, it caused existing deep water to mix upwards into the LIW. Klein etal. (1999) report... 

After the winter bloom, water column stratification occurs in March-April, which results in the formation of the DCM that is characteristic of the system for the remainder of the year. The stratification starts in the offshore area in the Southeast Levantine Basin and spreads from there to the north and west. In summer the DCM is typically greater than 100 m deep with the waters above it depleted in inorganic N and P but containing significant amounts of dissolved organic nitrogen and phosphorus. 

Kress, N. and Herut, B. (2001) Spatial and seasonal evolution of dissolved oxygen and nutrients in the Southern Levantine Basin (Eastern Mediterranean Sea) chemical characterisation of the water masses and inferences on the N P ratios. Deep-Sea Research I, 48, 2347-2372. 

---