Page 13 - ebsaws-2014-03

Page 12

Fig. 2-5. Sea floor topography of the Mediterranean Sea (Smith and Sandwell 1997).

The Mediterranean Sea has a negative hydrological balance, with loss through evaporation
exceeding the input of water through runoff and precipitation. This deficiency is mainly
compensated by the flow of Atlantic surface waters through the Strait of Gibraltar (about 35,000
km3 year-1). The major feature of the surface current system of the Mediterranean is the
movement of surface water from the Atlantic toward the east combined with numerous spin-off
eddies along the way (Miller 1983). Circulation patterns in the Mediterranean Sea, and the
progressive diversification of Atlantic surface water into a structured system of intermediate and
deep layers, are now well understood and described (e.g., Millot and Taupier-Letage 2004).
The Mediterranean circulation system also includes strong vertical convection currents that
determine the distribution of salinity and provide for vertical recycling of nutrients and other
dissolved substances. However, the sea has relatively low concentrations of nutrients even in
deeper waters. These chemicals are exported in the flow of deep water through the Strait of
Gibraltar that in turn receives nutrient-poor surface Atlantic water. No deep nutrient-rich Atlantic
waters take part in the Mediterranean circulation, and the input of nutrients is mostly due to river
input and agricultural runoff or pollution (Miller 1983).