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The role of the exchanges through the Strait of Gibraltar on the budget of elements in the Western Mediterranean Sea: consequences of human-induced modifications

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The role of the Strait of Gibraltar on the exchanges of substances between Mediterranean Sea and the Atlantic Ocean is reviewed.

The previous estimations have been recalculated by using a similar water flux and compared with the river and atmospheric inputs to the Western Mediterranean Sea. The man-induced changes in the dimensions of the Strait of Gibraltar increasing (planning the sill) or reducing of the cross-section by a total or partial dam are discussed. A total dam will control the sea-level rise in the Mediterranean Sea, but an annual increase of major nutrient concentrations of 1โ€“2% could be expected, lower than the rate of increase of the river and atmospheric inputs in the Western Mediterranean Sea. The increase of the cross-section of the Strait by increasing the depth (planning) at the sill could compensate the increase of the external nutrient inputs.

The Mediterranean Sea is undergoing a rapid alteration under the combined pressure of climate change and human impact (Jefti[1]c et al., 1992; Turley, 1999). The Strait of Gibraltar, the only significant link between the Mediterranean Sea and the worlds oceans (14 km width and less than 300 m depth at the sill), appears as a point of control of these exchanges and the modification of the biogeochemistry and circulation of the Mediterranean Sea and the North Atlantic Ocean. The basic circulation in the Strait of Gibraltar consists in an upper layer of warm, fresh surface Atlantic water (SAW) and North Atlantic central water (NACW) inflowing into the Mediterranean Sea, and an opposite deep current of colder, salty Mediterranean outflowing water (MOW). As the evaporation exceeds over the sum of precipitation and river discharges in the Mediterranean Sea, the Atlantic inflow slightly exceeds around 4โ€“5% the outflow of Mediterranean water to balance the net loss (Bryden et al., 1994).

The waters exchanges through the Strait have an influence on the formation of deep water (Reid, 1979; Baringer and Price, 1999), the surface circulation in the Atlantic Ocean (Ozgokmen et al., 2001) and in the biogeochemistry of the entire Mediterranean basin (Minas and Minas, 1993). Substances with higher concentrations in deep waters (nutrient-profile) present net loss through the Strait, experiencing a dilution in the Mediterranean Sea. The inverse estuarine circulation of the whole basin determinates a negative budget for the nutrients at the Gibraltar Strait, importing nutrient-poor surface waters from the Atlantic Ocean and exporting relatively nutrient-rich deep waters (Coste et al., 1988; Gomezet al., 2000b). On other hand, substances with higher concentration in the surface waters of the Gulf of Cadizsuch as several trace metals will be concentrated in an evaporative basin as the Mediterranean Sea (Elbaz-Poulichet et al., 2001ยช,b).

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