Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the western Alboran Sea

Abstract

Previous hydrological investigations of the Gibraltar Strait have left some questions unanswered as regards the composition of the Mediterranean outflow into the Atlantic Ocean. In the present study, nutrient, oxygen, and chlorophyll distributions are used to characterize the main water masses flowing through the Strait of Gibraltar and the Alboran Sea, with particular reference to Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW). High chlorophyll a concentrations, reaching 3 μg L−1 at the subsurface, are observed in the northern part of the anticyclonic Alboran Sea gyre. The salinity subsurface minimum (≃36), located temporarily in the strait, corresponds to high nutrient concentrations (7–8 μg at. L−1). Diagram analysis shows that in the upwelling area off the Spanish coast, nutrients are largely transported into the photic zone, and that photosynthetic nitrate consumption actually takes place. Consequently, the formation of the Alboran Sea O2 extraminimum can be related to the enhanced new production. Most of the lowest O2 values (<4.00 mL L−1) also correspond to a slight nutrient maximum (PO4 and NO3). Comparative diagram analysis of biologically affected parameters (oxygen and nutrients) versus salinity as a conservative property confirms and expands results from the classical θ‐S analysis. Oxygen and silicate exhibit the best tracer qualities for the circulation pattern, demonstrating in particular that during the period of observations, water rich in LIW feeds the Mediterranean outflow.

The N:Si:P molar ratio in the Strait of Gibraltar

“…It is well documented that when mixing events occur at or near the sill region of the Strait, the Atlantic water is enriched with nutrients and advected towards the Mediterranean [e.g., Minas et al, 1991;Echevarría et al, 2002]. Minas and Minas [1995] noted that Mediterranean water induces a hydrological recycling of high N:P water at the sill, back to the Mediterranean.…”

“…The Atlantic water is one component of this balance that is still poorly understood [Béthoux et al, 1998]. Despite the fact that the Atlantic water entering the Mediterranean Sea is depleted in nutrients [e. g., Minas et al, 1991;Turley, 1999], all existing descriptions of nutrients in the Strait suggest that these nutrients are related in the Redfield ratio [Béthoux et al, 2002]. Most of these observations were made on the western entrance of the Strait, and they were applied to the Atlantic water in the Mediterranean Sea [Béthoux et al, 2002].…”

Zonation patterns of benthic communities in an upwelling area from the western Medierranean (La Herradura, Alboran Sea)

“…Surface Atlantic waters enter the Mediterranean through the Strait of Gibraltar and reach the Alboran Sea poorly mixed with Mediterranean waters (Cano, 1978). Furthermore, Atlantic waters usually describe an anticyclonic gyre that generates an upwelling of deep waters along the coasts of Málaga and Granada (southern Spain; Lanoix, 1974;Parrilla and Kinder, 1987;Minas et al, 1991;Tintoré et al, 1991). Deep water upwellings are enhanced by the strong western winds that usually blow in this area (Rodríguez, 1990).…”

Variations of Mediterranean–Atlantic exchange across the late Pliocene climate transition

“…On its way, it mixes with upwelled MOW within the Strait of Gibraltar (Folkard et al, 1997) and with surface waters of the Alboran Sea, creating the Modified Atlantic Water with temperatures of 15-16 °C and salinity of 36.5 (Millot, 1999), which follows two anticyclonic gyres, the Western Alboran Gyre (WAG) and the Eastern Alboran Gyre (EAG) (Gascard and Richez, 1985;Vargas-Yáñez et al, 2002). The northern Alboran Sea is affected by upwelling along the Spanish coast providing nutrients and enhanced 80 primary productivity (Minas et al, 1991;Peeters et al, 2002;Sarhan et al, 2000). The bottom layer is represented by the Western Mediterranean Deep water (WMDW; water temperature ~ 13 °C, salinity ~ 38.5) below 1000 m, formed due to overturning in the Gulf of Lions (Bryden et al, 1994;Hernández-Molina et al, 2006;Millot, 1999;Rhein, 1995).…”