of submarine canyons could play a role in the exchange of waters between the
continental shelf and deep waters (Parrilla et al. 1986; Lafuente et al. 1999; Sarhan
et al. 2000). On the other hand, in the Alboran Sea, the intermediate Mediterranean
waters flow following the geometry of the north continental shelf, banking against
the slope (Parrilla et al. 1986) which facilitates its upwelling.
By far one of the most important fertilization mechanisms, due to its effect along
all the northern continental shelf of the Alboran Sea (from Europa Point to further
East of Cape Gata), is the upwelling induced by westerlies (Sarhan et al. 2000;
Baldacci et al. 2001; Garcia-Gorriz and Carr 1999, 2001; Bakun and Agostini 2001;
Mercado et al. 2012). Wind-driven upwelling is particularly important in coastal and
continental shelf waters (Sarhan et al. 2000). This type of upwelling is more intense
in spring coinciding with stronger westerlies (Garcia-Gorriz and Carr 1999, 2001;
Ramírez et al. 2005). Thus the wind regime has important consequences on the Chl-a
variability throughout the year. In the NW Alboran Sea, it has been reported that on
average 70% of nitrate and 83% of silicate temporal variability (Ramírez 2007) was
explained by the average seasonal zonal wind (E-W) component and seawater
temperature, over 12 seasonal cruises, using multiple regression model. Under
certain conditions, the upwelling can be attenuated or even inhibited by different
factors. Thus the presence of the Atlantic jet close to the Spanish coast may hamper
the upwelling of subsurface waters, caused by Ekman pumping due to westerlies
(Sarhan et al. 2000). On the African continental shelf, westerlies induce
downwelling of poor nutrient surface waters hampering phytoplankton growth
(Bakun and Agostini 2001). On the other hand, easterlies cause convergence and
downwelling of surface waters along the Spanish coast, while they produce upwelling along the African coast. During summer and autumn, when easterlies dominate
the wind regime, downwelling occurs along the Spanish coast and upwelling takes
place along the African coast (Bakun and Agostini 2001; Stanichny et al. 2005). The
presence of a marked thermocline from July to September can also hamper the
upwelling (Garcia-Gorriz and Carr 2001). Nevertheless, upwelling phenomena can
occur even in summer under strong favorable winds, leading to occasional blooms
(Ramírez 2007).
In addition to wind-driven upwelling, in this sector of the Alboran Sea, the path of
Atlantic jet leads to the formation of an intense geostrophic front located at the
northernmost limit of the WAG (Minas et al. 1991; Vargas-Yañez et al. 2002;
Garcia-Gorriz and Carr 2001; Vélez-Belchí et al. 2005). The instabilities and vertical
velocities associated to the agestrophic cross-fontal circulation promote the continuous vertical supply of nutrients to the photic zone (Tintoré et al. 1991; Gil and
Gomis 1994; Sarhan et al. 2000). Another fertilization mechanism is linked to the
variability of the Atlantic jet in the Western Alboran Sea. The position of the jet
show fluctuations over time caused by variations in both the inflow of Atlantic water
and the entrance angle of the Atlantic jet into the Alboran Sea (Sarhan et al. 2000;
Vargas-Yañez et al. 2002). Thus the jet can shift southwards several km in short time
periods (2–3 days) (Sarhan et al. 2000; Reul et al. 2005) leading to the upwelling of
subsurface waters north of the jet. This upwelling mechanism seems only to occur
7 The Biogeochemical Context of Marine Planktonic Ecosystems
213
continental shelf and deep waters (Parrilla et al. 1986; Lafuente et al. 1999; Sarhan
et al. 2000). On the other hand, in the Alboran Sea, the intermediate Mediterranean
waters flow following the geometry of the north continental shelf, banking against
the slope (Parrilla et al. 1986) which facilitates its upwelling.
By far one of the most important fertilization mechanisms, due to its effect along
all the northern continental shelf of the Alboran Sea (from Europa Point to further
East of Cape Gata), is the upwelling induced by westerlies (Sarhan et al. 2000;
Baldacci et al. 2001; Garcia-Gorriz and Carr 1999, 2001; Bakun and Agostini 2001;
Mercado et al. 2012). Wind-driven upwelling is particularly important in coastal and
continental shelf waters (Sarhan et al. 2000). This type of upwelling is more intense
in spring coinciding with stronger westerlies (Garcia-Gorriz and Carr 1999, 2001;
Ramírez et al. 2005). Thus the wind regime has important consequences on the Chl-a
variability throughout the year. In the NW Alboran Sea, it has been reported that on
average 70% of nitrate and 83% of silicate temporal variability (Ramírez 2007) was
explained by the average seasonal zonal wind (E-W) component and seawater
temperature, over 12 seasonal cruises, using multiple regression model. Under
certain conditions, the upwelling can be attenuated or even inhibited by different
factors. Thus the presence of the Atlantic jet close to the Spanish coast may hamper
the upwelling of subsurface waters, caused by Ekman pumping due to westerlies
(Sarhan et al. 2000). On the African continental shelf, westerlies induce
downwelling of poor nutrient surface waters hampering phytoplankton growth
(Bakun and Agostini 2001). On the other hand, easterlies cause convergence and
downwelling of surface waters along the Spanish coast, while they produce upwelling along the African coast. During summer and autumn, when easterlies dominate
the wind regime, downwelling occurs along the Spanish coast and upwelling takes
place along the African coast (Bakun and Agostini 2001; Stanichny et al. 2005). The
presence of a marked thermocline from July to September can also hamper the
upwelling (Garcia-Gorriz and Carr 2001). Nevertheless, upwelling phenomena can
occur even in summer under strong favorable winds, leading to occasional blooms
(Ramírez 2007).
In addition to wind-driven upwelling, in this sector of the Alboran Sea, the path of
Atlantic jet leads to the formation of an intense geostrophic front located at the
northernmost limit of the WAG (Minas et al. 1991; Vargas-Yañez et al. 2002;
Garcia-Gorriz and Carr 2001; Vélez-Belchí et al. 2005). The instabilities and vertical
velocities associated to the agestrophic cross-fontal circulation promote the continuous vertical supply of nutrients to the photic zone (Tintoré et al. 1991; Gil and
Gomis 1994; Sarhan et al. 2000). Another fertilization mechanism is linked to the
variability of the Atlantic jet in the Western Alboran Sea. The position of the jet
show fluctuations over time caused by variations in both the inflow of Atlantic water
and the entrance angle of the Atlantic jet into the Alboran Sea (Sarhan et al. 2000;
Vargas-Yañez et al. 2002). Thus the jet can shift southwards several km in short time
periods (2–3 days) (Sarhan et al. 2000; Reul et al. 2005) leading to the upwelling of
subsurface waters north of the jet. This upwelling mechanism seems only to occur
7 The Biogeochemical Context of Marine Planktonic Ecosystems
213