~1 and ~2. Below 100 m depth, the ratio tends to decrease slowly with depth. In the
NW Alboran Sea, at 300 m depth the average N:Si values ranged from ~1.5 to ~1.8
(Ramírez et al. 2005; Ramírez 2007). Crombet et al. (2011) found integrated
(0–100 m) N:Si ratios >1 at the Strait of Gibraltar and at the eastern border of the
Alboran Sea, which is consistent with the Si deficiency detected at shallow depths in
the NW Alboran Sea. The higher N:Si ratios found at !50 m in the NW Alboran Sea
could be due to a fast remineralization of nitrate in the water column compared to the
dissolution of biogenic silica, even if the latter is accelerated by bacterial colonization (Bidle and Azam 2001).
In the Almeria-Oran frontal area, Leblanc et al. (2004) observed a strong P
limitation in the photic layers. They found very low phosphate concentrations and
high integrated N:P ratios in the photic layer at the frontal zone, the jet, and the
adjacent waters. Thus, N:P ratios values of 24:1 were detected in the Atlantic waters
adjacent to the front, while the highest N:P ratio (90:1) was associated to Mediterranean waters adjacent to the front. These findings are consistent with observations
in the Algero-Balear basin, where P is in general the main limiting nutrient
(Thingstad et al. 1998; Moutin and Raimbault 2002). The N:Si values in the photic
layer of the Almeria-Oran front ranged from 0.32 to 0.83, while below the photic
zone they obtained N:Si values ranging from 1.4 to 2.0 (Leblanc et al. 2004).
7.3 Phytoplankton Productivity: Coupling Between
Physical, Biogeochemical, and Biological Features
7.3.1 Distribution Patterns of Chlorophyll-a and Primary
Production from In Situ Data
The Alboran Sea is one of the most productive basins of the Mediterranean Sea. As
previously discussed in this chapter, PP in the Alboran Sea is mainly related to windinduced upwelling (particularly that forced by westerlies), mostly on the northern
side of the WAG off the Spanish coast (Sarhan et al. 2000; Reul et al. 2005; Macías
et al. 2007), and to the frontal ageostrophic circulation, associated to the path of the
Atlantic jet, that induces upwelling at the periphery of the anticyclonic gyres (Sarhan
et al. 2000; Garcia-Gorriz and Carr 1999, 2001). Therefore, any changes in the wind
regime and the Atlantic jet characteristics may influence the dynamics and productivity of the Alboran Sea planktonic ecosystem (Garcia-Gorriz and Carr 2001; Ruiz
et al. 2001; Macías et al. 2009; Oguz et al. 2014; Kersting 2016).
A number of studies have reported in situ data on Chl-a and/or PP distribution in
the Alboran Sea (e.g., Morán and Estrada 2001; L’Helguen et al. 2002; Ramírez et al.
2005; Reul et al. 2005; Mercado et al. 2014; García-Martínez et al. 2019). Many of
them have been conducted in the NW sector of the Alboran Sea, in particular in the
area between Gibraltar and Marbella, as well as in the eastern side of the Strait of
Gibraltar and the northern side of the WAG, since these are the most productive
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T. Ramírez et al.
NW Alboran Sea, at 300 m depth the average N:Si values ranged from ~1.5 to ~1.8
(Ramírez et al. 2005; Ramírez 2007). Crombet et al. (2011) found integrated
(0–100 m) N:Si ratios >1 at the Strait of Gibraltar and at the eastern border of the
Alboran Sea, which is consistent with the Si deficiency detected at shallow depths in
the NW Alboran Sea. The higher N:Si ratios found at !50 m in the NW Alboran Sea
could be due to a fast remineralization of nitrate in the water column compared to the
dissolution of biogenic silica, even if the latter is accelerated by bacterial colonization (Bidle and Azam 2001).
In the Almeria-Oran frontal area, Leblanc et al. (2004) observed a strong P
limitation in the photic layers. They found very low phosphate concentrations and
high integrated N:P ratios in the photic layer at the frontal zone, the jet, and the
adjacent waters. Thus, N:P ratios values of 24:1 were detected in the Atlantic waters
adjacent to the front, while the highest N:P ratio (90:1) was associated to Mediterranean waters adjacent to the front. These findings are consistent with observations
in the Algero-Balear basin, where P is in general the main limiting nutrient
(Thingstad et al. 1998; Moutin and Raimbault 2002). The N:Si values in the photic
layer of the Almeria-Oran front ranged from 0.32 to 0.83, while below the photic
zone they obtained N:Si values ranging from 1.4 to 2.0 (Leblanc et al. 2004).
7.3 Phytoplankton Productivity: Coupling Between
Physical, Biogeochemical, and Biological Features
7.3.1 Distribution Patterns of Chlorophyll-a and Primary
Production from In Situ Data
The Alboran Sea is one of the most productive basins of the Mediterranean Sea. As
previously discussed in this chapter, PP in the Alboran Sea is mainly related to windinduced upwelling (particularly that forced by westerlies), mostly on the northern
side of the WAG off the Spanish coast (Sarhan et al. 2000; Reul et al. 2005; Macías
et al. 2007), and to the frontal ageostrophic circulation, associated to the path of the
Atlantic jet, that induces upwelling at the periphery of the anticyclonic gyres (Sarhan
et al. 2000; Garcia-Gorriz and Carr 1999, 2001). Therefore, any changes in the wind
regime and the Atlantic jet characteristics may influence the dynamics and productivity of the Alboran Sea planktonic ecosystem (Garcia-Gorriz and Carr 2001; Ruiz
et al. 2001; Macías et al. 2009; Oguz et al. 2014; Kersting 2016).
A number of studies have reported in situ data on Chl-a and/or PP distribution in
the Alboran Sea (e.g., Morán and Estrada 2001; L’Helguen et al. 2002; Ramírez et al.
2005; Reul et al. 2005; Mercado et al. 2014; García-Martínez et al. 2019). Many of
them have been conducted in the NW sector of the Alboran Sea, in particular in the
area between Gibraltar and Marbella, as well as in the eastern side of the Strait of
Gibraltar and the northern side of the WAG, since these are the most productive
226
T. Ramírez et al.