According to its recent origin, the WMDW should also have lower nutrient concentrations than intermediate waters. In accordance Béthoux et al. (1998) reported a
weak maximum for nitrate and phosphate in the Western Mediterranean associated
to intermediate waters, with averaged salinity !38.492, and slightly lower nitrate
and phosphate concentrations in deep waters. Similar profiles have been reported by
other authors in the NW Mediterranean (e.g., Karafistan et al. 2002). In contrast
silicate concentrations found in deep Mediterranean waters were slightly higher
compared to intermediate waters, which has been attributed to the slower
remineralization rates of silicate relative to nitrate and phosphate (Béthoux et al.
1998). However other studies have found that nutrient concentrations associated to
deep Mediterranean waters in the NW Mediterranean, as well as in the Alboran Sea,
are slightly higher than the concentrations found in the LIW (Manca et al. 2004).
These abnormally high nutrient concentrations in the NW Mediterranean have been
attributed to an increase of nutrient inputs from terrestrial and atmospheric sources
(Manca et al. 2004). During its transit from the Gulf Lions towards the Alboran Sea
the WMDW loses part of its biogeochemical signatures. In the Alboran Sea, the
average dissolved oxygen concentration associated to this water mass is 4.50 mlÁl
À1
(Manca et al. 2004). This value is similar to values reported in earlier studies (Minas
et al. 1991) in the Western Alboran Sea. The average nitrate and phosphate concentrations in the WMDW for the Alboran basin have been estimated to be 9.13 μM and
0.41 μM respectively (Manca et al. 2004), while the average silicate concentration
associated to WMDW in the Alboran Sea was 8.38 μM (Manca et al. 2004) (i.e.,
similar to the value found by these authors for the LIW in this basin). In the Western
Alboran Sea Minas et al. (1991) detected the higher silicate concentrations in the
WMDW, with values ~10 μM on the African continental slope, due to the lifting of
this deep water mass along the southern continental slope, while nitrate in deep
waters was uniform with values ~9.0 μM.
7.2.2 Fertilization Mechanisms: Sources of New Nutrients
to the Photic Layer
The most productive areas in the Alboran Sea are found in its NW sector, in
particular on the Iberian continental margin between Europa Point and Marbella as
well as in the proximities of the Strait of Gibraltar and the northern part of the
Western Anticyclonic Gyre (WAG) (Minas et al. 1991; Baldacci et al. 2001; Reul
et al. 2005). The enhanced productivity in this area, which is clearly distinguishable
from satellite images (Garcia-Gorriz and Carr 1999, 2001), is caused by different
fertilization mechanisms that induce the upwelling of colder subsurface nutrient-rich
waters and promote phytoplankton blooms (Minas et al. 1991; Sarhan et al. 2000;
Reul et al. 2005; Ramírez et al. 2005). The presence of cold surface waters in this
area is almost permanent (Renault et al. 2012) and is favored by the bottom
topography of the continental margin in the NW Alboran Sea, where the existence
212
T. Ramírez et al.
weak maximum for nitrate and phosphate in the Western Mediterranean associated
to intermediate waters, with averaged salinity !38.492, and slightly lower nitrate
and phosphate concentrations in deep waters. Similar profiles have been reported by
other authors in the NW Mediterranean (e.g., Karafistan et al. 2002). In contrast
silicate concentrations found in deep Mediterranean waters were slightly higher
compared to intermediate waters, which has been attributed to the slower
remineralization rates of silicate relative to nitrate and phosphate (Béthoux et al.
1998). However other studies have found that nutrient concentrations associated to
deep Mediterranean waters in the NW Mediterranean, as well as in the Alboran Sea,
are slightly higher than the concentrations found in the LIW (Manca et al. 2004).
These abnormally high nutrient concentrations in the NW Mediterranean have been
attributed to an increase of nutrient inputs from terrestrial and atmospheric sources
(Manca et al. 2004). During its transit from the Gulf Lions towards the Alboran Sea
the WMDW loses part of its biogeochemical signatures. In the Alboran Sea, the
average dissolved oxygen concentration associated to this water mass is 4.50 mlÁl
À1
(Manca et al. 2004). This value is similar to values reported in earlier studies (Minas
et al. 1991) in the Western Alboran Sea. The average nitrate and phosphate concentrations in the WMDW for the Alboran basin have been estimated to be 9.13 μM and
0.41 μM respectively (Manca et al. 2004), while the average silicate concentration
associated to WMDW in the Alboran Sea was 8.38 μM (Manca et al. 2004) (i.e.,
similar to the value found by these authors for the LIW in this basin). In the Western
Alboran Sea Minas et al. (1991) detected the higher silicate concentrations in the
WMDW, with values ~10 μM on the African continental slope, due to the lifting of
this deep water mass along the southern continental slope, while nitrate in deep
waters was uniform with values ~9.0 μM.
7.2.2 Fertilization Mechanisms: Sources of New Nutrients
to the Photic Layer
The most productive areas in the Alboran Sea are found in its NW sector, in
particular on the Iberian continental margin between Europa Point and Marbella as
well as in the proximities of the Strait of Gibraltar and the northern part of the
Western Anticyclonic Gyre (WAG) (Minas et al. 1991; Baldacci et al. 2001; Reul
et al. 2005). The enhanced productivity in this area, which is clearly distinguishable
from satellite images (Garcia-Gorriz and Carr 1999, 2001), is caused by different
fertilization mechanisms that induce the upwelling of colder subsurface nutrient-rich
waters and promote phytoplankton blooms (Minas et al. 1991; Sarhan et al. 2000;
Reul et al. 2005; Ramírez et al. 2005). The presence of cold surface waters in this
area is almost permanent (Renault et al. 2012) and is favored by the bottom
topography of the continental margin in the NW Alboran Sea, where the existence
212
T. Ramírez et al.