(Morán and Estrada 2001). As the distance to the center of the gyres increases the
nutricline uplifts rapidly, becoming shallower in the frontal areas associated to the
WAG and the EAG (Arin et al. 2002; Reul et al. 2005; Leblanc et al. 2004; Mercado
et al. 2014), due to the cross-frontal ageostrophic currents associated to the frontal
systems in the western and eastern Alboran basin (Tintore et al. 1988, 1991). In both
frontal areas the nutricline is usually found at <15 m depth (Morán and Estrada
2001; Reul et al. 2005; Leblanc et al. 2004).
In the continental margin of the NW Alboran Sea, the nutricline is roughly found
at similar depths for nitrate, phosphate, and silicate (Ramírez et al. 2005; Ramírez
2007) and it is notably shallower than in open waters of the Alboran Sea.
The banking of intermediate Mediterranean waters, circulating close to the border
of the continental shelf (Parrilla and Kinder 1987), and the remineralization process
in the water column could have a major role in the development of a shallower
nutricline in this sector. On the other hand, the nutricline in this area of the Alboran
Sea shows seasonal variations, being generally weak in winter, probably due to the
mixing in the water column and to the influence of low nutrient water masses at
intermediate depths (Ramírez 2007). The nutricline becomes steeper during the
course of the year as the water column stratification develops (Ramírez et al. 2005;
Ramírez 2007). In general, the nutricline in the continental margin is well developed
in summer and also in early autumn, when the vertical profiles of nitrate, phosphate,
and silicate are characterized by a relative maximum located around 50–75 m
(Ramírez et al. 2005; Ramírez 2007), while this nutrient maxima is absent in winter
and spring. At those maxima nutrient concentrations reach values close to ~7.0 μM
for nitrate, ~0.35–0.40 μM for phosphate and ~4.0 μM for silicate, coinciding with a
relative minimum of dissolved oxygen (Ramírez 2007). These findings suggest the
occurrence of intense remineralization processes in the water column at shallower
depths on the continental margin compared to remineralization processes in deeper
areas of the westernmost sector of the Alboran Sea (Minas et al. 1991). At
200–300 m depth nutrients also show a high variability (Ramírez 2007). Thus, on
the continental margin at 300 m depth the average nutrient concentrations are about
~8.5–10.0 μM for nitrate, ~0.4–0.5 μM for phosphate, and ~5.0–6.5 μM for silicate
(Ramírez 2007). Further offshore nitrate concentrations are higher, ~12 μM at 300 m
depth (García-Martínez et al. 2019).
On the other hand the vertical distribution of nitrite in the Alboran Sea shows a
conspicuous subsurface maximum (Bianchi et al. 1994; Ramírez et al. 2005;
Ramírez 2007; García-Martínez et al. 2019). In the Alboran Sea the presence of
nitrite maximum is frequently observed throughout the all year (Ramírez 2007;
García-Martínez et al. 2019). The highest values are usually found during the
stratification period, with an average nitrite concentration close to 0.3 μM (Ramírez
et al. 2005; Ramírez 2007; García-Martínez et al. 2019). In the NW Alboran Sea this
nitrite maximum is usually detected at depths ranging from 50 to 75 m (Fig. 7.2),
frequently associated to the end of the nutricline (Ramírez et al. 2005; Ramírez 2007;
García-Martínez et al. 2019). In the Almeria-Oran front area, the depth of the nitrite
maximum have been found to range from ~40 to 110 m (Bianchi et al. 1994), with
the highest concentrations (>0.5 μM) being found in the Atlantic waters adjacent to
220
T. Ramírez et al.
nutricline uplifts rapidly, becoming shallower in the frontal areas associated to the
WAG and the EAG (Arin et al. 2002; Reul et al. 2005; Leblanc et al. 2004; Mercado
et al. 2014), due to the cross-frontal ageostrophic currents associated to the frontal
systems in the western and eastern Alboran basin (Tintore et al. 1988, 1991). In both
frontal areas the nutricline is usually found at <15 m depth (Morán and Estrada
2001; Reul et al. 2005; Leblanc et al. 2004).
In the continental margin of the NW Alboran Sea, the nutricline is roughly found
at similar depths for nitrate, phosphate, and silicate (Ramírez et al. 2005; Ramírez
2007) and it is notably shallower than in open waters of the Alboran Sea.
The banking of intermediate Mediterranean waters, circulating close to the border
of the continental shelf (Parrilla and Kinder 1987), and the remineralization process
in the water column could have a major role in the development of a shallower
nutricline in this sector. On the other hand, the nutricline in this area of the Alboran
Sea shows seasonal variations, being generally weak in winter, probably due to the
mixing in the water column and to the influence of low nutrient water masses at
intermediate depths (Ramírez 2007). The nutricline becomes steeper during the
course of the year as the water column stratification develops (Ramírez et al. 2005;
Ramírez 2007). In general, the nutricline in the continental margin is well developed
in summer and also in early autumn, when the vertical profiles of nitrate, phosphate,
and silicate are characterized by a relative maximum located around 50–75 m
(Ramírez et al. 2005; Ramírez 2007), while this nutrient maxima is absent in winter
and spring. At those maxima nutrient concentrations reach values close to ~7.0 μM
for nitrate, ~0.35–0.40 μM for phosphate and ~4.0 μM for silicate, coinciding with a
relative minimum of dissolved oxygen (Ramírez 2007). These findings suggest the
occurrence of intense remineralization processes in the water column at shallower
depths on the continental margin compared to remineralization processes in deeper
areas of the westernmost sector of the Alboran Sea (Minas et al. 1991). At
200–300 m depth nutrients also show a high variability (Ramírez 2007). Thus, on
the continental margin at 300 m depth the average nutrient concentrations are about
~8.5–10.0 μM for nitrate, ~0.4–0.5 μM for phosphate, and ~5.0–6.5 μM for silicate
(Ramírez 2007). Further offshore nitrate concentrations are higher, ~12 μM at 300 m
depth (García-Martínez et al. 2019).
On the other hand the vertical distribution of nitrite in the Alboran Sea shows a
conspicuous subsurface maximum (Bianchi et al. 1994; Ramírez et al. 2005;
Ramírez 2007; García-Martínez et al. 2019). In the Alboran Sea the presence of
nitrite maximum is frequently observed throughout the all year (Ramírez 2007;
García-Martínez et al. 2019). The highest values are usually found during the
stratification period, with an average nitrite concentration close to 0.3 μM (Ramírez
et al. 2005; Ramírez 2007; García-Martínez et al. 2019). In the NW Alboran Sea this
nitrite maximum is usually detected at depths ranging from 50 to 75 m (Fig. 7.2),
frequently associated to the end of the nutricline (Ramírez et al. 2005; Ramírez 2007;
García-Martínez et al. 2019). In the Almeria-Oran front area, the depth of the nitrite
maximum have been found to range from ~40 to 110 m (Bianchi et al. 1994), with
the highest concentrations (>0.5 μM) being found in the Atlantic waters adjacent to
220
T. Ramírez et al.