(Fig. 7.2) (Bianchi et al. 1994; Claustre et al. 1994; Leblanc et al. 2004). Gil and
Gomis (1994) detected very high nitrate concentration >6.5 μM at 50 m in offshore
waters off the Almeria Bay, associated to a strong salinity gradient. Lower nitrate
concentrations were found by Bianchi et al. (1994) at 50 m in the frontal area (~3 μM
nitrate), while according to Leblanc et al. (2004) nitrate was ~1–2 μM at 50 m depth
in the Almeria-Oran front area. Figure 7.3b shows the climatology of the depthintegrated nitrate concentration in the upper 100 m during the month of May
(spring), one of the most productive months in the Alboran Sea (Lazzari et al.
2012). A marked north to south nitrate gradient in the Alboran Sea can be observed,
with higher concentrations in the Northern Alboran coasts and the lower values
observed in the Southern Alboran coasts (Fig. 7.3b). These marked differences are
due to the convergence of nutrient-poor Atlantic waters in the southern part, while in
the northern part the frequent upwellings lead to much higher concentrations.
In general, the vertical distribution of nutrients in the Alboran Sea follows a
typical vertical pattern with low concentrations in surface waters and below a
marked nutricline, that extends to depths around 250–300 m (Minas et al. 1991;
Béthoux et al. 1992) (Fig. 7.1). The marked nutricline reflects the influence of
Mediterranean waters below the nutrient-poor SAW layer (Minas et al. 1991). In
the Western Alboran Sea, below the nutricline nitrate concentrations remains nearly
constant (~9.0 μM) while phosphate concentration ranges between ~0.45 and
0.50 μM. In contrast, silicate concentration continues increasing more slowly
below the nutricline, reaching its higher values (~10 μM) at the bottom of the
basin in the Western Alboran Sea (Minas et al. 1991). However in areas close to
the Strait of Gibraltar, where the presence of a oxygen extraminimum has been
reported (Packard et al. 1988; Minas et al. 1991), the vertical profiles of nitrate and
phosphate present a weak maximum at the lower limit of the nutricline (200–250 m),
which is more conspicuous for nitrate (Minas et al. 1991). The maximum coincides
with a dissolved oxygen extraminimum in the water column, where concentrations
as low as 3.8 mlÁl
À1 are reached (Packard et al. 1988; Minas et al. 1991). This
extraminimum and the associated nitrate and phosphate maximum are the consequence of the intense export of organic matter from the nearby high productive areas
in the NW Alboran Sea towards the center of the WAG, where the organic matter
accumulates and sink promoting the growth of bacterial communities and the
respiration of the accumulated organic matter, leading to the oxygen extraminimum
and to the nutrient maxima (Minas et al. 1991).
At the center of the WAG and the EAG the subduction of SAW causes a
pronounced deepening of the 37.5 isohaline, which is considered the interface
between Atlantic and Mediterranean waters in this basin (Parrilla and Kinder
1987). As a result, a thick SAW layer (~150–200 m depth) is found at the center
of both gyres (Lafuente et al. 1998; Leblanc et al. 2004). The downwelling of poor
nutrient SAW at the center of both gyres is reflected in the nutricline, which reaches
its deepest locations at the core of both anticyclonic gyres. Accordingly, the
nitracline has been found at depths ranging from ~70 to 115 m at the center of the
anticyclonic gyres (Leblanc et al. 2004; Morán and Estrada 2001; Mercado et al.
2014), and the phosphacline has been found a depths varying from ~ 50 to ~170 m
218
T. Ramírez et al.
Gomis (1994) detected very high nitrate concentration >6.5 μM at 50 m in offshore
waters off the Almeria Bay, associated to a strong salinity gradient. Lower nitrate
concentrations were found by Bianchi et al. (1994) at 50 m in the frontal area (~3 μM
nitrate), while according to Leblanc et al. (2004) nitrate was ~1–2 μM at 50 m depth
in the Almeria-Oran front area. Figure 7.3b shows the climatology of the depthintegrated nitrate concentration in the upper 100 m during the month of May
(spring), one of the most productive months in the Alboran Sea (Lazzari et al.
2012). A marked north to south nitrate gradient in the Alboran Sea can be observed,
with higher concentrations in the Northern Alboran coasts and the lower values
observed in the Southern Alboran coasts (Fig. 7.3b). These marked differences are
due to the convergence of nutrient-poor Atlantic waters in the southern part, while in
the northern part the frequent upwellings lead to much higher concentrations.
In general, the vertical distribution of nutrients in the Alboran Sea follows a
typical vertical pattern with low concentrations in surface waters and below a
marked nutricline, that extends to depths around 250–300 m (Minas et al. 1991;
Béthoux et al. 1992) (Fig. 7.1). The marked nutricline reflects the influence of
Mediterranean waters below the nutrient-poor SAW layer (Minas et al. 1991). In
the Western Alboran Sea, below the nutricline nitrate concentrations remains nearly
constant (~9.0 μM) while phosphate concentration ranges between ~0.45 and
0.50 μM. In contrast, silicate concentration continues increasing more slowly
below the nutricline, reaching its higher values (~10 μM) at the bottom of the
basin in the Western Alboran Sea (Minas et al. 1991). However in areas close to
the Strait of Gibraltar, where the presence of a oxygen extraminimum has been
reported (Packard et al. 1988; Minas et al. 1991), the vertical profiles of nitrate and
phosphate present a weak maximum at the lower limit of the nutricline (200–250 m),
which is more conspicuous for nitrate (Minas et al. 1991). The maximum coincides
with a dissolved oxygen extraminimum in the water column, where concentrations
as low as 3.8 mlÁl
À1 are reached (Packard et al. 1988; Minas et al. 1991). This
extraminimum and the associated nitrate and phosphate maximum are the consequence of the intense export of organic matter from the nearby high productive areas
in the NW Alboran Sea towards the center of the WAG, where the organic matter
accumulates and sink promoting the growth of bacterial communities and the
respiration of the accumulated organic matter, leading to the oxygen extraminimum
and to the nutrient maxima (Minas et al. 1991).
At the center of the WAG and the EAG the subduction of SAW causes a
pronounced deepening of the 37.5 isohaline, which is considered the interface
between Atlantic and Mediterranean waters in this basin (Parrilla and Kinder
1987). As a result, a thick SAW layer (~150–200 m depth) is found at the center
of both gyres (Lafuente et al. 1998; Leblanc et al. 2004). The downwelling of poor
nutrient SAW at the center of both gyres is reflected in the nutricline, which reaches
its deepest locations at the core of both anticyclonic gyres. Accordingly, the
nitracline has been found at depths ranging from ~70 to 115 m at the center of the
anticyclonic gyres (Leblanc et al. 2004; Morán and Estrada 2001; Mercado et al.
2014), and the phosphacline has been found a depths varying from ~ 50 to ~170 m
218
T. Ramírez et al.