Atlantic inflow. Due to these processes, as was well as to phytoplankton consumption (Gómez et al. 2000), the elemental composition of Atlantic waters is modified
along the Strait of Gibraltar. Several studies have reported that the N:P molar ratio in
the Atlantic inflow is close to the Redfield ratio (Béthoux et al. 2002), while other
studies found that the N:P molar ratio in the Atlantic water inflow tends to be lower
than the Redfield ratio (16:1). Accordingly, Huertas et al. (2012) observed that the N:
P ratios in the Atlantic waters at the Strait of Gibraltar ranged from 11 to 12, while
the mean N:P ratio for the Mediterranean outflow was 17.5. The lower N:P ratios in
the Atlantic layer were attributed to a preferential consumption of nitrate by phytoplankton in the Strait of Gibraltar and adjacent waters. Nevertheless, nitrate and
phosphate in the Atlantic water inflow were higher than the half-saturation constant
for both nutrients, hence these waters cannot be considered nutrient depleted
(Huertas et al. 2012). Other studies found that the N:P ratios in the Atlantic water
layer were higher at the Mediterranean side of the Strait of Gibraltar (on average
23.6) than on the Atlantic side (on average 13.8) (Dafner et al. 2003).
For the N:Si ratio the opposite pattern has been observed, i.e., higher average
values on the western side of the Strait (1.46) and lower values on the eastern side
(0.81) (Dafner et al. 2003). These eastward changes across the Strait of Gibraltar in
the N:P and N:Si ratios within the Atlantic water layer are accompanied by changes
in the phytoplankton community, with dinoflagellates dominating in the Atlantic
side and diatoms at the eastern side (Gómez et al. 2000; Dafner et al. 2003). Dafner
et al. (2003) estimated that physical and biological processes at the Strait could
account for ~16% and ~84%, respectively, of the changes observed in the N:Si:P
ratio at the Strait of Gibraltar. These authors suggested that the increase of the N:Si
ratio at the eastern side of Strait could be due to uptake of Si by diatoms, similarly,
the low phosphate concentrations (<0.02 μM) at stations located at the eastern side
of the Strait was also attributed an intense consumption of P by phytoplankton
(Dafner et al. 2003). Nevertheless, the differences in the N:P values reported in the
literature for the upper layers in the Strait of Gibraltar could also be due to the high
spatio-temporal variability of hydrodynamic processes in this area (Gómez et al.
2000; Echevarría et al. 2002; Ramírez-Romero et al. 2014).
In the Alboran Sea, the nutrient molar ratios exhibit also high spatial and temporal
variability in concordance with the high variability of nutrients. Figure 7.4 illustrates
the spatial variability of the average integrated N:P ratio in the upper 100 m of the
water column in the Mediterranean and the Alboran Sea in spring (May), reflecting a
strong north-south gradient in the upper 100 m of the water column. However, as
already mentioned different studies in the NW Alboran Sea have reported a deficiency of N relative to P in the surface layers when compared to the Redfield ratio
(16:1) (Ramírez et al. 2005; Reul et al. 2005; Mercado et al. 2007). Thus, on the
continental margin of NW Alboran Sea the average N:P ratio in the upper layer
(0–20 m), i.e., at the depths where the Chl-a maximum is frequently found in this
area (Ramírez et al. 2005; Ramírez 2007; García-Martínez et al. 2019), has been
reported to range between ~2.0 and 14.0 (Ramírez et al. 2005; Ramírez 2007;
Mercado et al. 2007). Reul et al. (2005) found N:P ratios lower than 16:1 above
the nutricline and suggested a main role of N in regulating phytoplankton growth in
222
T. Ramírez et al.
along the Strait of Gibraltar. Several studies have reported that the N:P molar ratio in
the Atlantic inflow is close to the Redfield ratio (Béthoux et al. 2002), while other
studies found that the N:P molar ratio in the Atlantic water inflow tends to be lower
than the Redfield ratio (16:1). Accordingly, Huertas et al. (2012) observed that the N:
P ratios in the Atlantic waters at the Strait of Gibraltar ranged from 11 to 12, while
the mean N:P ratio for the Mediterranean outflow was 17.5. The lower N:P ratios in
the Atlantic layer were attributed to a preferential consumption of nitrate by phytoplankton in the Strait of Gibraltar and adjacent waters. Nevertheless, nitrate and
phosphate in the Atlantic water inflow were higher than the half-saturation constant
for both nutrients, hence these waters cannot be considered nutrient depleted
(Huertas et al. 2012). Other studies found that the N:P ratios in the Atlantic water
layer were higher at the Mediterranean side of the Strait of Gibraltar (on average
23.6) than on the Atlantic side (on average 13.8) (Dafner et al. 2003).
For the N:Si ratio the opposite pattern has been observed, i.e., higher average
values on the western side of the Strait (1.46) and lower values on the eastern side
(0.81) (Dafner et al. 2003). These eastward changes across the Strait of Gibraltar in
the N:P and N:Si ratios within the Atlantic water layer are accompanied by changes
in the phytoplankton community, with dinoflagellates dominating in the Atlantic
side and diatoms at the eastern side (Gómez et al. 2000; Dafner et al. 2003). Dafner
et al. (2003) estimated that physical and biological processes at the Strait could
account for ~16% and ~84%, respectively, of the changes observed in the N:Si:P
ratio at the Strait of Gibraltar. These authors suggested that the increase of the N:Si
ratio at the eastern side of Strait could be due to uptake of Si by diatoms, similarly,
the low phosphate concentrations (<0.02 μM) at stations located at the eastern side
of the Strait was also attributed an intense consumption of P by phytoplankton
(Dafner et al. 2003). Nevertheless, the differences in the N:P values reported in the
literature for the upper layers in the Strait of Gibraltar could also be due to the high
spatio-temporal variability of hydrodynamic processes in this area (Gómez et al.
2000; Echevarría et al. 2002; Ramírez-Romero et al. 2014).
In the Alboran Sea, the nutrient molar ratios exhibit also high spatial and temporal
variability in concordance with the high variability of nutrients. Figure 7.4 illustrates
the spatial variability of the average integrated N:P ratio in the upper 100 m of the
water column in the Mediterranean and the Alboran Sea in spring (May), reflecting a
strong north-south gradient in the upper 100 m of the water column. However, as
already mentioned different studies in the NW Alboran Sea have reported a deficiency of N relative to P in the surface layers when compared to the Redfield ratio
(16:1) (Ramírez et al. 2005; Reul et al. 2005; Mercado et al. 2007). Thus, on the
continental margin of NW Alboran Sea the average N:P ratio in the upper layer
(0–20 m), i.e., at the depths where the Chl-a maximum is frequently found in this
area (Ramírez et al. 2005; Ramírez 2007; García-Martínez et al. 2019), has been
reported to range between ~2.0 and 14.0 (Ramírez et al. 2005; Ramírez 2007;
Mercado et al. 2007). Reul et al. (2005) found N:P ratios lower than 16:1 above
the nutricline and suggested a main role of N in regulating phytoplankton growth in
222
T. Ramírez et al.