the NW Alboran Sea, while below the nutricline N:P ratios were usually above 16:1.
Higher N:P ratios (integrated values over 0–75 m) have been found by GarcíaMartínez et al. (2019) along the northern continental margin of the Alboran Sea,
where the average N:P ratio ranged between 13 and 16, depending on the time of the
year, with an overall mean value of 15. On the other hand, the N:Si ratio shows
values < 1:1 in the upper 20 m of the water column (Ramírez et al. 2005; Ramírez
2007). Thus, the average values for the N:Si ratio in the upper layers (0–20 m) in the
NW sector of the Alboran Sea vary between <0.5 and ~0.8, which are lower than the
ratio 1:1 for diatom growing under optimal conditions (Brzezinski 1985). The lowest
N:P an N:Si ratios are found during summer-early autumn, coinciding with an
intense stratification of the water column, while the higher ratios are usually
observed in winter, when the water column is mixed, as well as in those periods of
the year when the incidence of wind-driven upwelling driven is higher, usually
during spring (Ramírez et al. 2005; Ramírez 2007; Mercado et al. 2007; Macías et al.
2007).
These low N:P and N:Si ratios in the upper 20 m, together with the relatively low
nitrate concentrations during most time of the year, suggests that N plays a major
role limiting the phytoplankton growth in the upper layers of this basin during great
part of the year (Reul et al. 2005; Ramírez et al. 2005; Ramírez 2007). In addition,
nitrate concentrations found in upper layers of the Alboran Sea are generally lower
than the half-saturation constant (Ks) (1 μmolÁl
À1 ) for nitrate uptake by phytoplankton in coastal zones (MacIsaac and Dugdale 1969). All these findings support the
hypothesis that in the Alboran Sea nitrate is the main limiting nutrient for phytoplankton. Thus, nitrate would control phytoplankton growth in winter and autumn
when the Chl-a maximum is usually located at 0–20 m depth (Ramírez et al. 2005;
García-Martínez et al. 2019), and also in summer and autumn at those shallow
depths. However, during the stratification period, the Chl-a maximum becomes
deeper, particularly in areas out of the influence of upwellings, being usually
found at 50–75 m depth, i.e., below the seasonal thermocline and close to the limit
of the photic layer, coinciding with lower limit of the nutricline (Ramírez 2007). At
those depths nutrients should not be a limiting factor for phytoplankton, however,
limitation by light may occur (Mercado et al. 2008). The hypothesis of the
N-limitation in the NW Alboran Sea has been contrasted by additional experiments
(Ramírez 2007) and is also supported by other facts. Thus, based on a 3 years study
Ramírez (2007) found that on average the seasonal mean of the zonal wind component and nitrate concentration jointly explained ~80% of the temporal variability of
the average seasonal Chl-a values, while the relationship with phosphate was weak.
Likewise, Reul et al. (2005) found a correlation between cell (>2 μm) abundance
and nitrate, while they did not found a relationship with phosphate. Recent studies
(Lazzari et al. 2016) also found that, at difference from the rest of the Mediterranean,
N is the main limiting nutrient in the Alboran Sea.
During upwelling events, the N:P ratio in recent upwelled waters reaches values
>16:1 in the surface layers (0–20 m) while nitrate concentrations are usually >3 μM,
suggesting that there is no limitation by nitrate during the upwelling (Ramírez et al.
2005; Ramírez 2007). The intense upwellings in this area promote phytoplankton
224
T. Ramírez et al.
Higher N:P ratios (integrated values over 0–75 m) have been found by GarcíaMartínez et al. (2019) along the northern continental margin of the Alboran Sea,
where the average N:P ratio ranged between 13 and 16, depending on the time of the
year, with an overall mean value of 15. On the other hand, the N:Si ratio shows
values < 1:1 in the upper 20 m of the water column (Ramírez et al. 2005; Ramírez
2007). Thus, the average values for the N:Si ratio in the upper layers (0–20 m) in the
NW sector of the Alboran Sea vary between <0.5 and ~0.8, which are lower than the
ratio 1:1 for diatom growing under optimal conditions (Brzezinski 1985). The lowest
N:P an N:Si ratios are found during summer-early autumn, coinciding with an
intense stratification of the water column, while the higher ratios are usually
observed in winter, when the water column is mixed, as well as in those periods of
the year when the incidence of wind-driven upwelling driven is higher, usually
during spring (Ramírez et al. 2005; Ramírez 2007; Mercado et al. 2007; Macías et al.
2007).
These low N:P and N:Si ratios in the upper 20 m, together with the relatively low
nitrate concentrations during most time of the year, suggests that N plays a major
role limiting the phytoplankton growth in the upper layers of this basin during great
part of the year (Reul et al. 2005; Ramírez et al. 2005; Ramírez 2007). In addition,
nitrate concentrations found in upper layers of the Alboran Sea are generally lower
than the half-saturation constant (Ks) (1 μmolÁl
À1 ) for nitrate uptake by phytoplankton in coastal zones (MacIsaac and Dugdale 1969). All these findings support the
hypothesis that in the Alboran Sea nitrate is the main limiting nutrient for phytoplankton. Thus, nitrate would control phytoplankton growth in winter and autumn
when the Chl-a maximum is usually located at 0–20 m depth (Ramírez et al. 2005;
García-Martínez et al. 2019), and also in summer and autumn at those shallow
depths. However, during the stratification period, the Chl-a maximum becomes
deeper, particularly in areas out of the influence of upwellings, being usually
found at 50–75 m depth, i.e., below the seasonal thermocline and close to the limit
of the photic layer, coinciding with lower limit of the nutricline (Ramírez 2007). At
those depths nutrients should not be a limiting factor for phytoplankton, however,
limitation by light may occur (Mercado et al. 2008). The hypothesis of the
N-limitation in the NW Alboran Sea has been contrasted by additional experiments
(Ramírez 2007) and is also supported by other facts. Thus, based on a 3 years study
Ramírez (2007) found that on average the seasonal mean of the zonal wind component and nitrate concentration jointly explained ~80% of the temporal variability of
the average seasonal Chl-a values, while the relationship with phosphate was weak.
Likewise, Reul et al. (2005) found a correlation between cell (>2 μm) abundance
and nitrate, while they did not found a relationship with phosphate. Recent studies
(Lazzari et al. 2016) also found that, at difference from the rest of the Mediterranean,
N is the main limiting nutrient in the Alboran Sea.
During upwelling events, the N:P ratio in recent upwelled waters reaches values
>16:1 in the surface layers (0–20 m) while nitrate concentrations are usually >3 μM,
suggesting that there is no limitation by nitrate during the upwelling (Ramírez et al.
2005; Ramírez 2007). The intense upwellings in this area promote phytoplankton
224
T. Ramírez et al.