bloom and a notable increase of Chl-a which can reach values >5 μgÁl
À1 (Ramírez
et al. 2005; Ramírez 2007). After cessation of westerlies a sharp decrease of nitrate
has been reported, with concentrations dropping to undetectable levels at those
stations with higher Chl-a concentration. This fast decline of nitrate is accompanied
by a profound decrease of the N:P and N:Si ratios to values <2.0 and <0.2,
respectively (Ramírez et al. 2005; Ramírez 2007). This suggests that nitrate is
rapidly and preferentially removed by phytoplankton during intense phytoplankton
blooms, causing a rapid decline of both ratios. At the Strait of Gibraltar and the NW
Alboran Sea, a preferential uptake of nitrate has also been reported by other studies
(Reul et al. 2005; Huertas et al. 2012). The fast and preferential uptake of nitrate
would explain the low nitrate concentrations usually found in the upper layers
(0–20 m) in this sector of the Alboran Sea during large part of the year, except
when upwelling events take place. The limitation by nitrate in the upper layers
(0–20 m) is intermittently overcome by the upwelling events in the continental
margin (Ramírez et al. 2005; Reul et al. 2005) which lead to an temporal enhancement of the N:P ratio and nitrate concentrations in the surface waters (0–20 m).
Paradoxically, the fast and preferential uptake of nitrate during the blooms would
shift the system towards N-limitation in a few days after the cessation of the
upwelling event (Ramírez et al. 2005; Ramírez 2007).
The notable increase of the N:P and N:Si ratios in the upper layers (0–20 m)
observed during wind-driven upwelling events has its origin in the strong vertical
gradients of the N:P and N:Si molar ratios (Ramírez et al. 2005; Reul et al. 2005). In
this area the N:P and N:Si ratios show a sharp increase with depth down to
50–100 m. The vertical gradients of both molar ratios are more marked during the
stratification period. In contrast, vertical gradients are usually less marked in winter
and also during intense upwelling events (Ramírez et al. 2005; Ramírez 2007). At
50 m depth, the average N:P molar ratios vary between ~13 and 24, with most of the
values above 15, while at 100 m depth the ratio increase to values ranging from ~19
to 26 (Ramírez 2007). Therefore, in the NW Alboran Sea subsurface waters become
strongly deficient in phosphate at depths ~100 when compared to the Redfield ratio
(16:1). At greater depths, the average N:P ratios continue increasing although more
slowly, showing in general a weak maximum at 200 m where the average values
ranged from ~20 to ~27. At 300 m depth, the N:P ratio decreases slightly with
average ratios varying between ~19 and ~25 (Ramírez 2007), with most of these
values above 21. It is noticeable that the N:P ratios found at 200 m in the NW
Alboran Sea are higher than the values found in deep waters of the Algero-Balear
and Tyrrhenian basin (Ribera d’Alcalà et al. 2003). The high N:P values at shallower
depths in the NW Alboran Sea can be attributed to respiration processes (Minas et al.
1991; Ramírez et al. 2005, 2006) and to the upwelling of intermediate Mediterranean
waters on the continental slope during their transit towards the Strait of Gibraltar.
Similarly, the N:Si ratio also shows remarkable vertical gradients in the NW
Alboran Sea, reaching on average maximum values at depths ranging from 50 m to
100 m throughout the year (Ramírez et al. 2005; Ramírez 2007), with a peak
frequently observed in summer at 50 m. At that depth, the water column becomes
deficient in silicate in relation to nitrate, with the average N:Si ratio varying between
7 The Biogeochemical Context of Marine Planktonic Ecosystems
225
À1 (Ramírez
et al. 2005; Ramírez 2007). After cessation of westerlies a sharp decrease of nitrate
has been reported, with concentrations dropping to undetectable levels at those
stations with higher Chl-a concentration. This fast decline of nitrate is accompanied
by a profound decrease of the N:P and N:Si ratios to values <2.0 and <0.2,
respectively (Ramírez et al. 2005; Ramírez 2007). This suggests that nitrate is
rapidly and preferentially removed by phytoplankton during intense phytoplankton
blooms, causing a rapid decline of both ratios. At the Strait of Gibraltar and the NW
Alboran Sea, a preferential uptake of nitrate has also been reported by other studies
(Reul et al. 2005; Huertas et al. 2012). The fast and preferential uptake of nitrate
would explain the low nitrate concentrations usually found in the upper layers
(0–20 m) in this sector of the Alboran Sea during large part of the year, except
when upwelling events take place. The limitation by nitrate in the upper layers
(0–20 m) is intermittently overcome by the upwelling events in the continental
margin (Ramírez et al. 2005; Reul et al. 2005) which lead to an temporal enhancement of the N:P ratio and nitrate concentrations in the surface waters (0–20 m).
Paradoxically, the fast and preferential uptake of nitrate during the blooms would
shift the system towards N-limitation in a few days after the cessation of the
upwelling event (Ramírez et al. 2005; Ramírez 2007).
The notable increase of the N:P and N:Si ratios in the upper layers (0–20 m)
observed during wind-driven upwelling events has its origin in the strong vertical
gradients of the N:P and N:Si molar ratios (Ramírez et al. 2005; Reul et al. 2005). In
this area the N:P and N:Si ratios show a sharp increase with depth down to
50–100 m. The vertical gradients of both molar ratios are more marked during the
stratification period. In contrast, vertical gradients are usually less marked in winter
and also during intense upwelling events (Ramírez et al. 2005; Ramírez 2007). At
50 m depth, the average N:P molar ratios vary between ~13 and 24, with most of the
values above 15, while at 100 m depth the ratio increase to values ranging from ~19
to 26 (Ramírez 2007). Therefore, in the NW Alboran Sea subsurface waters become
strongly deficient in phosphate at depths ~100 when compared to the Redfield ratio
(16:1). At greater depths, the average N:P ratios continue increasing although more
slowly, showing in general a weak maximum at 200 m where the average values
ranged from ~20 to ~27. At 300 m depth, the N:P ratio decreases slightly with
average ratios varying between ~19 and ~25 (Ramírez 2007), with most of these
values above 21. It is noticeable that the N:P ratios found at 200 m in the NW
Alboran Sea are higher than the values found in deep waters of the Algero-Balear
and Tyrrhenian basin (Ribera d’Alcalà et al. 2003). The high N:P values at shallower
depths in the NW Alboran Sea can be attributed to respiration processes (Minas et al.
1991; Ramírez et al. 2005, 2006) and to the upwelling of intermediate Mediterranean
waters on the continental slope during their transit towards the Strait of Gibraltar.
Similarly, the N:Si ratio also shows remarkable vertical gradients in the NW
Alboran Sea, reaching on average maximum values at depths ranging from 50 m to
100 m throughout the year (Ramírez et al. 2005; Ramírez 2007), with a peak
frequently observed in summer at 50 m. At that depth, the water column becomes
deficient in silicate in relation to nitrate, with the average N:Si ratio varying between
7 The Biogeochemical Context of Marine Planktonic Ecosystems
225