the front, while in the rest of the areas nitrite maximum values were around 0.2 μM
(Bianchi et al. 1994). The nitrite maximum has been attributed to the oxidation of
ammonium by nitrifying bacteria (Bianchi et al. 1994; Ramírez et al. 2005; Ramírez
2007), although the contribution from exudation by phytoplankton during the
incomplete assimilatory reduction of nitrate cannot be disregarded (Ramírez
2007), since the nitrite maximum is frequently associated to the depth of the Chl-a
maximum (Ramírez 2007; García-Martínez et al. 2019). However, it is noteworthy
that the peak of nitrite is commonly found between the isohalines 37.0 and 37.5
(Ramírez et al. 2005; Ramírez 2007), which suggests that the AtlanticMediterranean interface could facilitate the accumulation of organic matter and the
nitrification processes at this layer.
7.2.4 Nutrient Molar Ratios: N or P Limitation?
It is accepted as a general paradigm that the molar ratio of dissolved inorganic N and
P in the global ocean follows the ratio N:P (16:1), which reflects the general
elemental composition of marine plankton (Redfield et al. 1963). In addition,
Brzezinski (1985) found that the N:Si ratio for diatoms growing under optimal
conditions was ~1:1. Based on these findings, it is widely assumed that the optimal
N:Si:P ratios for marine phytoplankton is 16:16:1. Deviations from this elemental
ratio have been extensively used to infer the potential limitation of phytoplankton
growth by nutrients (Howarth 1988; Nelson and Dortch 1996). Departures of the
theoretical Redfield ratio have been reported by many studies in different marine
regions, including the Mediterranean Sea which is characterized by the extremely
high N:P ratios (Krom et al. 1991; Ribera d’Alcalà et al. 2003). Thus, the N:P ratio in
the intermediate water layer in the Eastern Mediterranean usually varies from 24 to
51, while in the deep Mediterranean waters it ranges from 25 to 30 (Ribera d’Alcalà
et al. 2003). In addition, the intermediate and deep Mediterranean waters are also
characterized by low N:Si ratios compared to the ratio 1:1. The N:Si ratio is
generally lower than 1:1 in the Eastern Mediterranean, with values around ~0.90
for intermediate waters and around ~0.5–0.6 for deep waters (Ribera d’Alcalà et al.
2003). In the surface waters of the Eastern Mediterranean, where PP is strongly
limited by P (Thingstand et al. 2005), the N:P and N:Si ratios are very variable
ranging between <5–60, and 1.7–18.2, respectively (Ribera d’Alcalà et al. 2003).
At difference from the Eastern Mediterranean, where P is the main limiting
element for phytoplankton growth, the existing studies in the Alboran Sea and the
Strait of Gibraltar suggest that N is the main limiting nutrient for phytoplankton
growth in the surface layers (Dafner et al. 2003; Ramírez et al. 2005; Mercado et al.
2007, 2008; Ramírez 2007; Huertas et al. 2012). In the Gulf of Cadiz, the surface
waters are characterized by N:P ratios <16:1, which suggests potential limitation of
phytoplankton by N (Cravo et al. 2013). However, during its transits towards the
Alboran Sea, the Atlantic waters coming from the Gulf of Cadiz are mixed and
entrained with Mediterranean waters, while NACW cores are injected into the
7 The Biogeochemical Context of Marine Planktonic Ecosystems
221
(Bianchi et al. 1994). The nitrite maximum has been attributed to the oxidation of
ammonium by nitrifying bacteria (Bianchi et al. 1994; Ramírez et al. 2005; Ramírez
2007), although the contribution from exudation by phytoplankton during the
incomplete assimilatory reduction of nitrate cannot be disregarded (Ramírez
2007), since the nitrite maximum is frequently associated to the depth of the Chl-a
maximum (Ramírez 2007; García-Martínez et al. 2019). However, it is noteworthy
that the peak of nitrite is commonly found between the isohalines 37.0 and 37.5
(Ramírez et al. 2005; Ramírez 2007), which suggests that the AtlanticMediterranean interface could facilitate the accumulation of organic matter and the
nitrification processes at this layer.
7.2.4 Nutrient Molar Ratios: N or P Limitation?
It is accepted as a general paradigm that the molar ratio of dissolved inorganic N and
P in the global ocean follows the ratio N:P (16:1), which reflects the general
elemental composition of marine plankton (Redfield et al. 1963). In addition,
Brzezinski (1985) found that the N:Si ratio for diatoms growing under optimal
conditions was ~1:1. Based on these findings, it is widely assumed that the optimal
N:Si:P ratios for marine phytoplankton is 16:16:1. Deviations from this elemental
ratio have been extensively used to infer the potential limitation of phytoplankton
growth by nutrients (Howarth 1988; Nelson and Dortch 1996). Departures of the
theoretical Redfield ratio have been reported by many studies in different marine
regions, including the Mediterranean Sea which is characterized by the extremely
high N:P ratios (Krom et al. 1991; Ribera d’Alcalà et al. 2003). Thus, the N:P ratio in
the intermediate water layer in the Eastern Mediterranean usually varies from 24 to
51, while in the deep Mediterranean waters it ranges from 25 to 30 (Ribera d’Alcalà
et al. 2003). In addition, the intermediate and deep Mediterranean waters are also
characterized by low N:Si ratios compared to the ratio 1:1. The N:Si ratio is
generally lower than 1:1 in the Eastern Mediterranean, with values around ~0.90
for intermediate waters and around ~0.5–0.6 for deep waters (Ribera d’Alcalà et al.
2003). In the surface waters of the Eastern Mediterranean, where PP is strongly
limited by P (Thingstand et al. 2005), the N:P and N:Si ratios are very variable
ranging between <5–60, and 1.7–18.2, respectively (Ribera d’Alcalà et al. 2003).
At difference from the Eastern Mediterranean, where P is the main limiting
element for phytoplankton growth, the existing studies in the Alboran Sea and the
Strait of Gibraltar suggest that N is the main limiting nutrient for phytoplankton
growth in the surface layers (Dafner et al. 2003; Ramírez et al. 2005; Mercado et al.
2007, 2008; Ramírez 2007; Huertas et al. 2012). In the Gulf of Cadiz, the surface
waters are characterized by N:P ratios <16:1, which suggests potential limitation of
phytoplankton by N (Cravo et al. 2013). However, during its transits towards the
Alboran Sea, the Atlantic waters coming from the Gulf of Cadiz are mixed and
entrained with Mediterranean waters, while NACW cores are injected into the
7 The Biogeochemical Context of Marine Planktonic Ecosystems
221