Oceanography

Oceanography

Figure 5: Distribution patterns of nitrates, orthophosphates, orthosilicates and nitrites along the water column in the Strait of Sicily. centration of orthosilicates in the deep water is ∼2 µmol·l−1 , higher than the average concentration in the intermediate water. Also nitrates show a similar trend even if the increase in their average concentration is 1 µmol·l−1 . The measured average N:P is ∼10 in surface waters, while, in the intermediate and deep waters, it approximates ∼28 and 21 respectively. These values are comparable to those previously reported for the Strait of Sicily by Ribera d’Alcala et al. [4]. The average values of Si:N range between 3 in surface water and 1.1-0.96 in intermediate and deep waters, respectively. These ratios testify the important unbalance of Redfield ratios in the Mediterranean. Moreover, a high nu-

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tricline depth (150-200 m) is evident from Figures 6a, 6b, 6c, 6d. In the studied area a Deep Chlorophyll Maximum (DCM) at about 100-150 m, particularly in proximity of the Sicilian coastal stations (Figure 6e) occurs. Finally, an evident primary nitrite maximum (PNM) has been ascribed to a combination of incomplete assimilatory reduction of nitrate by phytoplankton and chemoautotrophic oxidation of ammonium by nitrifying bacteria (Figure 5d) [8].