The N:Si:P molar ratio in the Strait of Gibraltar

Abstract

All existing descriptions of nutrient distributions in the Strait of Gibraltar suggest that the Atlantic water brings to the Mediterranean Sea nutrients in the Redfield ratio (N:Si:P = 16:15:1). Here, the N:Si:P molar ratios (±Standard Error), obtained in April 1998, are used to show that in the Atlantic water at the western entrance of the Strait this ratio is lower (13.8(±0.5):12.1(±1.0):1) than the classical Redfield ratio; it is close to the Redfield ratio in the middle of the Strait (15.6(±0.6):10.7(±0.9):1), and increases dramatically to 23.6(±3.4):29.1(±4.5):1 at the eastern entrance of the Strait. In the Mediterranean water, the N:Si:P ratio has a quite similar trend with 31.5(±6.0):26.5(±3.6):1 in the east, 20.4(±0.2):31.5(±11.1):1 in the middle and 18.1(±0.6):17.6(±0.7):1 in the west of the Strait. The physical and biological processes that account for the observed spatial variability of the N:Si:P ratio along the Strait are identified. We estimated that in the Atlantic water entering the Mediterranean Sea, about 84% of the variability in N:Si:P molar ratio is due to biological and 16% to physical processes.

Anthropogenic and natural CO<sub>2</sub> exchange through the Strait of Gibraltar

“…Salinity values taken to define the interfacial layer on both sides of the Strait along with those derived from mixing analysis calculated along the GIFT leg are also illustrated. These values mark the balance of salinity in the Strait and following the scheme introduced by Dafner et al (2003) and considering the difference between the outflow and inflow at stations 6 and 8 respectively, a water entrainment equivalent to 0.044 Sv is attained. Similarly, a vertical mixing of 0.13 Sv between the upper Atlantic layer and the lower Mediterranean layer can be derived from this balance, a value that is coherent with preceding calculations in the channel .…”

“…Therefore, if the alkalinity budget for the Mediterranean Sea described in Schneider et al (2007) is taken into account, which considers a joint alkalinity entrance of 86 kmol s −1 into de basin from the Black Sea and rivers, and the alkalinity export through the Strait of Gibraltar obtained in our work is taken (−44 kmol s −1 ), 42 kmol s −1 of the alkalinity measured in the Mediterranean do not reach the Atlantic. Using the net export of nitrate through the Strait given by Dafner et al (2003), a consumption of 4 kmol s −1 of alkalinity is expected to be associated to the remineralization of the organic matter present in the Mediterranean Sea, and the most likely candidate for the loss of the remaining 38 kmol s −1 of alkalinity would be CaCO 3 sedimentation. Our results then imply a net carbonate sink of 19 kmol s −1 towards the seafloor.…”

“…Among these regions, it is a generally accepted opinion that the Mediterranean Sea plays a relevant role in the global circulation of the Atlantic Ocean (Hecht et al, 1997;Serra and Ambar, 2002;Carton et al, 2002), contributing significantly to the biogeochemical inventory of the North Atlantic (Dafner et al, 2001a;Dafner et al, 2003) and being recognized as a source of inorganic carbon for the Atlantic open ocean (Aït-Ameur and Goyet, 2006;de la Paz et al, 2008a). In fact, Ríos et al (2001) showed that the C ANT advected by Mediterranean Water (MW) was the most important contributor to the total C ANT advected into the Eastern North Atlantic, representing 59%.…”

Seasonal and inter-annual variability of the phytoplankton communities in an upwelling area of the Alborán Sea (SW Mediterranean Sea)

“…The interface between Atlantic and Mediterranean seawater is generally located at 40-70 m depth in the study area (Fernández, 1988), therefore it is reasonable to think that the nutrient concentration integrated value from the surface to 75 m depth must be strongly influenced by the characteristics of the Mediterranean seawater which is potentially upwelled. On the another hand, Dafner et al (2003), demonstrated that the surface Atlantic water which enters the Gibraltar Strait is depleted in nutrients and has a N:P ratio lower than 16:1. In contrast, Bethoux et al (2002), reported that Mediterranean seawater has high N:P and Si:P ratios, suggesting a deficiency of P in relation to N and Si.…”

The biogeochemistry of nutrients, dissolved oxygen and chlorophyll <em>a</em> in the Catalan Sea (NW Mediterranean Sea)

“…Dafner et al 2003), and the differences can be explained by the ratios being calculated at different depths. Moreover, as observed in the eastern Mediterranean Sea (Krom et al 1991, Kress andHerut 2001), the N:P and DIN:P scatterplots show that the relationship is non-linear at low and high N and P concentrations (Fig.…”