These regions are often affected by patches of enhanced chlorophyll near the islands, presumably associated with an eddy signature (Hernández-Guerra et al., 1993, Davenport et al., 1999). The filaments extend several hundred kilometres offshore, and cyclonic and anticyclonic eddies are visible in AVHRR SST and CZCS images as cold tongues and meanders, hot and cold patches near the islands, and regions with enhanced pigment concentration as compared with ambient levels. This regional activity is a result of the combined effects of the disturbance in the general South-westward flow of the Canary Current and the Trade winds together with the regular development of filament structures associated with the North-west African coastal upwelling at specific coastal positions (Cape Ghir, Cape Jubi and Cape Bojador). Recent research, satellite and field observations (Arístegui et al., 1994, Arístegui et al., 1997, Barton et al., 1998, Davenport et al., 1999) have revealed that the Canary Islands are affected by intense mesoscale structures.
The role that mesoscale processes could play as important vehicles for nutrient transport in the world's oceans has been debated for many years suggesting that mesoscale eddies constitute a significant nutrient supply to the euphotic zone in oligotrophic areas (Falkowski et al., 1991, McGillicuddy et al., 1998, McGillicuddy et al., 2003, Martin et al., 2002). It has been suggested that nitrogen fixation could be responsible for the remainder of the phytoplankton nutritional requirements (Hood et al., 2000). The entrainment of nutrients into the surface mixed layer caused by Wintertime convection, by diapycnal diffusion or by surface Ekman drift, account for under 50% of the new annual production (McGillicuddy et al., 2003). However, geochemical estimates of new production in the oligotrophic waters of the open ocean far surpass those which can be sustained by traditional mechanisms of nutrient supply (Jenkins and Goldman, 1985) thereby modifying the general perspective (Siegel et al., 1999). (2003) have described the presence of a seasonal thermocline at depths from 50 to 120 m which separates low-nutrient, low-chlorophyll surface waters from deep nutrient-rich waters for most of the year. The Eastern Atlantic waters around the Canary Islands manifest characteristics of oligotrophic systems. The sub-tropical gyres and the transition zones at their boundaries occupy 65% of the oceans' surface area (Koblentz-Mishke et al., 1970) and, thus, clearly play an important role in the carbon dioxide system. The reduced fCO 2 inside the cyclonic eddy, 15 μatm lower than that observed in non-affected surface water, was explained by thermodynamic aspects, biological activity, eddy upward pumping and diffusion and air–sea water exchange effects. The upward flux of inorganic carbon decreased the effect of the increased primary production on the carbon dioxide chemistry. An increase in the total inorganic carbon concentration in the upper layers inside the eddy field of 133 mmol C m − 2 d − 1 was determined. The fluxes were determined considering both the diffusive and convective contributions from the upward pumping and the corresponding horizontal transport of water outside the area. A model was applied to determine the net inorganic carbon balance in the cyclonic eddy. Nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies were also ascertained by upward displacement of dissolved inorganic carbon. Cyclonic and anticyclonic eddies were alternatively observed from the northwestern area to the central area of the Canary Islands. Measurements of surface partial pressure of CO 2 and water column alkalinity, pH T, nutrients, oxygen, fluorescence and hydrography were carried out, south of the Canary Islands during September 1998.
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