Influence of mesoscale processes on the distribution of pico- and nanoplankton south of Gran Canaria (Canary Islands)

Résumé du document

The Canary Islands are located in the northeast subtropical Atlantic waters and extend from 27°37' to 29°25' N and 18°10' to 13°20' W (Figure 1). The archipelago is composed of seven main islands distributed zonally across the eastern limb of the subtropical gyre of the North Atlantic (Barton et al., 1998).
For many years, the Canary region is described as oligotrophic. However, this area is characterized by a major mesoscale activity. The nearby African coast is cut by capes (Bojador and Jubi) and subject to an upwelling. The interaction between the coastal upwelling regime over the continental shelf and the offshore oceanic regime is highly variable in terms
of biological and physical water characteristics.

Extraits

[...] Nevertheless, their abundance and metabolic activity become weaker due to the attenuation of primary production (Zubkov et al., 1998), and so, the lack of dissolved organic matter Julien Canon August 2010 Figure 14 : Vertical profiles (means±SE) of abundance of heterotrophic bacteria et the eastern part of the Canary archipelago (Bode et al, 2001). In oligotrophic waters, the bacterial population size appears to be quasi-constant (Weisse and Scheffel-Möser, 1991). In fact, the substrate supply by the picophytoplankton and the grazing exert a balanced control on the bacterial community. The grazing is supported by the heterotrophic nanoflagellates (HNF). Hence, these HNF provide an important link in the microbial food-web by transferring carbon to higher trophic levels (Safi et al., 1997). This grazing has a positive effect on bacterial growth rates (Weisse and Scheffel-Möser, 1991). [...]

[...] These exchanges of properties may greatly influence the planktonic distribution and therefore the food-chain. Figure 10 : Sea surface AVHRR temperature image of 24 August 1999 showing, anticyclonic, cyclonic eddies and upwelling filaments in the Canary region (Barton et al., 2004) Julien Canon August Microorganisms The microbial community plays an important role in the pelagic ecosystem (Azam et al., 1983). This community is generally defined as the picoplankton. The term “picoplankton” has been introduced by Sieburth in 1979 (Zubkov et al., 2000). [...]

[...] Moreover, they are easily identifiable on a density section where the wake is characterised by a pool of low density surface water (Barton et al., 1998) (Figure 7). The wind intensity in the exposed region can exceed 15 m whereas values around 0 are found in the sheltered area (Barton et al., 2000). This implies a weak Ekman transport in the sheltered areas, compared to the net westward Ekman transport in the exposed regions. Therefore, convergent (downwelling) and divergent (upwelling) fronts are generated between the two regions (Basterretxea, et al., 2002). [...]

[...] However, its utilisation is restricted by a limited resolution and a small sample size analysable. Being based on the optical characteristics of cells, organisms can rarely be identified to genus or species. [...]

[...] (1985), studies of upwelling filaments have been conducted off Spain, northwest Africa and California (Alvarez-Salgado, 2007). Even if upwelling filaments are frequently observed, there is still no universal theory to explain their formation (Pacheco et al., 2001). What we know so far with certainty is that capes in coastal regions are associated to enhanced offshore material transport. The mechanism of filament formation has been investigated using laboratory models of the NACTZ. Several hypothesis have been suggested to explain the dynamical origin of these mesoscale structures (Barton et al., 1998) : 1. [...]