Zubkov et al. (2000) found that Prochlorococcus spp. were the dominant cyanobacteria in the northern and southern Atlantic gyres and the equatorial region, giving way to Synechococcus spp. in cooler waters. Synechococcus cells also become more numerous and even reach blooming densities near the tropical region affected by the Mauritanian upwelling. Finally, the concentrations of Picoeukaryotes tend to be at their height in temperate waters. The small coccoid prochlorophyte species, Prochlorococcus marinus, were found to be abundant in the North Atlantic (Veldhuis & Kraay, 1990), the tropical and subtropical Pacific Ocean (Campbell et al., 1994), the Mediterranean (Vaulot et al., 1990) and in the Red Sea (Veldhuis &.
