Factors responsible for the differences in satellite-based chlorophyll a concentration between the major global upwelling areas

The aim of this study was to identify the factors responsible for the differences in chlorophyll a concentration (Chl-a) observed between the California, Canary, Humboldt and Benguela upwelling areas. Monthly climatologic values of Chl-a obtained from satellite images, covering the years 1998–2004, revealed that this pigment was higher in the Benguela system than in the other areas. Upwelling intensity, as derived from offshore Ekman transport computations, was higher in the Benguela and Humboldt regions and, for the same upwelling intensity, Chl-a was higher in Benguela than in the other regions. Upwelling intensity appears to be able to drive Chl-a densities through nutrient supply, as nutrients are correlated to offshore Ekman transport. A linear regression model including the fraction of sea surface over the shelf in each 1°×1° box, nitrate, silicate, turbulence and variability of offshore Ekman transport explained the 84.8% of the variance in Chl-a among the areas. Differences in offshore Ekman transport explained the lower Chl-a observed in Canary and California and the higher Chl-a observed in Benguela and Peru-Humboldt. A narrow continental shelf and low water column stability also contribute to reducing phytoplankton pigment biomass in the Canary and California areas. The higher Chl-a values observed in Benguela compared to Humboldt-Peru are due to a wider extension of the continental shelf in the Benguela region.