Ocean Productivity in the Major Eastern Boundary Upwelling Ecosystems

Satellite-based net primary production (NPP) estimates among the major Eastern Boundary Upwelling Systems (EBUS), i.e., California, Canary, Humboldt and Benguela, were analyzed in order to investigate the importance of local environmental factors in determining the observed spatial differences among the areas. Data from an oligotrophic coastal upwelling area located in the central Mediterranean Sea were also included in the analysis for comparison purposes. NPP data used were generated by the standard algorithm for the Vertically Generalized Production Model (VGPM), a "chlorophyllbased" model that estimates net primary production from chlorophyll using a temperature-dependent description of chlorophyll-specific photosynthetic efficiency. Wind-induced upwelling intensity, as measured by offshore Ekman transport, is able to account for the differences between the lower NPP values in Canary and California and the higher NPP values observed in Benguela and Peru-Humboldt, but it does not explain the difference observed between these two more productive areas. Upwelling intensity appears to be able to drive NPP through nutrient supply, as nutrients are correlated to offshore Ekman transport. A linear multiple regression model including the fraction of sea surface over the shelf in each 1°x 1° box, nitrate, and variability of offshore Ekman transport explained over 75% of the variance in NPP among the areas. The extension of continental shelf explains the differences observed between Benguela (highest NPP values and wide shelf) and Humboldt Peru (lower NPP values than in Benguela and narrow shelf).