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The oligotrophic waters of the Subtropical Gyres cover >60% of the total ocean surface and contribute >30% of the global marine carbon fixation. Despite apparently uniform growth conditions over broad areas, primary production in these regions exhibits a remarkable degree of variability. In this study of 34 stations in the North and South Atlantic Subtropical Gyres, we found a 20 fold variation (from 18 to 362 mgC m-2 d-1) in water-column-integrated primary production rate (UPP), while chlorophyll biomass only varied by a factor of 3. The changes in productivity were not associated with variations in incident surface irradiance, chlorophyll concentration, phytoplankton C biomass or phytoplankton size structure. The rate of nutrient supply to the euphotic layer, as estimated from variations in the depth of nitracline, appeared as the most relevant environmental factor in explaining the observed variability in UPP. We found significant changes in the composition of the picophytoplankton community across the range of measured productivities. The relative biomass contribution of Synechococcus spp. and the picoeukaryotes tended to increase with increasing UPP, whereas the opposite was true for Prochlorococcus spp. Across the wide range of measured primary productivity rates, the persistent dominance of picophytoplankton indicates that the microbial loop and the microbial food web continued to be the most important trophic pathways. Our observations of the oligotrophic ocean reflect a dynamic ecosystem where the microbial community responds to environmental forcing with significant changes in biological rates rather than trophic organization.

High variability of primary production in oligotrophic waters of the Atlantic Ocean: uncoupling from phytoplankton biomass and size structure