To determine the effects of Saharan dust on the abundance, biomass, community structure, and metabolic activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30°N and 30°S in the central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged, different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The predominant type of metabolic response depended on the ecosystem's degree of oligotrophy and was modulated by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic waters, became less responsive to dust as the ecosystem's degree of oligotrophy increased. Given the divergent consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the ecosystem's degree of oligotrophy, stimulate or weaken biological CO 2 drawdown. Thus, the biogeochemical implications of changing dust fluxes might not be universal, but variable through both space and time.