Crustacean zooplankton, often dominated by copepods and euphausiids, are the major phytoplankton grazers in coastal upwelling systems. It has been argued that zooplankton grazing is a size-dependent process, such that models incorporating the size structure of zooplankton are appropriate for describing herbivore C-transfer. Here, based on the size-spectrum theory and on gut-fluorescence experiments, conducted with numerically dominant copepods from two upwelling sites off the Chilean coast, we show that C-specific ingestion rates of copepods are size-dependent. We further show that the size structure of the copepod community, synthesized by the slope of the normalized size spectrum, determines the impact of grazing on phytoplankton. C-specific ingestion rates, depending on species size, were in the range of 0.14-353.97 (ng C μg C-1 h-1). A modelled normalized biomass-spectra of a copepod community in the size range of 0.5 to 74.0 μg C showed that C-specific grazing impact can increase by a factor of 4 when small-sized species (0.1-10 μg C ind-1), such as Paracalanus cf. indicus, Acartia tonsa, Oncaea spp. and Corycaeus spp., dominate the community in terms of biomass. By contrast, when larger-sized copepods dominate (10-100 μg C ind-1), such as Calanus chilensis, Calanoidespatagoniensis and Rhyncalanus nasutus, total zooplankton biomass may increase, but with a sharp decrease in the efficiency of C transfer via herbivores. Our findings indicate that processes affecting the size structure of zooplankton communities can substantially impact the phytoplankton C flux through the pelagic food web, thus controlling production of higher trophic levels.