7 páginas, 3 figuras, 2 tablasIf the size dependence of species richness varies across ecosystems, it should beud reflected in the size distribution of total abundance. Using a database of phytoplankton abundance,ud species composition and cell size from coastal, shelf and open-ocean environments, we show that theud biogeographical patterns of phytoplankton size distribution in the ocean are a result of systematicud changes in the relationship between species richness and cell size. Our results indicate that, regardlessud of the environmental conditions, population abundance decreases consistently to the –3/4 powerud of cell size. By contrast, marine phytoplankton diversity peaks at small sizes in oligotrophic watersud but is either a log-normal function or independent of cell size in eutrophic systems. It is argued that,ud operating on evolutionary time scales, size-dependent biophysical constraints for resource acquisitionud are reflected in the size distribution of species richness and consequently in the size structure ofud phytoplankton communities in the ocean. These findings indicate that the way in which biologicalud diversity changes with body size is crucial to a better understanding of the structure and functioningud of microbial plankton communities and how energy flows through pelagic ecosystemsAMTud data collection was supported by the UK Natural Environmentalud Research Council through the Atlantic Meridionalud Transect consortium (NER/O/S/2001/00680). P.C. was supportedud by a Fulbright Postdoctoral Research Fellowship fromud the Spanish Ministry of Education and Science and a Marieud Curie Outgoing International Fellowship from the Europeanud UnionPeer reviewe