Species richness and cell-size distribution: size structure of phytoplankton communities

If the size dependence of species richness varies across ecosystems, it should be reflected in the size distribution of total abundance. Using a database of phytoplankton abundance, species composition and cell size from coastal, shelf and open-ocean environments, we show that the biogeographical patterns of phytoplankton size distribution in the ocean are a result of systematic changes in the relationship between species richness and cell size. Our results indicate that, regardless of the environmental conditions, population abundance decreases consistently to the -3/4 power of cell size. By contrast, marine phytoplankton diversity peaks at small sizes in oligotrophic waters but is either a log-normal function or independent of cell size in eutrophic systems. It is argued that, operating on evolutionary time scales, size-dependent biophysical constraints for resource acquisition are reflected in the size distribution of species richness and consequently in the size structure of phytoplankton communities in the ocean. These findings indicate that the way in which biological diversity changes with body size is crucial to a better understanding of the structure and functioning of microbial plankton communities and how energy flows through pelagic ecosystems.