We have determined the scaling relationship between photosynthesis rate and cell size in natural phytoplankton assemblages of contrasting marine environments. We found that phytoplankton photosynthesis in the ocean does not scale as the L-power of cell size, but scales approximately isometrically with cell size, indicating that a single model cannot predict the metabolism-size relationship in all photosynthetic organisms. The scaling relationship between cellular chlorophyll a content and cell size is also isometric. Taxonomical changes along the size spectrum may explain the deviation of phytoplankton photosynthesis from the general allometric rule. The size scaling exponent for photosynthesis is significantly higher (1.14) in coastal productive waters than in the oligotrophic open ocean (0.96), which provides a physiological basis to explain the dominance of larger cells in nutrient-rich environments. The size scaling exponent for phytoplankton abundance is significantly less negative in coastal productive waters (20.90) than in the oligotrophic open ocean (21.25). The observed size scaling relationships imply that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in productive ones. By controlling the metabolism-size scaling relationship, nutrient supply plays a major role in determining community size structure and the energy flow through the pelagic ecosystem.The relative importance of small and large phytoplankton is a key feature of the planktonic community, which strongly affects the fate of recently synthesized organic matter in the pelagic ecosystem (Kiørboe 1993;Legendre and Rassoulzadegan 1996;Falkowski et al. 1998). Small cells account for the bulk of phytoplankton biomass in open-ocean oligotrophic waters, where most of the newly produced organic carbon is recycled within the photic layer through complex microbial food webs. By contrast, larger cells dominate in nutrient-rich productive waters, where a major fraction of primary production is channeled through short food chains and exported toward the ocean interior, thus contributing to CO 2 sequestration. According to the allometric theory in biology (Peters 1983;Brown et al. 2000;Niklas and Enquist 2001), individual metabolic rates (M) scale with body size (V) as M 3 V 3/4 (the Lpower rule or Kleiber's law). A compilation of plant biomass production data covering 20 orders of magnitude in body size, and including laboratory measurements for microalgae, gives support to the view that the L-scaling rule applies to all photosynthetic organisms (Niklas and Enquist 2001). If the L-power rule holds, it follows that mass-specific metabolism and growth rates must scale as V 21/4 . This means that smaller cells should dominate all types of pelagic environments, on account of their faster metabolism and growth rates. However, trophic and hydrodynamic mechanisms may also play a role in controlling phytoplankton loss rates and, therefore, their size structure. Thus, the dominance of larger ph...