Does size matter? Pico-phytoplankton cell size affects biomass distribution and nutrient limitation in the oligotrophic Eastern Mediterranean Sea

Abstract. The Eastern Mediterranean Sea (EMS) is one of the most oligotrophic marine environments in the world, characterized by extreme nutrient scarcity and strong water-column stratification. In such systems, pico-phytoplankton typically dominate primary production, yet how cell size structures biomass distribution and nutrient limitation remains poorly constrained. Here, we examined the spatial and vertical variability of pico-phytoplankton cell size, abundance, and nutrient status in the easternmost Mediterranean Sea during oligotrophic summer stratification. Using flow cytometry and microscopy, we quantified the cell volumes of Prochlorococcus, Synechococcus, and pico-eukaryotes and combined these with ambient nutrient concentrations to estimate cellular nutrient quotas and Resource Supply Indices (RSI). All three groups exhibited increasing cell size with depth and from offshore to coastal waters, coinciding with higher nutrient availability and chlorophyll concentrations. Synechococcus and pico-eukaryotes were consistently larger in coastal and deeper waters, whereas Prochlorococcus maintained small and relatively constant cell size across environments. RSI analysis revealed widespread nitrogen limitation for Synechococcus and pico-eukaryotes (RSIₙ<1), while phosphorus was generally sufficient (RSIₚ>1). In contrast, Prochlorococcus remained largely unconstrained by either nitrogen or phosphorus, reflecting its low cellular nutrient demand and streamlined physiology. These results demonstrate that cell size is a powerful integrator of environmental forcing and ecological strategy in oligotrophic seas. The dominance of small Prochlorococcus cells under extreme nutrient scarcity imply how stable ocean stratification and nutrient decline may reshape microbial communities and biogeochemical cycling in the future oligotrophic oceans.