Large-sized phytoplankton sustain higher carbonspecific photosynthesis than smaller cells in a coastal eutrophic ecosystem

Size-fractionated, carbon-specific photosynthesis by natural phytoplankton assemblages has been analysed during a full annual cycle in a coastal, productive ecosystem. We observed high variability in the carbon-specific photosynthetic rates by the different phytoplankton size fractions. In surface waters, under high irradiance levels, large-sized phytoplankton showed consistently higher carbon-specific photosynthetic rates than small-sized cells. We suggest the possibility that interspecific differences in acquisition and use of light and nutrients account for this pattern. Under light-limited conditions at the bottom of the euphotic layer, differences in the carbon-specific photosynthesis between size fractions were lower or disappeared, which was likely due to the package effect on larger cells. Our results show that when the whole phytoplankton community is analysed, the relationship between carbon-specific photosynthesis and cell size is variable. Moreover, a higher carbon-specific photosynthesis by larger phytoplankton might represent an ecophysiological basis to explain their dominance in resource-rich environments.