Size dependence of coastal phytoplankton photosynthesis under vertical mixing conditions

We have determined the relationship between carbon-specific photosynthesis and phytoplankton cell size in a coastal ecosystem. The normalized size spectra of carbon (C) biomass and photosynthesis allow to determine both biomass and photosynthesis within any size class along the community size spectrum. By dividing the size spectra of photosynthesis and biomass, the size spectrum of C-specific photosynthesis is derived. Our results indicated a high variability in the slope of the C-specific photosynthesis size spectrum. Under favourable conditions for growth, in the upper euphotic layer, the slope was positive, indicating that larger phytoplankton attained higher C-specific photosynthesis rates than the smaller cells. This pattern represents a significant departure from the expected, literature value of 0.25 for the size-scaling of biomass-specific metabolism. We suggest that this change in the slope may be caused by the changes in the taxonomic composition along the community size spectrum. Towards the bottom of the euphotic layer, we observed a decrease in the slope of the C-specific photosynthesis size spectrum, which could be associated with an enhanced package effect in larger cells under light-limited conditions. These results question the applicability of single and overall exponents to describe the size scaling of photosynthesis in natural phytoplankton assemblages.