Size-fractionated biomass and primary productivity of Sargasso Sea phytoplankton

The size structure of phytoplankton communities greatly influences the function of pelagic food webs and, ultimately, the flux of material from the surface ocean to the deep sea. While the biomass and taxonomic composition of organisms in a specific size class are important characteristics of a food web, the activity of that size fraction is more directly relevant to trophic dynamics and biogeochemical cycling. Even if a size class dominates cell abundance or biomass, does it necessarily contribute the most to total primary productivity (PP)? We asked this question for phytoplankton communities from the Sargasso Sea. The picophytoplankton (0.7-2 mu m) accounted for 53-85% of the total integrated chl-a and 46-99% or more of the total integrated PP. The microphytoplankton (20-200 mu m) were responsible for up to 38% of the total PP, but accounted for no more than 22% of total chl-a. Variations in the picophytoplankton size-class explained 84% of the variance in total integrated chl-a and 87% of the variance in total PP. Size-dependent relative contributions to chl-a versus PP varied with depth, with differences generally driven by vertical variations in C:chl-a ratios and chl-specific rates of PP. Use of the relative contributions of picophytoplankton to biomass as a predictor of their contributions to total primary productivity resulted in an average underestimate of similar to 7%.