It has been known since the early eighties that picophytoplankton (50% in most of the studied area) and total phytoplankton carbon biomass (>20% in the open ocean), being indeed essential in determining the spatial variability of the former. However, this group's contribution to the diel variability in the cp-derived POC concentration was not significant (~10%). Daily rates of change (d-1) in picophytoplankton biomass, on the other hand, presented a significant positive correlation to those in cp (r = 0.7; p < 0.001). The usefulness of cp as a proxy for photosynthetic carbon biomass, compared to chlorophyll a, is briefly discussed. Picophytoeukaryotes carbon biomass was much more important than previously thought, equally or more important than that of Prochlorococcus in the open ocean. This group could therefore be playing a very important ecological and biogeochemical role in subtropical gyres, which extend over a vast area of the world's ocean.
