Size-fractionated phytoplankton biomass and production in the tropical Atlantic

Abstract

Two meridional transects were conducted in the tropical and subtropical Atlantic to describe (i) the spatial variability of total and size-fractionated (picophytoplankton and phytoplankton >2 mm) chlorophyll a (chl a) concentrationand primary production, (ii) the relative contribution of each phytoplankton size fraction to total biomass and carbon fixation, and (iii) the spatial variability of size-fractionated phytoplankton growth rate (P/B) and assimilation number (P/chl a) in the ocean. The highest chl a for both size fractions was observed in the Western Tropical Atlantic province (WTRA), while the lowest chl a was found in the upper mixed layer (UML) of the South Atlantic Tropical gyre (SATL). A similar pattern was found for carbon fixation. Within the SATL, the highest picophytoplankton contribution to total production was recorded at the Deep Chlorophyll Maximum (DCM), while the contribution of phytoplankton >2 mm was higher in the UML. Additionally, the relative contribution of large phytoplankton to total integrated primary production was higher than its contribution to total biomass. Both size fractions depicted maximum P/B and P/chl a in WTRA surface waters. In the SATL province, phytoplankton >2 mm showed the highest P/B and P/chl a along the UML, while picophytoplankton P/B and P/chl a peaked around the DCM. We suggest that the differential impact of light on small and large phytoplankton may help to explain the contrasting dynamics of the two size classes.