Size-fractionated biomass, photosynthesis and dark CO2 fixation in a tropical oceanic environment

This study examines the spatial distribution and size structure of phytoplankton biomass and productivity in relation to the vertical structrure of the Andaman Sea (northeastern Indian Ocean). This region was characterized by low concentrations of nutrients and high levels of insolation. Nitrogen availability appeared to control overall productivity with nitrate-based ‘new’ production accounting for 8–24% of the total primary production. Euphotic column chlorophyll (chl a ) averaged 52.5 mg m −2 of which a major portion was located as a subsurface chl a maximum (SCM) at ∼60–80m. Net, nano and picoplankton contributed an average of 39, 24 and 37% to euphotic column chl a , respectively. An inverse relationship was observed between the percentages of picoplankton and total chl a . On the other hand, net phytoplankton showed a decreasing trend with increasingly oligotrophic conditions. Of the total mean euphotic column production (0.17 g C m −2 day −1 ), 37% was attributable to picoplankton, whereas nano and net phytoplankton contributed 40 and 23%, respectively. In contrast to the generally accepted view that picoplankton are low-light adapted, no signs of photoinhibition were observed in surface populations of picoplankton photosynthesizing at high light intensities of ∼1500μE m −2 s −1 . Below the euphotic zone (100–200m), dark fixation of CO 2 was quite significant. The average column dark fixation of CO 2 was 0.045g C m −2 day −1 which is ∼19% of the euphotic column production. A substantial percentage of dark fixation of CO 2 was attributable to organisms in the picoplankton size class. Despite their low sinking rates, picoplankton may be the dominant contributors to organic carbon fluxes to deeper depths through the formation of aggregates with river-derived mineral particles.