Autotrophic carbon assimilation and biomass from size-fractionated phytoplankton in the surface waters across the subtropical frontal zone (Indian Ocean)

The composition of the phytoplanktonic communities in the surface waters of the La Reunion-Kerguelen transect (from 38 degrees 36S to 46 degrees 33S) has been investigated under spring conditions (Antares 3 cruise, France-JGOFS. 28 September-8 November, 1995). The study, conducted at six stations in the subtropical frontal zone, involved size fractionations (threshold: 2 mu m). The large variations in the overall biomass and autotrophic carbon fixation, calculated via Rubisco activity measurements and expressed respectively in terms of mu g chlorophyll (a + b + c) per liter and nmol fixed carbon dioxide per liter and per hour, were attributable only to phytoplanktonic cells of >2 mu m, with a peak observed in the frontal zone. The picophytoplankton (<2 mu m) biomass remained constant throughout the transect, but the evolution of the species composition of the picophytoplanktonic population, as calculated from flow cytometry measurements through this frontal zone, changed. This study provides evidence, for the first time in this area. of the disappearance of prochlorophytes from the south of the frontal zone (42-47 degrees S). Picoeukaryotes (<2 mu m) and cyanobacteria populations, resolved by flow cytometry, were present all along the transect. However, their abundance decreased southward up to the quasi-disappearance of cyanobacteria at the southernmost station (52 degrees S) that is characteristic of antarctic waters. The presence of prochlorophytes that is exclusive to the subtropical surface waters, and the low carbon fixation activity associated with these waters, may be linked to the specific hydrological features encountered. In contrast, the marked reduction in the cyanobacteria and the abundance of picoeukaryotes along the north-south transect is more likely to be a result of the reduction in temperature through the frontal zone.