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-JGO FS. 28 September-8 November, 1995). The study, conducted at six stations in the subtropical frontal zone, involved size fractionations (threshold: 2 m u m). The large variations in the overall biomass and autotrophic carbon fi xation, calculated via Rubisco activity measurements and expressed respecti vely in terms of mu g chlorophyll (a + b + c) per liter and nmol fixed carb on dioxide per liter and per hour, were attributable only to phytoplanktoni c cells of >2 mu m, with a peak observed in the frontal zone. The picophyto plankton (<2 mu m) biomass remained constant throughout the transect, but t he evolution of the species composition of the picophytoplanktonic populati on, as calculated from flow cytometry measurements through this frontal zon e, 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, t heir abundance decreased southward up to the quasi-disappearance of cyanoba cteria 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 associate d with these waters, may be linked to the specific hydrological features en countered. In contrast, the marked reduction in the cyanobacteria and the a bundance of picoeukaryotes along the north-south transect is more likely to be a result of the reduction in temperature through the frontal zone.