Abstract

The distribution of picoplankton (<2 to 3 μm in diameter) was determined on a transect across the eastern South Pacific Ocean from south of Tahiti to the coast of Chile along 32.5°S latitude during the early austral spring. The abundance of Synechococcus, picophytoeukaryotes and bacterioplankton increased from oligo- to eutrophic conditions, while that of Prochlorococcus decreased according to nutrient availability and hydrographic characteristics. Bacterioplankton dominated across the transect (>75% total picoplanktonic abundance). As expected, Prochlorococcus was the most numerically abundant phytoplankter under very oligotrophic (chlorophyll a concentration ≤0.1 mg m -3) and mesotrophic (>0.1 and ≤1 mg m -3) conditions. However, in contrast to other subtropical regions, picophytoeukaryotes appear to dominate the <2 μm autotrophic carbon biomass in this region of the South Pacific Ocean at this time of the year. In the upper 200 m of the water column, the integrated carbon biomass of Prochlorococcus, Synechococcus, picophytoeukaryotes and bacterioplankton were in the ratios of 9:1:14:11 and 3:1:8:6 under oligo- and mesotrophic conditions, respectively. Thus, picophytoeukaryotes were 1,4- to 2-fold higher in biomass than both cyanobacteria combined, and slightly more important (1.2- to 1.3-fold) than bacterioplankton. Picophytoeukaryotes could therefore play a dominant ecological and biogeochemical role in subtropical gyres, which extend over a vast area of the world's oceans. © Inter-Research 2007.