Spring phytoplankton assemblages in the Southern Ocean between Australia and Antarctica

Variations of phytoplankton assemblages were studied in November-December 2001, in surface waters of the Southern Ocean along a transect between the Sub-Antarctic Zone (SAZ) and the Seasonal Ice Zone (SIZ; 46.9 degrees-64.9 degrees S; 142 degrees-143 degrees E; CLlVAR-SR3 cruise). Two regions had characteristic but different phytoplankton assemblages. Nanoflagellates(< 20 mu m) and pico-plankton (similar to 2 mu m) occurred in similar concentrations along the transect, but were dominant in the SAZ, Sub-Antarctic Front (SAF), Polar Front Zone (PFZ) and the Inter-Polar Front Zone (IPFZ), (46.9 degrees-56.9 degrees S). Along the entire transect their average cell numbers in the upper 70 m of water column, varied from 3 x 10(5) to 1.1 x 10(6) cells 1(-1). Larger cells (> 20 mu m), diatoms and dinoflagellates, were more abundant in the Antarctic Zone-South (AZ-S) and the SIZ, (60.9 degrees-64.9 degrees S). In AZ-S and SIZ diatoms ranged between 2.7 x 10(5) and 1.2 x 10(6) cells 1(-1), dinoflagellates from 3.1 X 10(4) to 1.02 x 10(5) cells 1(-1). A diatom bloom was in progress in the AZ-S showing a peak of 1.8 x 10(6) cells 1(-1). Diatoms were dominated by Pseudo-nitzschia spp., Fragilariopsis spp., and Chaetoceros spp. Pseudo-nitzschia spp. outnumbered other diatoms in the AZ-S. Fragilaropsis spp. were most numerous in the SIZ. Dinoflagellates contained autotrophs (e.g. Prorocentrum) and heterotrophs (Gyrodiniuml Gymnodinium, Protoperidinium). Diatoms and dinoflagellates contributed most to the cellular carbon: 11-25 and 17-124 mu g C 1(-1), respectively. Small cells dominated in the northern region characterized by the lowest N-uptake and new production of the transect. Larger diatom cells were prevalent in the southern area with higher values of N-uptake and new production. Diatom and nanofiagellate cellular carbon contents were highly correlated with one another, with primary production, and productivity related parameters. They contributed up to 75% to the total autotrophic C biomass. Diatom carbon content was significantly correlated to nitrate uptake and particle export, but not to ammonium uptake, while flagellate carbon was well correlated to ammonium uptake, but not to export. Diatoms have contributed highly to particle export along the latitudinal transect, while flagellates played a minor role in the export.