Abundance, size structure and community composition of phytoplankton in the Southern Ocean in the austral summer 1999/2000

The abundance, size structure and community composition of phytoplankton in the Southern Ocean were studied, using flow cytometry, microscopy and pigment profiles on two transects one latitudinal (N) and one longitudinal (W) during December 1999 and January 2000. In both transects, the concentration of autotrophic eukaryotes of 2-10μm equivalent spherical diameter (ESD) commonly exceeded those10μm ESD were detected by flow cytometry (however microscopy showed cells>10μm in length). Throughout transect N, chlorophyll α concentrations were generally. South of the Antarctic Polar Front (APF), chlorophyll α concentrations increased southward. CHEMTAX allocation of pigment data (italicized) showed that Diatoms contributed most chlorophyll with Haptophytes sub-dominant. North of the APF, chlorophyll α concentrations tended to increase northward. Here, Haptophytes contributed most chlorophyll, followed by Diatoms, Chlorophytes and Cyanobacteria, except at the northernmost stations where Cyanobacteria dominated. In transect W, chlorophyll α concentrations were also in most cases, but variable. Higher concentrations occasionally occurred in the west. In this transect, Diatoms contributed most (mean=61±15%) of the chlorophyll α, followed by Haptophytes. Nanodiatoms (particularly Fragilariopsis spp.) numerically dominated the diatom community. Fecal pellets composed of these nanodiatoms were observed in the Antarctic water, probably originating from heterotrophic dinoflagellates, implying a significant contribution of nanodiatoms to the microbial food web. However they contributed little to total chlorophyll α and diatom carbon biomass, particularly when chlorophyll and carbon concentrations were high.