Wintertime abundance of picoplankton in the Atlantic sector of the Southern Ocean

Data on the distribution of picophytoplankton from the Southern Ocean are relatively scant and primarily collected during the austral spring and summer. During the ICEFISH (International Collaborative Expedition to collect and study Fish Indigenous to Sub-Antarctic Habitats) expedition conducted in the austral winter, 2004, we examined the abundance of picophytoplankton in surface waters along a 366 km W-E transect at similar to 55 degrees S latitude between the South Sandwich Islands and Bouvetoya Island, a 2780 kin S-N transect from Bouvetoya Island to Tristan da Cunha Island, and a 2050 kin W-E transect extending east from Tristan da Cunha toward Capetown, South Africa. The cruise track traversed a region of the Antarctic Circumpolar Current (ACC) that included four major frontal features: the Southern Antarctic Circumpolar Current Front (SACCF), the Antarctic Polar Front (APF), the Subantarctic Front (SAF), and the Subtropical Front (STF). In waters less than 1 degrees C, and south of the SACCF, picoeukaryotes represented more than 99% of the picophytoplankton in the community. Phycoerythrin-containing picoplankton 2 mu m in equivalent spherical diameter (ESD) were observed along our S-N transect once water temperatures exceeded 1.3 degrees C, placing their southernmost limit of distribution close to the Antarctic Polar Front. Substantial populations of these organisms, which had a flow cytometric signature comparable to those of PE-containing marine Synechococcus and other PE-containing picocyanobacteria, were seen in all samples collected in water >4 degrees C and north of the APF. Thus, both PE-containing picoplankton <2 mu m in equivalent spherical diameter (ESD) and picoeukaryotes appear to be part of the phytoplankton community in Antarctic polar waters year-round. In contrast, Prochlorococcus did not appear in water <10 degrees C or south of the southern expression of the STF, as expected from other reports. A strong linear relationship exists between log phytoplankton abundance and temperature, which is only observed when all functional groups are pooled. Picoplankton distributions show marked changes at frontal boundaries and support the hypothesis that water mass related phenomena, and not just temperature alone, determine phytoplankton community structure in the Southern Ocean.