Spatial variation of phytoplankton assemblages and biomass in the New Zealand sector of the Southern Ocean during the late austral summer 2008

Up until now, only the floristic composition of large phytoplankton species has been studied in the Southern Ocean immediately south of New Zealand. To fill the gaps in our knowledge of the Antarctic phytoplankton flora and biomass, in relation to physical and chemical environments, New Zealand’s IPY project sampled water between New Zealand and Antarctica and into the Ross Sea in late summer. Near surface phytoplankton samples, collected between New Zealand and Antarctica, and vertical stations, between Scott Island and the Ross Sea ice shelf, allowed us to contrast the flora and biomass in the Ross Sea/Antarctic and Subantarctic waters. Spatially, diatoms were found to be more diverse in both shelf and offshore Antarctic than in Subantarctic waters. Over the Ross Sea shelf, the pennate diatom, Fragilariopsis kerguelensis, and a colonial prymnesiophyte, Phaeocystis antarctica, dominated both numerically and in biomass, contributing to greatest levels of chlorophyll a and cell carbon biomass over the shelf. In both shelf and Antarctic waters, heterotrophic taxa accounted for a greater proportion of total dinoflagellate cell carbon (avg. 95.4 and 91.9 %, respectively) than autotrophic, while in the Subantarctic waters, the reverse was true (avg. 48.6 %). A small number of potentially harmful dinoflagellates, Karenia and Karlodinium spp., were found for the first time in the New Zealand sector of the Southern Ocean. This study represents the first thorough description of the spatial variation of the floristic composition and biomass of phytoplankton in relation to physical and chemical properties, from New Zealand to Antarctica.