The R.V. Polarstern cruise ANT X/6, part of the international Southern Ocean JGOFS programme, investigated phytoplankton spring bloom development and its biogeochemical effects in different water masses of the Atlantic sector of the Southern Ocean: the Polar Frontal region (PFr), the southern Antarctic Circumpolar Current zone (sACC), its boundary with the Weddell Gyre (AWB) and the marginal ice zone (MIZ). The relative roles of physical stability, iron limitation and grazing pressure in enhancing or constraining phytoplankton biomass accumulation were examined. Three sections were carried out between the PFr and the ice edge along the 6°W meridian from early October to late November 1992. This paper summarises the major findings of the cruise and discusses their implications for our understanding of Southern Ocean ecology and biogeochemistry. A major finding was the negligible build-up of plankton biomass and concomitant absence of CO2 drawdown associated with seasonal retreat of the ice cover. In striking contrast to this unexpected poverty of both the MIZ and the frontal region of the AWB, distinct phytoplankton blooms, dominated by different diatom species, accumulated in the PFr. Chlorophyll stocks in the sACC remained monotonously low throughout the study. Our findings confirm those of other studies that frontal regions are the major productive sites in the Southern Ocean and that input of meltwater and associated ice algae to the surface layer from a retreating ice edge is by itself an insufficient condition for induction of phytoplankton blooms. The blooms in the PFr developed under conditions of shallow mixing layers, high iron concentrations and relatively low grazing pressure. However, in all three blooms, high biomass extended to deeper than 70 m, which cannot be explained by either in situ growth or sinking out of a part of the population from the upper euphotic zone. Subduction of adjoining, shallower layers to explain depth distribution is invoked. Despite a clear CO2 drawdown in the Polar Frontal region, the highly variable conditions encountered render reliable estimation of annual CO2 fluxes in the Southern Ocean difficult.
