ECOLOGY AND BIOGEOCHEMISTRY OF THE ANTARCTIC CIRCUMPOLAR CURRENT DURING AUSTRAL SPRING - A SUMMARY OF SOUTHERN-OCEAN JGOFS CRUISE ANT X 6 OF RV POLARSTERN/

The R.V. Polarstern cruise ANT X/6, part of the international Southern Ocean JGOFS programme, investigated phytoplankton spring bloom develo pment 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 (AWE) and the marginal ice zone (MIZ). The relat ive roles of physical stability, iron limitation and grazing pressure in enhancing or constraining phytoplankton biomass accumulation were e xamined. Three sections were carried out between the PFr and the ice e dge along the 6 degrees W meridian from early October to late November 1992. This paper summarises the major findings of the cruise and disc usses their implications for our understanding of Southern Ocean ecolo gy and biogeochemistry. A major finding was the negligible build-up of plankton biomass and concomitant absence of CO2 drawdown associated w ith seasonal retreat of the ice cover. In striking contrast to this un expected poverty of both the MIZ and the frontal region of the AWE, di stinct phytoplankton blooms, dominated by different diatom species, ac cumulated in the PFr. Chlorophyll stocks in the sACC remained monotono usly low throughout the study. Our findings confirm those of other stu dies that frontal regions are the major productive sites in the Southe rn Ocean and that input of meltwater and associated ice algae to the s urface layer from a retreating ice edge is by itself an insufficient c ondition for induction of phytoplankton blooms. The blooms in the PFr developed under conditions of shallow mixing layers, high iron concent rations and relatively low grazing pressure. However, in all three blo oms, high biomass extended to deeper than 70 m, which cannot be explai ned by either in situ growth or sinking out of a part of the populatio n from the upper euphotic zone. Subduction of adjoining, shallower lay ers to explain depth distribution is invoked. Despite a clear CO2 draw down in the Polar Frontal region, the highly variable conditions encou ntered render reliable estimation of annual CO2 fluxes in the Southern Ocean difficult. (C) 1997 Elsevier Science Ltd.