SPRING DEVELOPMENT OF PHYTOPLANKTON BIOMASS AND COMPOSITION IN MAJOR WATER MASSES OF THE ATLANTIC SECTOR OF THE SOUTHERN-OCEAN

The distribution and composition of phytoplankton stocks in relation t o water masses were studied during the SO-JGOFS cruise of R.V. Polarst ern in the Atlantic sector of the Southern Ocean in October/November 1 992. The cruise comprised one west-to-east transect along the ice edge from 49 degrees W to 6 degrees W and several meridional transects alo ng 6 degrees W that extended from the closed pack ice of the Weddell S ea, across the southern Antarctic Circumpolar Current (ACC) and into t he Polar Frontal Zone. Chlorophyll (chl a concentrations, temperature and salinity were recorded continuously in surface water during the tr ansects. Vertical distribution and species composition of microplankto n were assessed microscopically in discrete water samples collected at stations. Contrary to expectations, no significant enhancement of phy toplankton biomass was found in the vicinity of the retreating ice cov er. Melt-water-influenced zones were indicated by low salinity but als o by abundance of characteristic sea-ice species such as Nitzschia clo sterium and N. prolongatoides, but chlorophyll concentrations averaged only 0.3 mg chi a m(-3) and barely increased during the spring. Value s were even lower and remained constant in the southern ACC (ca 0.2 mg chi a m(-3)). Ln contrast, large phytoplankton blooms developed durin g the 6 weeks of investigation in the region of the Polar Front (PFr), from 0.7 to >4 mg chi a m(-3). Three distinct blooms extended below 7 0 m depth, each dominated by a different diatom species (Fragilariopsi s kerguelensis, Corethron inerme and C. criophilum). We speculate that the large phytoplankton stocks below 40 m depth are a result of subdu ction of surface layers as sinking and in situ growth can be ruled out . The factors leading to the accumulation of high phytoplankton stocks in the PFr (up to 270 mg chi m(-2), but not in the meltwater zones or in the front between ACC and Weddell Gyre, are not clear, but higher iron concentrations in the former region seem to have played a role. ( C) 1997 Elsevier Science Ltd.