Spring development of phytoplankton biomass and composition in major water masses of the Atlantic sector of the Southern Ocean

Abstract

The distribution and composition of phytoplankton stocks in relation to water masses were studied during the SO-JGOFS cruise of R.V. Polarstern in the Atlantic sector of the Southern Ocean in October/November 1992. The cruise comprised one west-to-east transect along the ice edge from 49°W to 6°W and several meridional transects along 6°W that extended from the closed pack ice of the Weddell Sea, across the southern Antarctic Circumpolar Current (ACC) and into the Polar Frontal Zone. Chlorophyll (chi a concentrations, temperature and salinity were recorded continuously in surface water during the transects. Vertical distribution and species composition of microplankton were assessed microscopically in discrete water samples collected at stations. Contrary to expectations, no significant enhancement of phytoplankton biomass was found in the vicinity of the retreating ice cover. Melt-water-influenced zones were indicated by low salinity but also by abundance of characteristic sea-ice species such as Nitzschia closterium and N. prolongatoides, but chlorophyll concentrations averaged only 0.3 mg chi a m −3 and barely increased during the spring. Values were even lower and remained constant in the southern ACC ( ca 0.2 mg chi a m −3). In contrast, large phytoplankton blooms developed during 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 70 m depth, each dominated by a different diatom species ( Fragilariopsis kerguelensis, Corethron inerme and C. criophilum). We speculate that the large phytoplankton stocks below 40 m depth are a result of subduction 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.