Spring distribution of larger (>64 μm) protozoans in the Atlantic sector of the Southern Ocean

Distribution of larger protozoans (armoured dinoflagellates, tintinnids, heliozoans, radiolarians and foraminiferans > 64 mum) is presented for three major water masses of the Southern Ocean: the Polar Front region (PFr), the southern Antarctic Circumpolar Current (southern ACC) and the northern Weddell Gyre. Sampling took place during: the SO-JGOFS cruise ANT X/6 of R/V Polarstern (October-November 1992) along a meridional transect at 6 degreesW between 48 degrees 00'S and 59 degrees 30'S. Multinet samples (64 mum mesh size) were taken at six stations from the surface down to 500 m depth at five different depth intervals. In the upper 100 m of the water column abundances of larger protozoans varied between 94 and 10,930 ind, m(-3), with highest abundances in the PFr, where phytoplankton blooms occurred, and lowest values in the Antarctic Circumpolar Current-Weddell Gyre Boundary (AWB). Foraminiferans and polycystine and smaller ( < 300 m) phaeodarian radiolarians dominated larger protozoan assemblages in the PFr. In open water of the southern ACC, tintinnids, armoured dinoflagellates, foraminiferans and smaller ( < 300 m) phaeodarian radiolarians were equally important. The heliozoans Sticholonche spp. and nassellarian radiolarians dominated assemblages in the Weddell Gyre and AS VB. Larger protozoan biomasses ranged between 2 and 674 mug C m(-3) and were always dominated by larger (>300 mum) phaeodarians. Highest biomasses were found in the AWE between 200 and 500 m depth. Standing stocks of larger protozoans constituted a negligible fraction of zooplankton biomass in the upper 200 m of the water column. In deeper layers of the ice-covered Weddell Gyre and AWE their biomasses, dominated by larger ( > 300 mum) phaeodarians, was significant contributing up to 45% to total larger protozoan and metazoan biomass. Analysis of correlation between distribution patterns and environmental conditions at the stations sampled indicate that spring distribution patterns of heterotrophic armoured dinoflagellates, polycystine radiolarians and foraminiferans follow productivity in the water column. Of the protozoan groups studied the smaller (< 300 m) phaeodarian radiolarians also showed a significant correlation with productivity during spring, however, results from previous studies do not suggest a consistent pattern. Spring distribution patterns of other larger protozoans were not related to differences in productivity in the water column, and effects such as ice-cover, grazing or silica limitation might be determining. Dead radiolarian skeletons constituted on average 27, 8 and 11% of the population of nassellarians, spumellarians and smaller ( < 300 m) phaeodarians, respectively. The contribution of dead radiolarian skeletons to total radiolarian stocks varied with depth and mater mass. Differences between live and skeleton assemblages composition mere observed. These differences should be taken into consideration when interpreting the geological record. (C) 2001 Elsevier Science Ltd. All rights reserved.