Horizontal and vertical distribution pattern of living radiolarians along a transect from the Southern Ocean to the South Atlantic subtropical region

The horizontal and vertical distribution of living polycystine and phaeodarian radiolarians was investigated at seven locations on a transect from the Antarctic Zone (54°S) to the subtropical region (30°S) in the southeastern South Atlantic during austral autumn. Four or five depth intervals from 0 to 1000 m depth were sampled with opening/closing nets in combination with a hydrographic CTD survey. Highest abundances of radiolarians occurred in the Antarctic Zone south of the Polar Frontal Zone (up to 1.9 × 104/m3), representing the highest abundances yet reported from the Southern Ocean. There, two species of phaeodarians dominated the assemblages, exceeding polycystine abundances by one to two orders of magnitude. Our results combined with results of previous studies indicate that abundances of both groups of radiolarians south of the Polar Front can vary seasonally and geographically over four orders of magnitude. Phaeodarian abundances decreased to the north in the Polar Frontal Zone, while polycystine abundances decreased (to < 102/m3) further north in the Subantarctic Zone. Thus, phaeodarian abundances were markedly highest in the cold waters south of the Polar Frontal Zone. Abundances of both groups were low (ca 10/m3 in the nutrient-poor subtropical region, and polycystine abundances increased in the Namibian upwelling region. Gradients of temperature, silica concentration and nutrients along the transect probably influence the latitudinal abundance pattern of both radiolarian groups. nces of living polycystines were generally highest from 50 to 200 m, and abundance maxima occurred either above the thermocline or directly below the mixed layer, in contrast to the distribution pattern from North Pacific and equatorial Atlantic waters. Abundances of spumellarians and nassellarians also showed a more variable relationship wit depth than has been found for the equatorial Atlantic. Living phaeodarians were most abundant in the upper 200–300 m in the Antarctic Zone, and below 100 m further north; abundance maxima showed no consistent relation to the mixed layer. Phaeodarian abundances were highest in Antarctic Surface Water, followed by the Circumpolar Deep Water, followed by the Antarctic Intermediate Water, and were lowest in the Subantarctic Surface Water and the South Atlantic Central Water. ost all depths, living polycystines outnumbered polycystine skeletons, and there was a distinct pattern of almost exclusively living forms in the upper 100 m. Living phaeodarians also almost always outnumbered phaeodarian skeletons. In contrast to polycystines, significant numbers of phaeodarian skeletons occurred in the upper 100 m at the southernmost station. No latitudinal pattern of abundance of radiolarian skeletons was discernable. A comparison with data on skeletal standing stocks from three sites in the Pacific Ocean suggests that vertical distributions of radiolarian skeletons differ geographically.