Summer Weddell sea microplankton: Assemblage structure, distribution and abundance, with special emphasis on the Tintinnina

Abstract

Silicoflagellates, large thecate dinoflagellates and tintinnids were counted and measured in screened (26 μm pore-size) pump-samples retrieved from a depth of ca. 9 m in February-March 1987 along a two-leg north-south transect in the Weddell Sea (approx. 62°S to 78°S). Fourteen tintinnid taxa were identified and their individual abundances and biomasses were estimated. Highest biomass, in terms of organic carbon, was recorded for the Tintinnina, which averaged 4.001 mg C m-3, with mean cell-numbers of 161 ind. l-1 (maximum: 859 ind. l-1); followed by the dinoflagellates (1.018 mg C m-3, mean: 192 ind. l-1; max.: 1176 ind. l-1); and the silicoflagellates (0.391 mg C m-3, mean: 467 ind. l-1, max.: 3123 ind. l-1). Conspicuous abundance and settling volume peaks were recorded at some distance off the edge of the ice-pack (at approx. 69°S to 72°S), and in the ice-covered area south of 74–75°S. This pattern was paralleled by changes in the specific makeup of tintinnid assemblages: in the ice-covered southern area Cymatocylis drygalskii and Laackmanniella prolongata were dominant, while in ice-free waters north of 73°S Codonellopsis gaussi, Cd. glacialis and Cymatocylis affinis/convallaria were the main components of the fauna. Overall tintinnid abundances were higher than those reported for many (but not all) extrapolar areas, and the average size of the species present was considerably larger than elsewhere; tintinnid carbon figures were therefore very high, oscillating around 2 mg C m-3 in the northern ice-free area, and 20 mg C m-3 in the southern ice-covered zone.