Diatom assemblages in the surface water of the Indian Sector of the Antarctic Surface Water in summer of 1999/2000

Abundance and species composition of diatom assemblages in surface water of the Indian Sector of the Antarctic Surface Water of the Southern Ocean were examined using samples gathered during the JARE-41 Cruise of the icebreaker Shirase in the summer of 1999/2000. On the westward leg (Leg 1), abundance of the diatom assemblage was geographically rather uniform, while species composition was different geographically in mosaic manner. Dominant species on Leg 1 were the pennate Pseudo-nitzschia spp. and the centric Dactyliosolen tenuijunctus, Chaetoceros sp. cf. dichaeta and Chaetoceros neglectus. On the eastward leg (Leg 2), abundance and species composition were different east and west of 60°E. The community to the west was large in abundance and dominated by D. tenuijunctus, while that to the east was small and dominated by the pennate Fragilariopsis species. These diatom assemblages were grouped into four clusters on Leg 1 and three on Leg 2. Mosaic distribution of the clusters was again evident on Leg 1, while rather simple east-west difference was the case on Leg 2. These geographical variations seem to be affected by local sea ice dynamics