Leblanc¹, K., B. Quéguiner², S. Blain³ and M. Fiala⁴

¹College of Marine Studies, 700 Pilottown Road, 19958 Lewes, Delaware, USA, Tel: 302-645-4079, Fax: 302-645-4007, E-mail: leblanc@udel.edu, ²Centre d’Océanologie de Marseille, Laboratoire d’Océanographie et de Biogéochimie, Campus de Luminy, Case 901, 13288 Marseille Cedex 9, France, ³Institut Universitaire Européen de la mer, UMR CNRS 6539, Place Nicolas Copernic, 29280 Plouzané, France and ⁴Observatoire Océanologique de Banyuls, Université P. et M. Curie, UMR CNRS 7621, 66651 Banyuls-sur-mer, France

Dynamics of the siliceous components in the frontal zone of the ACC (Antarctic Circumpolar Current) – Bottom-up control and Si/C decoupling processes

Three sectors of the ACC were studied during the ANTARES 4 cruise, which occurred in Jan-Feb 1999 in the indian sector of the Southern Ocean: the Polar Front Zone (PFZ), the Sub-Antarctic Zone (SAZ) and the Sub-Tropical Zone (STZ). The area was characterized as an HNLSiLC (High Nutrient Low Silicate Low Chlorophyll) area. The distribution of particulate matter such as biogenic silica and chlorophyll a were strongly influenced by the frontal dynamics, showing an accumulation of biomass along the Sub-Tropical Front. Both diatom concentrations in terms of biogenic silica and silica production rates were low, reflecting the end of the productive period. Diatoms appeared Si-limited both in the SAZ and the STZ where surface values were quite low (0.1-1.4 µM). In the STZ, NO₃ availability was most prone to limit algal accumulation and production rates. Enrichment experiments suggested Fe-limitation in all three sub-systems, and Si/Fe co-limitation processes in the PFZ/SAZ. Si production was 3 times higher in the STZ than in the PFZ, while the highest exportation rates were measured on the contrary in the latter region, with respectively 2% and 18% of the biogenic silica produced in the surface layer exported to 200 m. This discrepancy may be explained by a faster dissolution of biogenic silica in the surface layer in the STZ due to higher temperature and bacterial degradation, while the increased grazing rates in the PFZ may have contributed to export more silica through fecal pellets.