Biogeochemical dynamics and the silicon cycle in the Atlantic sector of the Southern Ocean during austral spring 1992

High biogenic silica (BSi) concentrations (maximum: 11.7 mu mol l(-1)) were recorded during late November at the southern border of the Polar Frontal region (PFr). Position of the BSi maximum at depth suggested the occurrence of a sinking diatom population. By contrast, siliceous biomass was low (BSi <0.6 mu mol l(-1)) in the Marginal Ice Zone (MIZ) despite a sea-ice retreat of 200 km during the study period. Diatoms released from the receding ice were not actively growing. The Permanently Open Ocean Zone also showed very low BSi biomass (<0.5 mu mol l(-1)) and appeared as an area where phytoplankton are not dominated by siliceous organisms, especially in its middle part where BSi/POC (particulate organic carbon) molar ratios ranged between 0.04 and 0.06 at 53 degrees S, from surface to 200 m depth. At the southern border of the PFZ, the bloom coincided with an area of high lithogenic silica concentrations probably of aeolian origin. In addition, BSi/POC molar ratios measured in the PFZ were the highest ever recorded in the surface waters of the Southern Ocean (maximum: 1.75). This could be due to the presence of heavily silicified diatoms such as Fragilariopsis kerguelensis or also could reflect the more rapid recycling of POC as compared to BSi. Within the bloom area BSi concentrations were positively correlated to pyrophaeophytin pigments, possibly indicating the occurrence of a senescent diatom population. High concentrations of BSi (>1.5 mu mol Si l(-1)) extended to 200 m between 49 degrees S and 51 degrees S. Numerous empty frustules also were observed, suggesting significant sedimentation of siliceous particles between 49 degrees S and 51 degrees S. Estimates of the BSi production of the Polar Frontal region are derived from C-14 primary production and appropriate BSilPOC ratios, and implications for the total annual production of BSi for the Southern Ocean are discussed. (C) 1997 Elsevier Science Ltd.