Biogenic silica production rates and particulate organic matter distribution in the Atlantic sector of the Southern Ocean during austral spring 1992

Several of the components of the silicon cycle—orthosilicic acid (Si(OH)4), biogenic silica (BSi), and biogenic silica production rates (ρSi)—have been investigated, together with the distribution of particulate organic carbon (POC), particulate organic nitrogen (PON) and carbon primary production (ρC), on a series of transects across three sub-systems in the Atlantic sector of the Southern Ocean (6°W): the seasonal ice zone (SIZ), the permanently open ocean zone (POOZ), and the southern boundary of the polar frontal zone (PFZ). The study was conducted in Spring 1992 as part of the European SO-JGOFS cruise aboard the R.V. Polarstern. High BSi concentrations (maximum: 11.7 μmol Si l−1) were recorded in late November at the southern border of the PFZ. In contrast, no large BSi biomass was found in the other subsystems studied. In the SIZ, no diatom bloom was observed, despite a sea-ice retreat of 200 km during the study period, and BSi biomass never exceeded 0.6 μmol Si l−1. The POOZ also showed very low BSi biomass (<0.5 μmol Si l−1), and low BSi/POC molar ratios from the surface to 200 m (0.04–0.06 at 53°S) suggest that this was an area where phytoplankton were not dominated by siliceous organisms. At the southern border of the PFZ, BSi/POC molar ratios were among the highest ever recorded in the surface waters of the Southern Ocean (maximum: 1.33). This could be a result of the presence of heavily silicified diatoms or also could reflect a more rapid recycling of POC as compared to BSi. High concentrations of BSi (>1.5 μmol Si l−1) extended well below the euphotic zone to 200 m depth between 49°S and 51°S, suggesting significant sedimentation of siliceous particles in that area. High values of ρSi also were observed in the PFZ (29.6–60.7 mmol Si m−2 d−1, during the production maximum) indicating that this subsystem is important in the biogeochemical budget of the Southern Ocean. High depth-integrated ρSi/ρC (0.25–0.46) and BSi/POC (0.53–0.85) in the PFZ imply the production of diatoms rich in silica compared to organic matter. The high rates of silica production observed in the PFZ support the recent hypothesis that the formation of the abyssal siliceous oozes that encircle much of Antartica form primarily as the result of high levels of silica production in surface waters rather than as a result of high rates of opal preservation as has been suggested in the past.