PRODUCTION OF BIOGENIC SILICA IN THE WEDDELL-SCOTIA SEAS MEASURED WITH SI-32

During leg 2 (November 1988-January 1989) of the European Polarstern study (EPOS) in the Weddell-Scotia Confluence zone, we implemented a new, simple, rapid, and sensitive method based on Si-32 (a beta-emitter in equilibrium with P-32) to determine biogenic silica (BSi) production. This method provides simultaneous determination of the production of organic P. Rates of BSi production ranged from 0.09 to 0.95 mmol Si m-2 h-1, and the BSi content of the photic layer ranged from 7 to 220 mmol m-2. The Si:C production molar ratios varied between 0.08 (small cell density of diatoms in communities dominated by cryptophyceans and dinophyceans) and 0.81 (diatom-dominated populations), while the C:P production ratios varied between 47 and 151 with an average of 105, in agreement with the Redfield ratio. We give here the first direct evidence that in the well-mixed waters of the Scotia Sea highly silicified diatom blooms may occur and that unlike earlier descriptions, because of the intense grazing pressure of krill in the marginal ice zone (MIZ) of the Weddell Sea, the remaining siliceous phytoplankton can grow with only moderate Si production rates. We conclude that the contribution of the MIZ to the total BSi production of the Southern Ocean must not be overstated.