Macronutrient and trace-metal geochemistry of an in situ iron-induced Southern Ocean bloom

We have investigated the effect of iron supply and increased phytoplankton growth on the cycling of the macronutrients phosphate, nitrate and silicic acid as well as the micronutrients copper (Cu), nickel (Ni), zinc (Zn) and cadmium (Cd). Nutrient levels were measured in situ in an iron-induced phyt oplankton bloom at 61 degreesS 140 degreesE in the Southern Ocean Iron Rele ase experiment (SOIREE). Nutrient ratios upon arrival at the study site ind icate that much of the seasonal phytoplankton productivity was by iron-limi ted diatoms growing at low mean light levels. The addition of iron (Fe) ind uced a bloom that led to a draw-down in the macronutrients in ratios consis tent with the growth of diatoms under iron-replete conditions, None of the bioutilised trace metals Cu, Ni, Zn or Cd showed any indication of co-limit ation, with Fe, of phytoplankton growth. Zn concentrations did not decrease by algal uptake as expected. Cd was partitioned to the particulate phase i ndicating consumption by the algae. Cd was preferentially utilised with res pect to P with a alpha (Cd/P) = 5.8. Interpretation of the Cd/Ca data from the sedimentary record using this higher induced alpha (Cd/P) value would i mply even higher Southern Ocean surface water P during the last glacial max imum. (C) 2001 Elsevier Science Ltd. All rights reserved.