Mesoscale biogeochemical responses to iron fertilization in the upper layers of the Southern Ocean Iron Experiment areas

During the Southern Ocean Iron Experiment (SOFeX), January-February 2002, two iron fertilization experiments were conducted at the south (66.45 degrees S, 171.8 degrees W) and north (56.23 degrees S, 172 degrees W) patches. The south patch was replete with all macronutrients, whereas the north patch was nearly depleted of silicate. Using a towed water sampling/measurement system, high resolution three-dimensional observations of temperature, salinity, CO2 partial pressure (pCO(2)) and the concentrations of total CO2 (TCO2), nitrate, phosphate and silicate in the upper 100 m were obtained 10 and 28 days after initial iron additions in the south and north patches, respectively. TCO2 and nutrient drawdowns observed in the south patch were small due to wind events that limited biological response in the early postfertilization period. At the north patch, drawdown signals were stronger, resulting from the longer time since fertilization. The north patch was dispersed along a frontal zone between two distinct waters, with the denser subducted under the lighter. Subduction limited CO2 uptake from the air, but provided alternative mechanisms for vertical carbon export. Based on the biogeochemical drawdowns and published estimates of the rate of the entrainment of nonfertilized waters into the patch, the mean net community productivity over 28 days was estimated to be 94 +/- 12 mmol carbon m(-2) day(-1), to which the air-to-sea flux contributed only 2%. Despite low silicate concentrations and potential limitation of growth of silicifying plankton in the north patch, our estimates of the enhanced carbon uptake are similar to estimates from higher silicate environments.