Large decrease in ocean-surface CO2 fugacity in response to in situ iron fertilization

Abstract

THE equatorial Pacific Ocean is a 'high-nitrate, low-chlorophyll' region where nitrate and phosphate are abundant all year round. These nutrients cannot therefore be limiting to phytoplankton production. It has been suggested that the bioavailability of iron—a micronutrient—may be preventing full biological utilization of the major nutrients1–3. The results of a previous in situ iron fertilization experiment in this region provided support for this hypothesis4, but the observed biological response resulted in only a small decrease in surface-water CO2 fugacity5. Here we report a much larger, biologically induced uptake of surface-water CO2 that occurred during a second study6. The fugacity of CO2 in the centre of the (iron-fertilized) patch of surface ocean fell from a background value near 510 μatm to approximately 420 μatm, corresponding to a transient 60% decrease in the natural ocean-to-atmosphere CO2 flux. We conclude that iron supply to this ocean region can strongly modulate the local short-term source of CO2 to the atmosphere, but has little long-term influence on atmospheric CO2 partial pressure. However, if such a modulation also occurs in the Southern Ocean, then iron bioavailability at high southern latitudes could have a significant effect on atmospheric CO2 partial pressure7–11, for example over glacial–interglacial periods.