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 nutrients^1–3. 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 pressure^7–11, for example over glacial–interglacial periods.