Rapid establishment of the CO₂ sink associated with Kerguelen's bloom observed during the KEOPS2/OISO20 cruise

Abstract. Iron and light are the main factors limiting the biological pump of CO₂ in the Southern Ocean. Iron fertilization experiments have demonstrated the potential for increased uptake of atmospheric CO₂, but little is known about the evolution of fertilized environnements. This paper presents observations collected in one of the largest phytoplankton bloom of the Southern Ocean sustained by iron originating from the Kerguelen Plateau. We first complement previous studies by investigating the mechanisms that control air–sea CO₂ fluxes over and downstream of the Kerguelen Plateau at the onset of the bloom based on measurements obtained in October–November 2011. These new observations show the rapid establishment of a strong CO₂ sink in waters fertilized with iron as soon as vertical mixing is reduced. The magnitude of the CO₂ sink was closely related to chlorophyll a and iron concentrations. Because iron concentration strongly depends on the distance from the iron source and the mode of delivery, we identified lateral advection as the main mechanism controlling air–sea CO₂ fluxes downtream the Kerguelen Plateau during the growing season. In the southern part of the bloom, situated over the Plateau (iron source), the CO₂ sink was stronger and spatially more homogeneous than in the plume offshore. However, we also witnessed a substantial reduction in the uptake of atmospheric CO₂ over the Plateau following a strong winds event. Next, we used all the data available in this region in order to draw the seasonal evolution of air–sea CO₂ fluxes. The CO₂ sink is rapidly reduced during the course of the growing season, which we attribute to iron and silicic acid depletion. South of the Polar Front, where nutrients depletion is delayed, we suggest that the amplitude and duration of the CO₂ sink is mainly controlled by vertical mixing. The impact of iron fertilization on air–sea CO₂ fluxes is revealed by comparing the uptake of CO₂ integrated over the productive season in the bloom, between 1 and 1.5 mol C m⁻² yr⁻¹, and in the iron-poor HNLC waters, where we found a typical value of 0.4 mol C m⁻² yr⁻¹. Extrapolating our results to the ice-free Southern Ocean (~50–60° S) suggests that iron fertilization of the whole area would increase the contemporay oceanic uptake of CO₂ by less than 0.1 Pg C yr⁻¹, i.e., less than 1% of the current anthropogenic CO₂ emissions.