Carbon, oxygen and biological productivity in the Southern Ocean in and out the Kerguelen plume: CARIOCA drifter results

Abstract. The Kerguelen Plateau region in the Indian sector of the Southern Ocean supports annually a large-scale phytoplankton bloom which is naturally fertilized with iron. As part of the second Kerguelen Ocean and Plateau compared Study expedition (KEOPS2) in austral spring (October–November 2011), one CARbon Interface OCean Atmosphere (CARIOCA) buoy was deployed east of the Kerguelen Plateau. It drifted eastward downstream along the Kerguelen plume. Hourly surface measurements of pCO₂, O₂ and ancillary observations were collected between 1 November 2011 and 12 February 2012 with the aim of characterizing the spatial and temporal variability of the biological net community production, NCP, downstream the Kerguelen Plateau, assessing the impact of iron-induced productivity on the biological inorganic carbon consumption and consequently on the CO₂ flux exchanged at the air–sea interface. The trajectory of the buoy up to mid-December was within the longitude range 72–83° E, close to the polar front and then in the polar frontal zone, PFZ, up to 97° E. From 17 November to 16 December, the buoy drifted within the Kerguelen plume following a filament carrying dissolved iron, DFe, for a total distance of 700 km. In the first part of the trajectory of the buoy, within the iron plume, the ocean surface waters were always a sink for CO₂ and a source for O₂, with fluxes of respective mean values equal to −8 mmol CO₂ and +38 mmol O₂ m⁻² d⁻¹. To the east, as the buoy escaped the iron-enriched filament, the fluxes were in the opposite direction, with respective mean values of +5 mmol CO₂ and −48 mmol O₂ m⁻² d⁻¹. These numbers clearly indicate the strong impact of biological processes on the biogeochemistry in the surface waters within the Kerguelen plume in November–mid-December, while it is undetectable to the east in the PFZ from mid- December to mid-February. While the buoy follows the Fe-enriched filament, simultaneous observations of dissolved inorganic carbon (DIC) and dissolved oxygen (O₂) highlight biological events lasting from 2 to 4 days. Stoichiometric ratios, O₂ / C, between 1.1 and 1.4 are observed indicating new and regenerated production regimes. NCP estimates range from 60 to 140 mmol C m⁻² d⁻¹.