ALBERT

Description: Carbon, oxygen and biological productivity in the Southern Ocean in and out the Kerguelen plume: CARIOCA drifter results Biogeosciences Discussions, 11, 16877-16909, 2014 Author(s): L. Merlivat, J. Boutin, and F. d'Ovidio 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 Carioca buoy was deployed east of the Kerguelen plateau. It drifted eastward downstream in the Kerguelen plume. Hourly surface measurements of p CO 2 , O 2 and ancillary observations were collected between 1 November 2011 to 12 February 2012 with the aim of characterizing the spatial and temporal variability of the biological Net Community Production (NCP) downstream the Kerguelen plateau, assess the impact of iron-induced productivity on the biological carbon consumption and consequently on the CO 2 flux exchanged at the air–sea interface. The trajectory of the buoy until mid-December was within the longitude range, 72–83° E, close to the polar front and then in the polar frontal zone, PFZ, until 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, the ocean surface waters are a sink for CO 2 and a source for CO 2 , with fluxes of respective mean values equal to −8 and +38 mmol CO 2 m −2 d −1 . Eastward, as the buoy escapes the iron enriched filament, the fluxes are in opposite direction, with respective mean values of +5 and −48 mmol O 2 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 eastward 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, O 2 , highlight biological events lasting from 2 to 4 days. Stoichiometric ratios, O 2 /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 . Based on the relationship between the time a water parcel has left the plateau and its iron content, we have highlighted that the main control on the value of NCP is the availability of iron in the upper water column, with the largest NCP occurring in waters that have recently left the plateau and presented the highest iron concentrations.