A seasonal carbon budget for a naturally iron-fertilized bloom over the Kerguelen Plateau in the Southern Ocean

During the Kerguelen Ocean and Plateau compared Study (KEOPS, January–February 2005), a high-resolution distribution of surface fugacity of carbon dioxide (fCO2) was obtained from underway measurements. The stations in the core of the naturally iron-fertilized bloom were characterized by low fCO2 (311±8 μatm) compared to the atmosphere, thus representing a large CO2 sink. This contrasted with stations typical of high-nutrient low-chlorophyll (HNLC) conditions where the surface water was roughly in equilibrium with the atmosphere (fCO2=372±5 μatm). The vertical distribution of dissolved inorganic carbon (DIC) also was obtained at stations within and outside the bloom. Based on this data set, we constructed a carbon budget for the mixed layer that allowed us to determine the seasonal net community production (NCPseason) and the seasonal carbon export in two contrasting environments. The robustness of the approach and the errors also were estimated. The NCPseason in the core of the bloom was 6.6±2.2 mol m−2, typical of productive areas of the Southern Ocean. At the HNLC station the NCPseason was 3 times lower than in the bloom. Our estimate of the daily net community production (NCPdaily) within the bloom compares well with shipboard measurements of NCP. The NCPdaily obtained above the Kerguelen Plateau was of the same order as the estimates from Southern Ocean artificial iron-fertilization experiments (SOIREE and EisenEx). The seasonal carbon export was derived from NCPseason after subtraction of the seasonal accumulation of particulate and dissolved organic carbon. In the bloom, the carbon export (5.4±1.9 mol m−2) was 3-fold higher than at the HNLC station (1.7±0.4 mol m−2). Comparison of our results to artificial iron-fertilization experiments shows that the biological pump is enhanced by natural iron fertilization.