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This paper presents results on the spatial and temporal distribution patterns of mesozooplankton in the naturally fertilized region to the east of the Kerguelen Islands (Southern Ocean) visited at early bloom stage during the KEOPS2 survey (15 October to 20 November 2011). The aim of this study was to compare the zooplankton response in contrasted environments localized over the Kerguelen Plateau in waters of the east shelf and shelf edge and in productive oceanic deep waters characterized by conditions of complex circulation and rapidly changing phytoplankton biomass. The mesozooplankton community responded to the spring bloom earlier on the plateau than in the oceanic waters, where complex mesoscale circulation stimulated initial more or less ephemeral blooms before a broader bloom extension. Taxonomic compositions showed a high degree of similarity across the whole region, and the populations initially responded to spring bloom with a large production of larval forms increasing abundances, without biomass changes. Taxonomic composition and stable isotope ratios of size-fractionated zooplankton indicated the strong domination of herbivores, and the total zooplankton biomass values over the survey presented a significant correlation with the integrated chlorophyll concentrations in the mixed layer. The biomass stocks observed at the beginning of the KEOPS2 cruise were around 1.7 g C m−2 above the plateau and 1.2 g C m−2 in oceanic waters. Zooplankton biomass in oceanic waters remained on average below 2 g C m−2 over the study period, except for one station in the Polar Front zone (F-L), whereas zooplankton biomasses were around 4 g C m−2 on the plateau at the end of the survey. The most remarkable feature during the sampling period was the stronger increase in abundance in the oceanic waters (25 × 103 to 160 × 103 ind m−2) than on the plateau (25 × 103 to 90 × 103 ind m−2). The size structure and taxonomic distribution patterns revealed a cumulative contribution of

While production of aggregates and their subsequent sinking is known to be one pathway for the downward movement of organic matter from the euphotic zone, the rapid transition from non-aggregated to aggregated particles has not been reported previously. We made one vertical profile of particle size distributions (PSD; sizes ranging from 0.052 to several millimeters in equivalent spherical diameter) at pre-bloom stage and seven vertical profiles 3 weeks later over a 48 h period at early bloom stage using the Underwater Vision Profiler during the Kerguelen Ocean and Plateau Compared Study cruise 2 (KEOPS2, October-November 2011). In these naturally iron-fertilized waters southeast of Kerguelen Island (Southern Ocean), the total particle numerical abundance increased by more than fourfold within this time period. A massive total volume increase associated with particle size distribution changes was observed over the 48 h survey, showing the rapid formation of large particles and their accumulation at the base of the mixed layer. The results of a one-dimensional particle dynamics model support coagulation as the mechanism responsible for the rapid aggregate formation and the development of the V-T subsurface maxima. The comparison of VT profiles between early bloom stage and pre-bloom stage indicates an increase of particulate export below 200 m when bloom has developed. These results highlight the role of coagulation in forming large particles and triggering carbon export at the early stage of a naturally iron-fertilized bloom, while zooplankton grazing may dominate later in the season. The rapid changes observed illustrate the critical need to measure carbon export flux with high sampling temporal resolution. Our results are the first published in situ observations of the rapid accumulation of marine aggregates and their export and the general agreement of this rapid event with a model of phytoplankton growth and coagulation.

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 (31178 matm) 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 ¼ 37275 matm). 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 6.672.2 molm2, 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 (5.471.9 molm2) was 3-fold higher than at the HNLC station (1.770.4 molm2). Comparison of our results to artificial iron-fertilization experiments shows that the biological pump is enhanced by natural iron fertilization.

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 (31178 matm) 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 ¼ 37275 matm). 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 6.672.2 molm2, 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 (5.471.9 molm2) was 3-fold higher than at the HNLC station (1.770.4 molm2). Comparison of our results to artificial iron-fertilization experiments shows that the biological pump is enhanced by natural iron fertilization.

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 (31178 matm) 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 ¼ 37275 matm). 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 6.672.2 molm2, 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 (5.471.9 molm2) was 3-fold higher than at the HNLC station (1.770.4 molm2). Comparison of our results to artificial iron-fertilization experiments shows that the biological pump is enhanced by natural iron fertilization.

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 (31178 matm) 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 ¼ 37275 matm). 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 6.672.2 molm2, 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 (5.471.9 molm2) was 3-fold higher than at the HNLC station (1.770.4 molm2). Comparison of our results to artificial iron-fertilization experiments shows that the biological pump is enhanced by natural iron fertilization.