The Southern Ocean is regarded as a key region in understanding the role of biogeochemical cycling on the variation of global climate. Although this Ocean is characterized by HNLC (High Nutrient Low Chlorophyll) conditions, areas of high biomass do occur including the Kerguelen-Heard one. These bloom occurrences are attributed to natural fertilization due to micronutrient inputs from the Kerguelen archipelago and plateau, which is therefore an ideal laboratory to study the mechanisms of natural iron fertilization in the Ocean. Better defining these mechanisms was the main aim of the KErguelen Ocean and Plateau compared Study project (KEOPS, PI S. Blain). For this purpose, a good understanding of the parameters forcing the biological activity, the particle dynamics and advection processes in this area was required. Two cruises were realized onboard R/V Marion-Dufresne (IPEV): KEOPS 1 at the end of summer (19 January to 13 February 2005) and centered on the plateau and KEOPS 2 (10 October–20 November 2011) at the beginning of spring and which track was extended to the fertilized plume in the wake of Kerguelen.
Main results :
During KEOPS 1, the mechanism of natural iron fertilization was clearly demonstrated. Diapycnal mixing enhanced by internal wave activity is one of the mechanisms that made the deep iron seen over the plateau available for phytoplankton in surface waters (Park et al., 2008a). The large-scale circulation above and around the Kerguelen plateau (Park et al., 2008b) shows that the bloom region was characterized by weak currents, leading to a water-mass residence time of several months. This enhanced residence time allows the bloom to develop and persist in response to natural iron fertilization throughout the entire season (Blain et al., 2008). The iron-enriched bloom region was dominated by microphytoplankton (diatoms), which contributed 80–90% to the total primary production whereas in the HNLC area, the primary production was mainly achieved by small diatoms and nanoflagellates (Armand et al., 2008). Jouandet et al. (2008) found the bloom area to be a deep CO2 sink compared to the HNLC waters that were close to equilibrium with the atmosphere. However, the efficiency of the export, defined as the ratio of the particulate organic carbon (POC) to primary production, was variable above the plateau, but on average lower (28%) than in surrounding HNLC waters (58%). Observations from KEOPS 1 also evidenced a strong decoupling between the major elemental cycles (nitrate and silicic acid) in the naturally fertilized region (Mosseri et al., 2008).
Main results :
During KEOPS 2 moored sediment trap measured a strong POC flux attenuation over the Kerguelen plateau compared to the open ocean. This « High biomass Low export » (HBLE) scenario may reflect the transfer of carbon biomass to higher and mobile trophic groups that fuel large mammal and bird populations rather than the classical remineralization controlled attenuation characterizing open-ocean environments (Rembauville et al., 2015). Diatom‘s resting spore would accounted for more than 60% of the POC flux by escaping « grazing » pressure. KEOPS 2 proposed the first seasonal estimate of the Si biogeochemical budget above the Kerguelen Plateau based on direct measurements (Closset et al., 2014; Lasbleiz et al., 2014). A shift from a biogenic silica production regime based on the regeneration of H4SiO4 during the early stages of bloom onset to a regime based on a new production during the bloom development was observed. This confirmed the decoupling of the Si and N cycles, which in part explains the strong H4SiO4 depletion of surface water by late summer.