Mesopelagic carbon remineralization during the European Iron Fertilization Experiment

The impact of iron fertilization on mesopelagic carbon (C) remineralization was assessed during the European Iron Fertilization Experiment (EIFEX) in the Southern Ocean by following the temporal change of excess particulate barium (also called biogenic or Baxs) in the mesopelagic waters of a mesoscale eddy. Before the iron infusion the site was already sustaining a significant vertical flux of particles leading to organic remineralization in the mesopelagic. Approximately 35 d after the fertilization, mesopelagic Baxs contents provided evidence of changes in surface particulate export and of enhanced and particularly fast remineralization extending down to 1000 m. At the end of the experiment (day 36) both export and remineralization decreased. Organic carbon remineralization took place mainly in the lower part of the mesopelagic zone (500–1000 m) and increased more than 5‐fold during EIFEX, reaching values up to 92 ± 15 mg C m−2 d−1. However, such remineralization rates are not particularly high when compared to other values reported for the natural Southern Ocean during summer. Though export and remineralization reached peak values around day 34, remineralization integrated over the 150 to 1000 m layer accounted only for 13 ± 1.4% of the export at 150 m, suggesting that a substantial amount of exported carbon reached deeper in the water column. Compared to natural blooms in high‐nutrient low‐chlorophyll (HNLC) waters of the Southern Ocean, the Fe‐induced bloom during EIFEX resulted in a lower ratio of remineralized organic carbon over exported carbon, similar to what we recently observed for natural Fe‐replete conditions prevailing above the Kerguelen Plateau. Unexpected similarities and phase lags of profiles in the mesopelagic between casts sampled inside and outside the fertilized patch during EIFEX indicated that eddy dynamics were a determinant in setting particle patterns on a broad spatial scale exceeding the extent of the fertilization, thereby homogenizing the signals of mesopelagic remineralization.