Preprints
https://doi.org/10.5194/bg-2021-283
https://doi.org/10.5194/bg-2021-283

  03 Nov 2021

03 Nov 2021

Review status: this preprint is currently under review for the journal BG.

The impact of the South-East Madagascar bloom on the oceanic CO2 sink

Nicolas Metzl1, Claire Lo Monaco1, Coraline Leseurre1, Céline Ridame1, Jonathan Fin1, Claude Mignon1, Marion Gehlen2, and Thi Tuyet Trang Chau2 Nicolas Metzl et al.
  • 1Laboratoire LOCEAN/IPSL, Sorbonne Université-CNRS-IRD-MNHN, Paris, 75005, France
  • 2Laboratoire LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay Gif-sur-Yvette, 91191, France

Abstract. We described new sea surface CO2 observations in the southwestern Indian Ocean obtained in January 2020 when a strong bloom event occurred south-east of Madagascar and extended eastward in the oligotrophic Indian Ocean subtropical domain. Compared to previous years (1991–2019) we observed very low fCO2 and dissolved inorganic carbon concentrations (CT) in austral summer 2020, indicative of a biologically driven process. In the bloom the anomaly of fCO2 and CT reached respectively −33 µatm and −42 µmol.kg-1 whereas no change is observed for alkalinity (AT). In January 2020 we estimated a local maximum of air-sea CO2 flux at 27° S of −6.9 mmol.m-2.d-1 (ocean sink) and −4.3 mmol.m-2.d-1 when averaging the flux in the band 26–30° S. In the domain 25–30° S/50–60° E we estimated that the bloom led to a regional carbon uptake of about −1 TgC.month-1 in January 2020 whereas this region was previously recognized as an ocean CO2 source or near equilibrium during this season. Using a neural network approach that reconstructs the monthly fCO2 fields we estimated that when the bloom was at peak in December 2019 the CO2 sink reached −3.1 (±1.0) mmol.m-2.d-1 in the band 25–30° S, i.e. the model captured the impact of the bloom. Integrated in the domain restricted to 25–30° S/50–60° E the region was a CO2 sink in December 2019 of −0.8 TgC.month-1 compared to a CO2 source of +0.12 (± 0.10) TgC.month-1 in December when averaged over the period 1996–2018. Consequently in 2019 this region was a stronger CO2 annual sink of −8.8 TgC.yr-1 compared to −7.0 (±0.5) TgC.yr-1 averaged over 1996–2018. In austral summer 2019/2020, the bloom was likely controlled by relatively deep mixed-layer depth during preceding winter (July–September 2019) that would supply macro and/or micro-nutrients as iron to the surface layer to promote the bloom that started in November 2019 in two large rings in the Madagascar Basin. Based on measurements in January 2020, we observed relatively high N2 fixation rates (up to 18 nmol N.L-1.d-1) suggesting that diazotrophs could play a role on the bloom in the nutrient depleted waters. The bloom event in austral summer 2020, along with the new carbonate system observations, represents a benchmark case for complex biogeochemical model sensitivity studies (including N2-fixation process and iron supplies) for a better understanding on the origin and termination of this still <q>mysterious</q> sporadic bloom and its impact on ocean carbon uptake in the future.

Nicolas Metzl et al.

Status: open (until 15 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-283', Meric Srokosz, 25 Nov 2021 reply

Nicolas Metzl et al.

Nicolas Metzl et al.

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Short summary
During an oceanographic cruise conducted in January 2020 in the south-western Indian Ocean we observed very low CO2 concentrations associated to a strong phytoplankton bloom that occurred south-east of Madagascar. This biological event leaded to a strong regional CO2 ocean sink not previously observed.
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