Preprints
https://doi.org/10.5194/bg-2021-315
https://doi.org/10.5194/bg-2021-315
 
26 Nov 2021
26 Nov 2021
Status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site)

Liliane Merlivat1, Michael Hemming2, Jacqueline Boutin1, David Antoine3,4, Vincenzo Vellucci5, Melek Golbol5, Gareth A. Lee6, and Laurence Beaumont7 Liliane Merlivat et al.
  • 1Sorbonne Université, CNRS/IRD/MNHN, LOCEAN, IPSL, Paris, France
  • 2Coastal and Regional Oceanography Lab, School of Mathematics and Statistics, University of New South Wales, Sydney, New South Wales, Australia
  • 3Remote Sensing and Satellite Research Group, School of Earth and Planetary Sciences, Curtin University, Perth, WA 6845, Australia
  • 4Sorbonne Université, CNRS, Laboratoire d’Océanographie de Villefranche, Villefranche sur Mer 06230, France
  • 5Sorbonne Université, CNRS, Institut de la Mer de Villefranche, Villefranche sur Mer 06230, France
  • 6Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK
  • 7Division Technique INSU-CNRS, 92195 Meudon CEDEX, France

Abstract. Several trigger mechanisms have been proposed for the onset of the phytoplankton spring bloom. Among these, that phytoplankton cells begin to bloom when they experience higher average light levels in shallower mixed layers, a result of the surface net heat fluxes becoming positive and wind strength decreasing. We study the impact of these two forcings in the northwestern Mediterranean Sea. We take advantage of hourly measurements of oceanic and atmospheric parameters collected at two neighboring moorings during the months of March and April in the years 2016 to 2019, combined with glider data in 2016. The onset of phytoplankton growth is concomitant with the start of significant biological activity detected by a sudden decrease in dissolved inorganic concentrations derived from measurements in the upper 10 m of the water column. A rapid reduction in wind stress following high-wind events is observed at the same time. A resulting shallow mixing layer favors carbon uptake by phytoplankton lasting a few days. Simultaneously, the air-sea net heat flux switches from negative to positive because of changes in the latent air-sea heat flux, which is proportional to the wind speed. This results in an increased thermal stratification of the ocean's surface layers. In 2016, glider data show that the mixing layer is significantly shallower than the mixed layer at the onset of the phytoplankton bloom. We conclude that decreases in the mixing and mixed layer depths lead to the onset of the phytoplankton bloom due to the relaxation of wind speed following storms. We estimate net daily community production in the mixing layer over periods of 3 days between 2016 and 2019 as between 38 mmol C m−2 and 191 mmol C m−2. These results have important implications on the oceanic carbon cycle and biological productivity estimates in the Mediterranean Sea in a scenario of climate-driven changes of the wind.

Liliane Merlivat et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-315', Anonymous Referee #1, 16 Dec 2021
    • AC1: 'Reply on RC1', Liliane Merlivat, 23 Feb 2022
  • RC2: 'Comment on bg-2021-315', Emmanuel Boss, 25 Dec 2021
    • RC3: 'Reply on RC2', Emmanuel Boss, 28 Dec 2021
      • AC3: 'Reply on RC3', Liliane Merlivat, 06 Mar 2022
    • AC2: 'Reply on RC2', Liliane Merlivat, 23 Feb 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-315', Anonymous Referee #1, 16 Dec 2021
    • AC1: 'Reply on RC1', Liliane Merlivat, 23 Feb 2022
  • RC2: 'Comment on bg-2021-315', Emmanuel Boss, 25 Dec 2021
    • RC3: 'Reply on RC2', Emmanuel Boss, 28 Dec 2021
      • AC3: 'Reply on RC3', Liliane Merlivat, 06 Mar 2022
    • AC2: 'Reply on RC2', Liliane Merlivat, 23 Feb 2022

Liliane Merlivat et al.

Liliane Merlivat et al.

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Latest update: 08 Aug 2022
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Short summary
We use in-situ high temporal measurements of dissolved inorganic carbon and atmospheric parameters at the air-sea interface to analyze the initiation of the phytoplankton bloom identified as net rate of biological carbon uptake in the Mediterranean Sea. The shift from wind-driven mixing to buoyancy-driven creates the conditions for blooms to begin. Active mixing at the air-sea interface lead to the onset of the phytoplankton bloom due to the relaxation of wind speed following storms.
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