The particulate carbon flux and the link with the phytoplankton community : an approach with BioGeoChemical-Argo profiling floats

• Main Author: Terrats, Louis
• Other Authors: Laboratoire d'océanographie de Villefranche (LOV), Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut de la Mer de Villefranche (IMEV), Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Sorbonne Université, Hervé Claustre
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2022
• Subjects: Carbon cycle; Biological Carbon Pum; Gravitational Pump; Phytoplankton; BioGeoChemical-Argo (BGC-Argo) floats; Coccolithophores; Cycle de carbone; Pompe Biologique de Carbone; Pompe gravitationnelle; Phytoplancton; Flotteurs BioGeoChimique-Argo; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Southern Ocean
• Online Access: https://theses.hal.science/tel-04333533; https://theses.hal.science/tel-04333533/document; https://theses.hal.science/tel-04333533/file/TERRATS_Louis_these_2022.pdf

The ocean plays a key role in the climate by exchanging large quantities of carbon with the atmosphere. Atmospheric carbon is fixed at the ocean surface by phytoplankton that transforms it into biogenic carbon, part of which is transported to the deep ocean by physical and biological mechanisms; this is the Biological Carbon Pump (BCP). A tiny fraction of this biogenic carbon reaches sufficient depths to be sequestered for several centuries before it returns to the atmosphere, thus regulating concentrations of atmospheric CO2. Today, we know enough about the BCP to recognize its importance in climate, but our knowledge of its functioning is limited due to insufficient sampling of biogenic carbon fluxes. Here, we used BioGeoChimical-Argo floats, observational platforms designed to solve the undersampling problem, to explore a major mechanism of the BCP called the gravitational pump. The gravitational pump is the transport of biogenic carbon in the form of organic particles (POC) that sink from the surface into the deep ocean. Our study of the gravitational pump is divided into three axes. The first axis consisted of developing a method to detect blooms of coccolithophores, a major phytoplankton group that potentially has an important control on the transport of POC at depth. The second axis focused on the seasonal and regional variability of POC fluxes in the Southern Ocean, an undersampled area in which several floats have been deployed with an optical sediment trap (OST). Only ten floats were equipped with an OST, which is low compared to the whole BGC-Argo fleet (i.e. several hundred floats). Therefore, in the third axis, we developed a method to estimate the POC flux with the standard sensors of BGC-Argo floats. This method was then applied to hundreds of floats to describe the seasonal variability of the POC flux in many regions. In this study, we also highlighted the link between the POC flux and the nature of surface particles. For example, we calculated relationships between phytoplankton community ...