Diatom biodiversity and the modulation of biogeochemical fluxes of carbon and silicon in pelagic polar ecosystems

• Main Author: Lafond, Augustin
• Other Authors: Institut méditerranéen d'océanologie (MIO), Institut de Recherche pour le Développement (IRD)-Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Toulon (UTLN)-Centre National de la Recherche Scientifique (CNRS), Aix Marseille Université, Bernard Quéguiner, Karine Leblanc
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2021
• Subjects: Diatoms; Biological pump; Community structure; Baffin Bay; Kerguelen Plateau; Arctic; Southern Ocean; Biogeochemistry; Diatomées; Pompe biologique; Structure des communautés; Baie de Baffin; Plateau des Kerguelen; Arctique; Océan Austral; Biogéochimie; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDE.BE]Environmental Sciences/Biodiversity and Ecology; Arctique*; Baffin; ice pack; Phytoplankton; Sea ice
• Online Access: https://theses.hal.science/tel-03137420; https://theses.hal.science/tel-03137420/document; https://theses.hal.science/tel-03137420/file/These_LAFOND.pdf

Polar regions are particularly affected by the ongoing climate changes which have profound consequences on the development of phytoplankton and on the functioning of the biological pump (vertical transfer of particulate organic carbon from the surface to the deep ocean). Plankton community structure and species-specific properties (size, shape, cellular content in Si and C, physiological state, life stage, etc.) can strongly influence biogeochemical cycles and the export of organic matter (e.g. C, N) and biomineral (e.g. Si). In high latitudes, diatoms are a major contributor to primary production and exhibit a great diversity of species. The objectives of this PhD are (1) to identify the factors controlling the development and the structure of diatom communities, (2) to assess the influence of species diversity on the composition and stoichiometry of particulate matter, and (3) to understand mortality and export modes in the deep ocean. This work is based on the study of two subpolar regions located in both the northern and southern hemispheres: (1) the Baffin Bay, a seasonal ice zone in the Arctic, and (2) the Kerguelen region, a permanently open ocean zone in the Southern Ocean. In the latter region, we studied two contrasted environments: a low production HNLC (High Nutrient Low Chlorophyll) area versus a high production iron-fertilized area located above the Kerguelen plateau. In Baffin Bay, we highlight a diatom community succession in relation with sea ice melting and the strong west-to-east water masses gradient prevailing in the bay. A low productive community dominated by pennate diatoms was found beneath the consolidated ice pack (pre-bloom conditions), in ice-free Pacific influenced waters, and on the easternside of the bay (post-bloom conditions). In contrast, an actively growing diatom bloom composed of a mixed assemblage of centric (e.g. Chaetoceros, Thalassiosira) and pennate diatoms was found in the marginal ice zone. These results highlight the importance of the melting sea ice for the ...