Estimation of seasonal and inter-annual variations in biomass and phytoplankton composition in the Southern Ocean over the past two decades and evaluation of their ecological consequences

• Main Author: Le Ster, Loïc
• 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), Centre d'Études Biologiques de Chizé - UMR 7372 (CEBC), La Rochelle Université (ULR)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE), Sorbonne Université, Christophe Guinet, Hervé Claustre
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2023
• Subjects: Phytoplankton; Southern Ocean; Chlorophyll a fluorescence; Ocean Colour; seasonal cycle; sensor calibration; southern elephant seal; bio-logging; Phytoplancton; Océan Austral; Fluorescence de la chlorophylle a; Couleur de l'eau; cycle saisonnier; calibration de capteur; éléphant de mer; [SDE.BE]Environmental Sciences/Biodiversity and Ecology; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Austral; Indian; Elephant Seal; Mirounga leonina; Southern Elephant Seal
• Online Access: https://hal.science/tel-04255749; https://hal.science/tel-04255749/document; https://hal.science/tel-04255749/file/LE_STER_Loic.pdf

Discipline: Océan Atmosphère, Climat et Observations SpatialesSpécialité: Océanographie The main objective of this thesis is to examine the timescales associated with variations in phytoplankton concentration in the Southern Ocean and provide key data to understand how external forcing conditions phytoplankton growth at various spatio-temporal scales. Furthermore, this thesis opens the way to the assessment of the influence of variations in primary production on the ecosystem. The first objective of this thesis was to disentangle the different timescales of variation present in long-term time series of chlorophyll-a concentration estimated from satellite data. The first objective was specifically to characterize the different types of phytoplankton phenologies in the Southern Ocean, to study their spatial distribution and to assess their evolution in time. Secondly, the work in this thesis focused on the Indian sector of the Southern Ocean to studymore specifically the linkages between primary production dynamics and ecosystem functioning. In the second objective, we defined bioregions based on phenology of satellite chlorophyll-a concentration estimates. The dynamics of these bioregions were first studied in terms of phytoplankton community composition using a satellite model. Then, indicators derived from tracking data of a top predator - the southern elephant seal (Mirounga leonina) - were used to infer potential influence of variations in primary production on higher trophic levels. Finally, in a third objective, we proposed a method based on a cost-effective optical variable - namely, lightattenuation in the water column - to improve accuracy and spatial resolution of bio-logging-derived chlorophyll a fluorescence measurements. In the context of upcoming high-resolution oceancirculation satellite missions, the method shows promising for the studying of bio-physical interactions at (sub)mesoscale in the Southern Ocean. L'objectif principal de cette thèse est d'examiner les échelles de temps associées aux ...